MKSH(1) MidnightBSD General Commands Manual MKSH(1)

NAME

mksh, sh — MirBSD Korn shell

SYNOPSIS

mksh [−+abCefhiklmnprUuvXx] [−T /dev/ttyCn ] [−+o option] [

−c string | −s |  file  [argument ... ]]

DESCRIPTION

mksh is a command interpreter intended for both interactive and shell script use. Its command language is a superset of the sh(C) shell language and largely compatible to the original Korn shell.

The options are as follows:

−c string

mksh will execute the command(s) contained in string.

−i

Interactive shell. A shell is ‘‘interactive’’ if this option is used or if both standard input and standard error are attached to a tty(4). An interactive shell has job control enabled, ignores the SIGINT, SIGQUIT, and SIGTERM signals, and prints prompts before reading input (see the PS1 and PS2 parameters). It also processes the ENV parameter or $HOME/.mkshrc (see below). For non-interactive shells, the trackall option is on by default (see the set command below).

−l

Login shell. If the basename the shell is called with (i.e. argv[0]) starts with ‘−’ or if this option is used, the shell is assumed to be a login shell and the shell reads and executes the contents of /etc/profile and $HOME/.profile if they exist and are readable.

−p

Privileged shell. A shell is ‘‘privileged’’ if this option is used or if the real user ID or group ID does not match the effective user ID or group ID (see getuid(2) and getgid(2)). A privileged shell does not process $HOME/.profile nor the ENV parameter or $HOME/.mkshrc (see below). Instead, the file /etc/suid_profile is processed. Clearing the privileged option causes the shell to set its effective user ID (group ID) to its real user ID (group ID).

−r

Restricted shell. A shell is ‘‘restricted’’ if this option is used. The following restrictions come into effect after the shell processes any profile and ENV files:

The cd command is disabled.

The SHELL, ENV, and PATH parameters cannot be changed.

Command names can’t be specified with absolute or relative paths.

The −p option of the built-in command command can’t be used.

Redirections that create files can’t be used (i.e. ‘>’, ‘>|’, ‘>>’, ‘<>’).

−s

The shell reads commands from standard input; all non-option arguments are positional parameters.

−T tty

Spawn mksh on the tty(4) device given. Superuser only. If tty is a dash, detach from controlling terminal (daemonise) instead.

In addition to the above, the options described in the set built-in command can also be used on the command line: both [−+abCefhkmnuvXx] and [−+o option] can be used for single letter or long options, respectively.

If neither the −c nor the −s option is specified, the first non-option argument specifies the name of a file the shell reads commands from. If there are no non-option arguments, the shell reads commands from the standard input. The name of the shell (i.e. the contents of $0) is determined as follows: if the −c option is used and there is a non-option argument, it is used as the name; if commands are being read from a file, the file is used as the name; otherwise, the basename the shell was called with (i.e. argv[0]) is used.

If the ENV parameter is set when an interactive shell starts (or, in the case of login shells, after any profiles are processed), its value is subjected to parameter, command, arithmetic, and tilde (‘~’) substitution and the resulting file (if any) is read and executed. If the ENV variable is unset or empty, the file $HOME/.mkshrc is read and processed like above instead, leaving ENV unchanged. This processing does not occur if ENV is set to a non-existing filename.

The exit status of the shell is 127 if the command file specified on the command line could not be opened, or non-zero if a fatal syntax error occurred during the execution of a script. In the absence of fatal errors, the exit status is that of the last command executed, or zero, if no command is executed.

Command syntax
The shell begins parsing its input by removing any backslash-newline combinations, then breaking it into words. Words (which are sequences of characters) are delimited by unquoted whitespace characters (space, tab, and newline) or meta-characters (

‘<’, ‘>’, ‘|’, ‘;’, ‘(’, ‘)’, and ‘&’ ). Aside from delimiting words, spaces and tabs are ignored, while newlines usually delimit commands. The meta-characters are used in building the following tokens: ‘<’, ‘<&’, ‘<<’, ‘<<<’, ‘>’, ‘>&’, ‘>>’, ‘&>’, etc. are used to specify redirections (see Input/output redirection below); ‘|’ is used to create pipelines; ‘|&’ is used to create co-processes (see Co-processes below); ‘;’ is used to separate commands; ‘&’ is used to create asynchronous pipelines; ‘&&’ and ‘||’ are used to specify conditional execution; ‘;;’ is used in case statements; ‘(( .. ))’ is used in arithmetic expressions; and lastly, ‘( .. )’ is used to create subshells.

Whitespace and meta-characters can be quoted individually using a backslash (‘\’), or in groups using double (‘"’) or single (‘'’) quotes. Note that the following characters are also treated specially by the shell and must be quoted if they are to represent themselves: ‘\’, ‘"’, ‘'’, ‘#’, ‘$’, ‘`’, ‘~’, ‘{’, ‘}’, ‘*’, ‘?’, and ‘[’. The first three of these are the above mentioned quoting characters (see Quoting below); ‘#’, if used at the beginning of a word, introduces a comment – everything after the ‘#’ up to the nearest newline is ignored; ‘$’ is used to introduce parameter, command, and arithmetic substitutions (see Substitution below); ‘`’ introduces an old-style command substitution (see Substitution below); ‘~’ begins a directory expansion (see Tilde expansion below); ‘{’ and ‘}’ delimit csh(1)-style alterations (see Brace expansion below); and finally, ‘*’, ‘?’, and ‘[’ are used in file name generation (see File name patterns below).

As words and tokens are parsed, the shell builds commands, of which there are two basic types: simple-commands, typically programmes that are executed, and compound-commands, such as for and if statements, grouping constructs, and function definitions.

A simple-command consists of some combination of parameter assignments (see Parameters below), input/output redirections (see Input/output redirections below), and command words; the only restriction is that parameter assignments come before any command words. The command words, if any, define the command that is to be executed and its arguments. The command may be a shell built-in command, a function, or an external command (i.e. a separate executable file that is located using the PATH parameter; see Command execution below). Note that all command constructs have an exit status: for external commands, this is related to the status returned by wait(2) (if the command could not be found, the exit status is 127; if it could not be executed, the exit status is 126); the exit status of other command constructs (built-in commands, functions, compound-commands, pipelines, lists, etc.) are all well-defined and are described where the construct is described. The exit status of a command consisting only of parameter assignments is that of the last command substitution performed during the parameter assignment or 0 if there were no command substitutions.

Commands can be chained together using the ‘|’ token to form pipelines, in which the standard output of each command but the last is piped (see pipe(2)) to the standard input of the following command. The exit status of a pipeline is that of its last command. A pipeline may be prefixed by the ‘!’ reserved word which causes the exit status of the pipeline to be logically complemented: if the original status was 0, the complemented status will be 1; if the original status was not 0, the complemented status will be 0.

Lists of commands can be created by separating pipelines by any of the following tokens: ‘&&’, ‘||’, ‘&’, ‘|&’, and ‘;’. The first two are for conditional execution: ‘‘cmd1 && cmd2’’ executes cmd2 only if the exit status of cmd1 is zero; ‘||’ is the opposite – cmd2 is executed only if the exit status of cmd1 is non-zero. ‘&&’ and ‘||’ have equal precedence which is higher than that of ‘&’, ‘|&’, and ‘;’, which also have equal precedence. Note that the ‘&&’ and ‘||’ operators are "left-associative". For example, both of these commands will print only "bar":

$ false && echo foo || echo bar
$ true || echo foo && echo bar

The ‘&’ token causes the preceding command to be executed asynchronously; that is, the shell starts the command but does not wait for it to complete (the shell does keep track of the status of asynchronous commands; see Job control below). When an asynchronous command is started when job control is disabled (i.e. in most scripts), the command is started with signals SIGINT and SIGQUIT ignored and with input redirected from /dev/null (however, redirections specified in the asynchronous command have precedence). The ‘|&’ operator starts a co-process which is a special kind of asynchronous process (see Co-processes below). Note that a command must follow the ‘&&’ and ‘||’ operators, while it need not follow ‘&’, ‘|&’, or ‘;’. The exit status of a list is that of the last command executed, with the exception of asynchronous lists, for which the exit status is 0.

Compound commands are created using the following reserved words. These words are only recognised if they are unquoted and if they are used as the first word of a command (i.e. they can’t be preceded by parameter assignments or redirections):

case else function then !
do esac if time [[
done fi in until {
elif for select while }

Note: Some shells (but not this one) execute control structure commands in a subshell when one or more of their file descriptors are redirected, so any environment changes inside them may fail. To be portable, the exec statement should be used instead to redirect file descriptors before the control structure.

In the following compound command descriptions, command lists (denoted as list) that are followed by reserved words must end with a semicolon, a newline, or a (syntactically correct) reserved word. For example, the following are all valid:

$ { echo foo; echo bar; }
$ { echo foo; echo bar<newline>}
$ { { echo foo; echo bar; } }

This is not valid:

$ { echo foo; echo bar }

(list)

Execute list in a subshell. There is no implicit way to pass environment changes from a subshell back to its parent.

{ list; }

Compound construct; list is executed, but not in a subshell. Note that ‘{’ and ‘}’ are reserved words, not meta-characters.

case word in [
([]  pattern [| pattern] ... ) list ;; ] ... esac

The case statement attempts to match word against a specified pattern; the list associated with the first successfully matched pattern is executed. Patterns used in case statements are the same as those used for file name patterns except that the restrictions regarding ‘.’ and ‘/’ are dropped. Note that any unquoted space before and after a pattern is stripped; any space within a pattern must be quoted. Both the word and the patterns are subject to parameter, command, and arithmetic substitution, as well as tilde substitution. For historical reasons, open and close braces may be used instead of in and esac e.g. case $foo { *) echo bar; }. The exit status of a case statement is that of the executed list; if no list is executed, the exit status is zero.

for name [
in word ...]; do list; done

For each word in the specified word list, the parameter name is set to the word and list is executed. If in is not used to specify a word list, the positional parameters ($1, $2, etc.) are used instead. For historical reasons, open and close braces may be used instead of do and done e.g. for i; { echo $i; }. The exit status of a for statement is the last exit status of list; if list is never executed, the exit status is zero.

if list; then list; [
elif list; then list;] ... [
else list;] fi

If the exit status of the first list is zero, the second list is executed; otherwise, the list following the elif, if any, is executed with similar consequences. If all the lists following the if and elifs fail (i.e. exit with non-zero status), the list following the else is executed. The exit status of an if statement is that of non-conditional list that is executed; if no non-conditional list is executed, the exit status is zero.

select name [
in word ...]; do list; done

The select statement provides an automatic method of presenting the user with a menu and selecting from it. An enumerated list of the specified word(s) is printed on standard error, followed by a prompt (

PS3: normally ‘#? ’ ). A number corresponding to one of the enumerated words is then read from standard input, name is set to the selected word (or unset if the selection is not valid), REPLY is set to what was read (leading/trailing space is stripped), and list is executed. If a blank line (i.e. zero or more IFS characters) is entered, the menu is reprinted without executing list.

When list completes, the enumerated list is printed if REPLY is NULL, the prompt is printed, and so on. This process continues until an end-of-file is read, an interrupt is received, or a break statement is executed inside the loop. If ‘‘in word ...’’ is omitted, the positional parameters are used (i.e. $1, $2, etc.). For historical reasons, open and close braces may be used instead of do and done e.g. select i; { echo $i; }. The exit status of a select statement is zero if a break statement is used to exit the loop, non-zero otherwise.

until list; do list; done

This works like while, except that the body is executed only while the exit status of the first list is non-zero.

while list; do list; done

A while is a pre-checked loop. Its body is executed as often as the exit status of the first list is zero. The exit status of a while statement is the last exit status of the list in the body of the loop; if the body is not executed, the exit status is zero.

function name { list; }

Defines the function name (see Functions below). Note that redirections specified after a function definition are performed whenever the function is executed, not when the function definition is executed.

name() command

Mostly the same as function (see Functions below).

time [−p] [pipeline]

The time reserved word is described in the Command execution section.

(( expression ))

The arithmetic expression expression is evaluated; equivalent to ‘‘let expression’’ (see Arithmetic expressions and the let command, below).

[[ expression ]]

Similar to the test and [ ... ] commands (described later), with the following exceptions:

Field splitting and file name generation are not performed on arguments.

The −a (AND) and −o (OR) operators are replaced with ‘&&’ and ‘||’, respectively.

Operators (e.g. ‘−f’, ‘=’, ‘!’) must be unquoted.

The second operand of the ‘!=’ and ‘=’ expressions are patterns (e.g. the comparison [[ foobar = f*r ]] succeeds).

The single argument form of test, which tests if the argument has a non-zero length, is not valid; explicit operators must always be used, e.g. instead of [ str ] use [[ −n str ]].

Parameter, command, and arithmetic substitutions are performed as expressions are evaluated and lazy expression evaluation is used for the ‘&&’ and ‘||’ operators. This means that in the following statement, $(<foo) is evaluated if and only if the file foo exists and is readable:

$ [[ −r foo && $(<foo) = b*r ]]

Quoting
Quoting is used to prevent the shell from treating characters or words specially. There are three methods of quoting. First, ‘\’ quotes the following character, unless it is at the end of a line, in which case both the ‘\’ and the newline are stripped. Second, a single quote (‘'’) quotes everything up to the next single quote (this may span lines). Third, a double quote (‘"’) quotes all characters, except ‘$’, ‘`’ and ‘\’, up to the next unquoted double quote. ‘$’ and ‘`’ inside double quotes have their usual meaning (i.e. parameter, command, or arithmetic substitution) except no field splitting is carried out on the results of double-quoted substitutions. If a ‘\’ inside a double-quoted string is followed by ‘\’, ‘$’, ‘`’, or ‘"’, it is replaced by the second character; if it is followed by a newline, both the ‘\’ and the newline are stripped; otherwise, both the ‘\’ and the character following are unchanged.

Aliases
There are two types of aliases: normal command aliases and tracked aliases. Command aliases are normally used as a short hand for a long or often used command. The shell expands command aliases (i.e. substitutes the alias name for its value) when it reads the first word of a command. An expanded alias is re-processed to check for more aliases. If a command alias ends in a space or tab, the following word is also checked for alias expansion. The alias expansion process stops when a word that is not an alias is found, when a quoted word is found, or when an alias word that is currently being expanded is found.

The following command aliases are defined automatically by the shell:

autoload='typeset −fu'
functions='typeset −f'
hash='alias −t'
history='fc −l'
integer='typeset −i'
local='typeset'
login='exec login'
nohup='nohup '
r='fc −e −'
stop='kill −STOP'
suspend='kill −STOP $$'
type='whence −v'

Tracked aliases allow the shell to remember where it found a particular command. The first time the shell does a path search for a command that is marked as a tracked alias, it saves the full path of the command. The next time the command is executed, the shell checks the saved path to see that it is still valid, and if so, avoids repeating the path search. Tracked aliases can be listed and created using alias −t. Note that changing the PATH parameter clears the saved paths for all tracked aliases. If the trackall option is set (i.e. set −o trackall or set −h), the shell tracks all commands. This option is set automatically for non-interactive shells. For interactive shells, only the following commands are automatically tracked: cat(1), cc(1), chmod(1), cp(1), date(1), ed(1), emacs(1), grep(1), ls(1), make(1), mv(1), pr(1), rm(1), sed(1), sh(1), vi(1), and who(1).

Substitution
The first step the shell takes in executing a simple-command is to perform substitutions on the words of the command. There are three kinds of substitution: parameter, command, and arithmetic. Parameter substitutions, which are described in detail in the next section, take the form $name or ${...}; command substitutions take the form $(command) or (deprecated) `command`; and arithmetic substitutions take the form $((expression)).

If a substitution appears outside of double quotes, the results of the substitution are generally subject to word or field splitting according to the current value of the IFS parameter. The IFS parameter specifies a list of characters which are used to break a string up into several words; any characters from the set space, tab, and newline that appear in the IFS characters are called ‘‘IFS whitespace’’. Sequences of one or more IFS whitespace characters, in combination with zero or one non-IFS whitespace characters, delimit a field. As a special case, leading and trailing IFS whitespace and trailing IFS non-whitespace are stripped (i.e. no leading or trailing empty field is created by it); leading non-IFS whitespace does create an empty field.

Example: If IFS is set to ‘‘<space>:’’, and VAR is set to ‘‘<space>A<space>:<space><space>B::D’’, the substitution for $VAR results in four fields: ‘A’, ‘B’, ‘’ (an empty field), and ‘D’. Note that if the IFS parameter is set to the NULL string, no field splitting is done; if the parameter is unset, the default value of space, tab, and newline is used.

Also, note that the field splitting applies only to the immediate result of the substitution. Using the previous example, the substitution for $VAR:E results in the fields: ‘A’, ‘B’, ‘’, and ‘D:E’, not ‘A’, ‘B’, ‘’, ‘D’, and ‘E’. This behavior is POSIX compliant, but incompatible with some other shell implementations which do field splitting on the word which contained the substitution or use IFS as a general whitespace delimiter.

The results of substitution are, unless otherwise specified, also subject to brace expansion and file name expansion (see the relevant sections below).

A command substitution is replaced by the output generated by the specified command which is run in a subshell. For $(command) substitutions, normal quoting rules are used when command is parsed; however, for the deprecated `command` form, a ‘\’ followed by any of ‘$’, ‘`’, or ‘\’ is stripped (a ‘\’ followed by any other character is unchanged). As a special case in command substitutions, a command of the form <file is interpreted to mean substitute the contents of file. Note that $(<foo) has the same effect as $(cat foo), but it is carried out more efficiently because no process is started.

Note: $(command) expressions are currently parsed by finding matching parentheses, regardless of quoting. This should be fixed soon.

Arithmetic substitutions are replaced by the value of the specified expression. For example, the command print $((2+3*4)) displays 14. See Arithmetic expressions for a description of an expression.

Parameters
Parameters are shell variables; they can be assigned values and their values can be accessed using a parameter substitution. A parameter name is either one of the special single punctuation or digit character parameters described below, or a letter followed by zero or more letters or digits (

‘_’ counts as a letter ). The latter form can be treated as arrays by appending an array index of the form [expr] where expr is an arithmetic expression. Array indices are currently limited to the range 0 through 4294967295, (for mksh only; portable maximum is 1023), inclusive. That is, they are a 32-bit unsigned integer. Parameter substitutions take the form $name, ${name}, or ${name[
expr
]} where name is a parameter name. If substitution is performed on a parameter (or an array parameter element) that is not set, a null string is substituted unless the nounset option (
set −o nounset
or set −u ) is set, in which case an error occurs.

Parameters can be assigned values in a number of ways. First, the shell implicitly sets some parameters like ‘#’, ‘PWD’, and ‘$’; this is the only way the special single character parameters are set. Second, parameters are imported from the shell’s environment at startup. Third, parameters can be assigned values on the command line: for example, FOO=bar sets the parameter ‘‘FOO’’ to ‘‘bar’’; multiple parameter assignments can be given on a single command line and they can be followed by a simple-command, in which case the assignments are in effect only for the duration of the command (such assignments are also exported; see below for the implications of this). Note that both the parameter name and the ‘=’ must be unquoted for the shell to recognise a parameter assignment. The fourth way of setting a parameter is with the export, readonly, and typeset commands; see their descriptions in the Command execution section. Fifth, for and select loops set parameters as well as the getopts, read, and set −A commands. Lastly, parameters can be assigned values using assignment operators inside arithmetic expressions (see Arithmetic expressions below) or using the ${name=value} form of the parameter substitution (see below).

Parameters with the export attribute (set using the export or typeset −x commands, or by parameter assignments followed by simple commands) are put in the environment (see environ(7)) of commands run by the shell as name=value pairs. The order in which parameters appear in the environment of a command is unspecified. When the shell starts up, it extracts parameters and their values from its environment and automatically sets the export attribute for those parameters.

Modifiers can be applied to the ${name} form of parameter substitution:

${name:−word}

If name is set and not NULL, it is substituted; otherwise, word is substituted.

${name:+word}

If name is set and not NULL, word is substituted; otherwise, nothing is substituted.

${name:=word}

If name is set and not NULL, it is substituted; otherwise, it is assigned word and the resulting value of name is substituted.

${name:?word}

If name is set and not NULL, it is substituted; otherwise, word is printed on standard error (preceded by name:) and an error occurs (normally causing termination of a shell script, function, or script sourced using the ‘.’ built-in). If word is omitted, the string ‘‘parameter null or not set’’ is used instead. Currently a bug, if word is a variable which expands to the null string, the error message is also printed.

In the above modifiers, the ‘:’ can be omitted, in which case the conditions only depend on name being set (as opposed to set and not NULL). If word is needed, parameter, command, arithmetic, and tilde substitution are performed on it; if word is not needed, it is not evaluated.

The following forms of parameter substitution can also be used:

${#name}

The number of positional parameters if name is ‘*’, ‘@’, or not specified; otherwise the length of the string value of parameter name.

${#name[*]}
${#name[@]}

The number of elements in the array name.

${name#pattern}
${name##pattern}

If pattern matches the beginning of the value of parameter name, the matched text is deleted from the result of substitution. A single ‘#’ results in the shortest match, and two of them result in the longest match.

${name%pattern}
${name%%pattern}

Like ${..#..} substitution, but it deletes from the end of the value.

${name/pattern/string}
${name//pattern/string}

Like ${..#..} substitution, but it replaces the longest match of pattern, anchored anywhere in the value, with string. If pattern begins with ‘#’, it is anchored at the beginning of the value; if it begins with ‘%’, it is anchored at the end. A single ‘/’ replaces the first occurence of the search pattern, and two of them replace all occurences. If /string is omitted, the pattern is replaced by the empty string, i.e. deleted.

${name:pos:len}

The first len bytes of name, starting at position pos, are substituted. Both pos and :len are optional. If pos is negative, counting starts at the end of the string; if it is omitted, it defaults to 0. If len is omitted or greater than the length of the remaining string, all of it is substituted. Both pos and len are evaluated as arithmetic expressions. Currently, pos must start with a space, opening parenthesis or digit to be recognised.

The following special parameters are implicitly set by the shell and cannot be set directly using assignments:

!

Process ID of the last background process started. If no background processes have been started, the parameter is not set.

#

The number of positional parameters ($1, $2, etc.).

$

The PID of the shell, or the PID of the original shell if it is a subshell. Do NOT use this mechanism for generating temporary file names; see mktemp(1) instead.

The concatenation of the current single letter options (see the set command below for a list of options).

?

The exit status of the last non-asynchronous command executed. If the last command was killed by a signal, $? is set to 128 plus the signal number.

0

The name of the shell, determined as follows: the first argument to mksh if it was invoked with the −c option and arguments were given; otherwise the file argument, if it was supplied; or else the basename the shell was invoked with (i.e. argv[0]). $0 is also set to the name of the current script or the name of the current function, if it was defined with the function keyword (i.e. a Korn shell style function).

1 ... 9

The first nine positional parameters that were supplied to the shell, function, or script sourced using the ‘.’ built-in. Further positional parameters may be accessed using ${number}.

*

All positional parameters (except parameter 0) i.e. $1, $2, $3, ... If used outside of double quotes, parameters are separate words (which are subjected to word splitting); if used within double quotes, parameters are separated by the first character of the IFS parameter (or the empty string if IFS is NULL).

@

Same as $*, unless it is used inside double quotes, in which case a separate word is generated for each positional parameter. If there are no positional parameters, no word is generated. $@ can be used to access arguments, verbatim, without losing NULL arguments or splitting arguments with spaces.

The following parameters are set and/or used by the shell:

_ (underscore)

When an external command is executed by the shell, this parameter is set in the environment of the new process to the path of the executed command. In interactive use, this parameter is also set in the parent shell to the last word of the previous command.

CDPATH

Search path for the cd built-in command. It works the same way as PATH for those directories not beginning with ‘/’ in cd commands. Note that if CDPATH is set and does not contain ‘.’ or contains an empty path, the current directory is not searched. Also, the cd built-in command will display the resulting directory when a match is found in any search path other than the empty path.

COLUMNS

Set to the number of columns on the terminal or window. Always set, defaults to 80, unless the value as reported by stty(1) is non-zero and sane enough; similar for LINES. This parameter is used by the interactive line editing modes, and by the select, set −o, and kill −l commands to format information columns.

ENV

If this parameter is found to be set after any profile files are executed, the expanded value is used as a shell startup file. It typically contains function and alias definitions.

ERRNO

Integer value of the shell’s errno variable. It indicates the reason the last system call failed. Not yet implemented.

EXECSHELL

If set, this parameter is assumed to contain the shell that is to be used to execute commands that execve(2) fails to execute and which do not start with a ‘‘#!shell’’ sequence.

FCEDIT

The editor used by the fc command (see below).

FPATH

Like PATH, but used when an undefined function is executed to locate the file defining the function. It is also searched when a command can’t be found using PATH. See Functions below for more information.

HISTFILE

The name of the file used to store command history. When assigned to, history is loaded from the specified file. Also, several invocations of the shell running on the same machine will share history if their HISTFILE parameters all point to the same file.

Note: If HISTFILE isn’t set, no history file is used. This is different from AT&T UNIX ksh.

HISTSIZE

The number of commands normally stored for history. The default is 500.

HOME

The default directory for the cd command and the value substituted for an unqualified ~ (see Tilde expansion below).

IFS

Internal field separator, used during substitution and by the read command, to split values into distinct arguments; normally set to space, tab, and newline. See Substitution above for details.

Note: This parameter is not imported from the environment when the shell is started.

KSH_VERSION

The name and version of the shell (read-only). See also the version commands in Emacs editing mode and Vi editing mode sections, below.

LINENO

The line number of the function or shell script that is currently being executed.

LINES

Set to the number of lines on the terminal or window. Always set, defaults to 24.

OLDPWD

The previous working directory. Unset if cd has not successfully changed directories since the shell started, or if the shell doesn’t know where it is.

OPTARG

When using getopts, it contains the argument for a parsed option, if it requires one.

OPTIND

The index of the next argument to be processed when using getopts. Assigning 1 to this parameter causes getopts to process arguments from the beginning the next time it is invoked.

PATH

A colon separated list of directories that are searched when looking for commands and files sourced using the ‘.’ command (see below). An empty string resulting from a leading or trailing colon, or two adjacent colons, is treated as a ‘.’ (the current directory).

PGRP

The process ID of the shell’s process group leader.

PPID

The process ID of the shell’s parent.

PS1

The primary prompt for interactive shells. Parameter, command, and arithmetic substitutions are performed, and ‘!’ is replaced with the current command number (see the fc command below). A literal ‘!’ can be put in the prompt by placing ‘!!’ in PS1.

The default prompt is ‘$ ’ for non-root users, ‘# ’ for root. If mksh is invoked by root and PS1 does not contain a ‘#’ character, the default value will be used even if PS1 already exists in the environment.

The mksh distribution comes with a sample dot.mkshrc containing a sophisticated example, but you might like the following one (note that ${HOSTNAME:=$(hostname −s)} and the root-vs-user distinguishing clause are (in this example) executed at PS1 assignment time, while the $USER and $PWD are escaped and thus will be evaluated each time a prompt is displayed):

PS1='${USER:=$(id −un)}'"@${HOSTNAME:=$(hostname −s)}:\$PWD $(

if (( USER_ID )); then print \$; else print \#; fi) "

Note that since the command-line editors try to figure out how long the prompt is (so they know how far it is to the edge of the screen), escape codes in the prompt tend to mess things up. You can tell the shell not to count certain sequences (such as escape codes) by prefixing your prompt with a character (such as control-A) followed by a carriage return and then delimiting the escape codes with this character. Any occurences of that character in the prompt are not printed. By the way, don’t blame me for this hack; it’s derived from the original ksh88(1), which did print the delimiter character so you were out of luck if you did not have any non-printing characters.

Since Backslashes and other special characters may be interpreted by the shell, to set PS1 either escape the backslash itself, or use double quotes. The latter is more practical. This is a more complex example, avoiding to directly enter special characters (for example with ^V in the emacs editing mode), which embeds the current working directory, in reverse video, in the prompt string:

x=$(print \\001)
PS1="$x$(print \\r)$x$(tput smso)$x\$PWD$x$(tput rmso)$x> "

PS2

Secondary prompt string, by default ‘> ’, used when more input is needed to complete a command.

PS3

Prompt used by the select statement when reading a menu selection. The default is ‘#? ’.

PS4

Used to prefix commands that are printed during execution tracing (see the set −x command below). Parameter, command, and arithmetic substitutions are performed before it is printed. The default is ‘+ ’.

PWD

The current working directory. May be unset or NULL if the shell doesn’t know where it is.

RANDOM

Every time RANDOM is referenced, it is assigned a 15-bit pseudo-random number, i.e. between 0 and 32767, first. See the description of set −o arc4random below for details.

REPLY

Default parameter for the read command if no names are given. Also used in select loops to store the value that is read from standard input.

SECONDS

The number of seconds since the shell started or, if the parameter has been assigned an integer value, the number of seconds since the assignment plus the value that was assigned.

TMOUT

If set to a positive integer in an interactive shell, it specifies the maximum number of seconds the shell will wait for input after printing the primary prompt (PS1). If the time is exceeded, the shell exits.

TMPDIR

The directory temporary shell files are created in. If this parameter is not set, or does not contain the absolute path of a writable directory, temporary files are created in /tmp.

USER_ID

The effective user id of the shell.

Tilde expansion
Tilde expansion which is done in parallel with parameter substitution, is done on words starting with an unquoted ‘~’. The characters following the tilde, up to the first ‘/’, if any, are assumed to be a login name. If the login name is empty, ‘+’, or ‘−’, the value of the HOME, PWD, or OLDPWD parameter is substituted, respectively. Otherwise, the password file is searched for the login name, and the tilde expression is substituted with the user’s home directory. If the login name is not found in the password file or if any quoting or parameter substitution occurs in the login name, no substitution is performed.

In parameter assignments (such as those preceding a simple-command or those occurring in the arguments of alias, export, readonly, and typeset), tilde expansion is done after any assignment (i.e. after the equals sign) or after an unquoted colon (‘:’); login names are also delimited by colons.

The home directory of previously expanded login names are cached and re-used. The alias −d command may be used to list, change, and add to this cache (e.g. alias −d fac=/usr/local/facilities; cd ~fac/bin).

Brace expansion (alteration)
Brace expressions take the following form:

prefix{str1,...,strN}suffix

The expressions are expanded to N words, each of which is the concatenation of prefix, stri, and suffix (e.g. ‘‘a{c,b{X,Y},d}e’’ expands to four words: ‘‘ace’’, ‘‘abXe’’, ‘‘abYe’’, and ‘‘ade’’). As noted in the example, brace expressions can be nested and the resulting words are not sorted. Brace expressions must contain an unquoted comma (‘,’) for expansion to occur (e.g. {} and {foo} are not expanded). Brace expansion is carried out after parameter substitution and before file name generation.

File name patterns
A file name pattern is a word containing one or more unquoted ‘?’, ‘*’, ‘+’, ‘@’, or ‘!’ characters or ‘‘[..]’’ sequences. Once brace expansion has been performed, the shell replaces file name patterns with the sorted names of all the files that match the pattern (if no files match, the word is left unchanged). The pattern elements have the following meaning:

?

Matches any single character.

*

Matches any sequence of characters.

[..]

Matches any of the characters inside the brackets. Ranges of characters can be specified by separating two characters by a ‘−’ (e.g. ‘‘[a0−9]’’ matches the letter ‘a’ or any digit). In order to represent itself, a ‘−’ must either be quoted or the first or last character in the character list. Similarly, a ‘]’ must be quoted or the first character in the list if it is to represent itself instead of the end of the list. Also, a ‘!’ appearing at the start of the list has special meaning (see below), so to represent itself it must be quoted or appear later in the list.

[!..]

Like [..], except it matches any character not inside the brackets.

*(pattern|...|pattern)

Matches any string of characters that matches zero or more occurrences of the specified patterns. Example: The pattern *(foo|bar) matches the strings ‘‘’’, ‘‘foo’’, ‘‘bar’’, ‘‘foobarfoo’’, etc.

+(pattern|...|pattern)

Matches any string of characters that matches one or more occurrences of the specified patterns. Example: The pattern +(foo|bar) matches the strings ‘‘foo’’, ‘‘bar’’, ‘‘foobar’’, etc.

?(pattern|...|pattern)

Matches the empty string or a string that matches one of the specified patterns. Example: The pattern ?(foo|bar) only matches the strings ‘‘’’, ‘‘foo’’, and ‘‘bar’’.

@(pattern|...|pattern)

Matches a string that matches one of the specified patterns. Example: The pattern @(foo|bar) only matches the strings ‘‘foo’’ and ‘‘bar’’.

!(pattern|...|pattern)

Matches any string that does not match one of the specified patterns. Examples: The pattern !(foo|bar) matches all strings except ‘‘foo’’ and ‘‘bar’’; the pattern !(*) matches no strings; the pattern !(?)* matches all strings (think about it).

Note that mksh (

and pdksh ) never matches ‘.’ and ‘..’, but AT&T UNIX ksh, Bourne sh, and GNU bash do.

Note that none of the above pattern elements match either a period (‘.’) at the start of a file name or a slash (‘/’), even if they are explicitly used in a [..] sequence; also, the names ‘.’ and ‘..’ are never matched, even by the pattern ‘.*’.

If the markdirs option is set, any directories that result from file name generation are marked with a trailing ‘/’.

The POSIX character classes (i.e. [:class-name:] inside a [..] expression) are not yet implemented.

Input/output redirection
When a command is executed, its standard input, standard output, and standard error (file descriptors 0, 1, and 2, respectively) are normally inherited from the shell. Three exceptions to this are commands in pipelines, for which standard input and/or standard output are those set up by the pipeline, asynchronous commands created when job control is disabled, for which standard input is initially set to be from /dev/null, and commands for which any of the following redirections have been specified:

> file

Standard output is redirected to file. If file does not exist, it is created; if it does exist, is a regular file, and the noclobber option is set, an error occurs; otherwise, the file is truncated. Note that this means the command cmd < foo > foo will open foo for reading and then truncate it when it opens it for writing, before cmd gets a chance to actually read foo.

>| file

Same as >, except the file is truncated, even if the noclobber option is set.

>> file

Same as >, except if file exists it is appended to instead of being truncated. Also, the file is opened in append mode, so writes always go to the end of the file (see open(2)).

< file

Standard input is redirected from file, which is opened for reading.

<> file

Same as <, except the file is opened for reading and writing.

<< marker

After reading the command line containing this kind of redirection (called a ‘‘here document’’), the shell copies lines from the command source into a temporary file until a line matching marker is read. When the command is executed, standard input is redirected from the temporary file. If marker contains no quoted characters, the contents of the temporary file are processed as if enclosed in double quotes each time the command is executed, so parameter, command, and arithmetic substitutions are performed, along with backslash (‘\’) escapes for ‘$’, ‘`’, ‘\’, and ‘\newline’, but not for ‘"’. If multiple here documents are used on the same command line, they are saved in order.

<<− marker

Same as <<, except leading tabs are stripped from lines in the here document.

<<< word

Same as <<, except that word is the here document. This is called a here string.

<& fd

Standard input is duplicated from file descriptor fd. fd can be a number, indicating the number of an existing file descriptor; the letter ‘p’, indicating the file descriptor associated with the output of the current co-process; or the character ‘−’, indicating standard input is to be closed. Note that fd is limited to a single digit in most shell implementations.

>& fd

Same as <&, except the operation is done on standard output.

&> file

Same as > file 2>&1. This is a GNU bash extension supported by mksh which also supports the preceding explicit fd number, for example, 3&> file is the same as 3> file 2>&3 in mksh but a syntax error in GNU bash.

&>| file, &>> file, &>& fd

Same as >| file, >> file, or >& fd, followed by 2>&1, as above. These are mksh extensions.

In any of the above redirections, the file descriptor that is redirected (i.e. standard input or standard output) can be explicitly given by preceding the redirection with a number (portably, only a single digit). Parameter, command, and arithmetic substitutions, tilde substitutions, and (if the shell is interactive) file name generation are all performed on the file, marker, and fd arguments of redirections. Note, however, that the results of any file name generation are only used if a single file is matched; if multiple files match, the word with the expanded file name generation characters is used. Note that in restricted shells, redirections which can create files cannot be used.

For simple-commands, redirections may appear anywhere in the command; for compound-commands (

if statements, etc. ), any redirections must appear at the end. Redirections are processed after pipelines are created and in the order they are given, so the following will print an error with a line number prepended to it:

$ cat /foo/bar 2>&1 > /dev/null | cat −n

File descriptors created by input/output redirections are private to the Korn shell, but passed to sub-processes if −o posix is set.

Arithmetic expressions
Integer arithmetic expressions can be used with the let command, inside $((..)) expressions, inside array references (e.g. name[expr]), as numeric arguments to the test command, and as the value of an assignment to an integer parameter.

Expressions are calculated using signed arithmetic and the mksh_ari_t type (a 32-bit signed integer), unless they begin with a sole ‘#’ character, in which case they use mksh_uari_t (

a 32-bit unsigned integer ).

Expressions may contain alpha-numeric parameter identifiers, array references, and integer constants and may be combined with the following C operators (listed and grouped in increasing order of precedence):

Unary operators:

+ − ! ~ ++ −−

Binary operators:

,
= *= /= %= += −= <<= >>= &= ^= |=
||
&&
|
^
&
== !=
< <= >= >
<< >>
+ −
* / %

Ternary operators:

?: (precedence is immediately higher than assignment)

Grouping operators:

( )

Integer constants and expressions are calculated using the mksh_ari_t (

if signed ) or mksh_uari_t (
if unsigned ) type, and are limited to 32 bits. Overflows wrap silently. Integer constants may be specified with arbitrary bases using the notation base#number, where base is a decimal integer specifying the base, and number is a number in the specified base. Additionally, integers may be prefixed with ‘0X’ or ‘0x’ (specifying base 16), similar to AT&T UNIX ksh, or ‘0’ (base 8), as an mksh extension, in all forms of arithmetic expressions, except as numeric arguments to the test command. As a special mksh extension, numbers to the base of one are treated as either (8-bit transparent) ASCII or Unicode codepoints, depending on the shell’s utf8−mode flag (current setting). Note that NUL bytes (integral value of zero) cannot be used. In Unicode mode, raw octets are mapped into the range EF80..EFFF as in OPTU-8, which is in the PUA and has been assigned by CSUR for this use. If more than one octet in ASCII mode, or a sequence of more than one octet not forming a valid and minimal CESU-8 sequence is passed, the behaviour is undefined (usually, the shell aborts with a parse error, but rarely, it succeeds, e.g. on the sequence C2 20). That’s why you should always use ASCII mode unless you know that the input is well-formed UTF-8 in the range of 0000..FFFD.

The operators are evaluated as follows:

unary +

Result is the argument (included for completeness).

unary −

Negation.

!

Logical NOT; the result is 1 if argument is zero, 0 if not.

~

Arithmetic (bit-wise) NOT.

++

Increment; must be applied to a parameter (not a literal or other expression). The parameter is incremented by 1. When used as a prefix operator, the result is the incremented value of the parameter; when used as a postfix operator, the result is the original value of the parameter.

−−

Similar to ++, except the parameter is decremented by 1.

,

Separates two arithmetic expressions; the left-hand side is evaluated first, then the right. The result is the value of the expression on the right-hand side.

=

Assignment; the variable on the left is set to the value on the right.

*= /= += −= <<= >>= &= ^= |=

Assignment operators. 〈

var〉〈op〉=〈expr〉 is the same as 〈
var
〉=〈var〉〈op〉〈expr〉, with any operator precedence in 〈expr〉 preserved. For example, ‘‘var1 *= 5 + 3’’ is the same as specifying ‘‘var1 = var1 * (5 + 3)’’.

||

Logical OR; the result is 1 if either argument is non-zero, 0 if not. The right argument is evaluated only if the left argument is zero.

&&

Logical AND; the result is 1 if both arguments are non-zero, 0 if not. The right argument is evaluated only if the left argument is non-zero.

|

Arithmetic (bit-wise) OR.

^

Arithmetic (bit-wise) XOR (exclusive-OR).

&

Arithmetic (bit-wise) AND.

==

Equal; the result is 1 if both arguments are equal, 0 if not.

!=

Not equal; the result is 0 if both arguments are equal, 1 if not.

<

Less than; the result is 1 if the left argument is less than the right, 0 if not.

<= >= >

Less than or equal, greater than or equal, greater than. See <.

<< >>

Shift left (right); the result is the left argument with its bits shifted left (right) by the amount given in the right argument.

+ − * /

Addition, subtraction, multiplication, and division.

%

Remainder; the result is the remainder of the division of the left argument by the right. The sign of the result is unspecified if either argument is negative.

arg1〉?〈arg2〉:〈arg3

If 〈arg1〉 is non-zero, the result is 〈arg2〉; otherwise the result is 〈arg3〉.

Co-processes
A co-process (which is a pipeline created with the ‘|&’ operator) is an asynchronous process that the shell can both write to (using print −p) and read from (using read −p). The input and output of the co-process can also be manipulated using >&p and <&p redirections, respectively. Once a co-process has been started, another can’t be started until the co-process exits, or until the co-process’s input has been redirected using an exec n>&p redirection. If a co-process’s input is redirected in this way, the next co-process to be started will share the output with the first co-process, unless the output of the initial co-process has been redirected using an exec n<&p redirection.

Some notes concerning co-processes:

The only way to close the co-process’s input (so the co-process reads an end-of-file) is to redirect the input to a numbered file descriptor and then close that file descriptor: exec 3>&p; exec 3>&−

In order for co-processes to share a common output, the shell must keep the write portion of the output pipe open. This means that end-of-file will not be detected until all co-processes sharing the co-process’s output have exited (when they all exit, the shell closes its copy of the pipe). This can be avoided by redirecting the output to a numbered file descriptor (as this also causes the shell to close its copy). Note that this behaviour is slightly different from the original Korn shell which closes its copy of the write portion of the co-process output when the most recently started co-process (instead of when all sharing co-processes) exits.

print −p will ignore SIGPIPE signals during writes if the signal is not being trapped or ignored; the same is true if the co-process input has been duplicated to another file descriptor and print −un is used.

Functions
Functions are defined using either Korn shell function function-name syntax or the Bourne/POSIX shell function-name() syntax (see below for the difference between the two forms). Functions are like .-scripts (i.e. scripts sourced using the ‘.’ built-in) in that they are executed in the current environment. However, unlike .-scripts, shell arguments (i.e. positional parameters $1, $2, etc.) are never visible inside them. When the shell is determining the location of a command, functions are searched after special built-in commands, before regular and non-regular built-ins, and before the PATH is searched.

An existing function may be deleted using unset −f function-name. A list of functions can be obtained using typeset +f and the function definitions can be listed using typeset −f. The autoload command (which is an alias for typeset −fu) may be used to create undefined functions: when an undefined function is executed, the shell searches the path specified in the FPATH parameter for a file with the same name as the function which, if found, is read and executed. If after executing the file the named function is found to be defined, the function is executed; otherwise, the normal command search is continued (i.e. the shell searches the regular built-in command table and PATH). Note that if a command is not found using PATH, an attempt is made to autoload a function using FPATH (this is an undocumented feature of the original Korn shell).

Functions can have two attributes, ‘‘trace’’ and ‘‘export’’, which can be set with typeset −ft and typeset −fx, respectively. When a traced function is executed, the shell’s xtrace option is turned on for the function’s duration; otherwise, the xtrace option is turned off. The ‘‘export’’ attribute of functions is currently not used. In the original Korn shell, exported functions are visible to shell scripts that are executed.

Since functions are executed in the current shell environment, parameter assignments made inside functions are visible after the function completes. If this is not the desired effect, the typeset command can be used inside a function to create a local parameter. Note that special parameters (e.g. $$, $!) can’t be scoped in this way.

The exit status of a function is that of the last command executed in the function. A function can be made to finish immediately using the return command; this may also be used to explicitly specify the exit status.

Functions defined with the function reserved word are treated differently in the following ways from functions defined with the () notation:

The $0 parameter is set to the name of the function (Bourne-style functions leave $0 untouched).

Parameter assignments preceding function calls are not kept in the shell environment (executing Bourne-style functions will keep assignments).

OPTIND is saved/reset and restored on entry and exit from the function so getopts can be used properly both inside and outside the function (Bourne-style functions leave OPTIND untouched, so using getopts inside a function interferes with using getopts outside the function).

In the future, the following differences will also be added:

A separate trap/signal environment will be used during the execution of functions. This will mean that traps set inside a function will not affect the shell’s traps and signals that are not ignored in the shell (but may be trapped) will have their default effect in a function.

The EXIT trap, if set in a function, will be executed after the function returns.

Command execution
After evaluation of command-line arguments, redirections, and parameter assignments, the type of command is determined: a special built-in, a function, a regular built-in, or the name of a file to execute found using the PATH parameter. The checks are made in the above order. Special built-in commands differ from other commands in that the PATH parameter is not used to find them, an error during their execution can cause a non-interactive shell to exit, and parameter assignments that are specified before the command are kept after the command completes. Regular built-in commands are different only in that the PATH parameter is not used to find them.

The original ksh and POSIX differ somewhat in which commands are considered special or regular:

POSIX special commands

., :, break, continue, eval, exec, exit, export, readonly, return, set, shift, trap, unset

Additional mksh special commands

builtin, times, typeset

Very special commands (non-POSIX)

alias, readonly, set, typeset

POSIX regular commands

alias, bg, cd, command, false, fc, fg, getopts, jobs, kill, read, true, umask, unalias, wait

Additional mksh regular commands

[, echo, let, mknod, print, pwd, test, ulimit, whence

In the future, the additional mksh special and regular commands may be treated differently from the POSIX special and regular commands.

Once the type of command has been determined, any command-line parameter assignments are performed and exported for the duration of the command.

The following describes the special and regular built-in commands:

. file [arg ...]

This is called the ‘‘dot’’ command. Execute the commands in file in the current environment. The file is searched for in the directories of PATH. If arguments are given, the positional parameters may be used to access them while file is being executed. If no arguments are given, the positional parameters are those of the environment the command is used in.

: [...]

The null command. Exit status is set to zero.

alias [
−d
| −t [
−r
] | +−x] [−p] [+] [
name
[=value] ...]

Without arguments, alias lists all aliases. For any name without a value, the existing alias is listed. Any name with a value defines an alias (see Aliases above).

When listing aliases, one of two formats is used. Normally, aliases are listed as name=value, where value is quoted. If options were preceded with ‘+’, or a lone ‘+’ is given on the command line, only name is printed.

The −d option causes directory aliases which are used in tilde expansion to be listed or set (see Tilde expansion above).

If the −p option is used, each alias is prefixed with the string ‘‘alias ’’.

The −t option indicates that tracked aliases are to be listed/set (values specified on the command line are ignored for tracked aliases). The −r option indicates that all tracked aliases are to be reset.

The −x option sets (+x clears) the export attribute of an alias, or, if no names are given, lists the aliases with the export attribute (exporting an alias has no effect).

bg [job ...]

Resume the specified stopped job(s) in the background. If no jobs are specified, %+ is assumed. See Job control below for more information.

bind [−l]

The current bindings are listed. If the −l flag is given, bind instead lists the names of the functions to which keys may be bound. See Emacs editing mode for more information.

bind [−m] string=[substitute] ...
bind
string=[editing-command] ...

The specified editing command is bound to the given string, which should consist of a control character optionally preceded by one of the two prefix characters and optionally succeded by a tilde character. Future input of the string will cause the editing command to be immediately invoked. If the −m flag is given, the specified input string will afterwards be immediately replaced by the given substitute string which may contain editing commands. If a tilde postfix is given, a tilde trailing the one or two prefices and the control character is ignored, any other trailing character will be processed afterwards.

Control characters may be written using caret notation i.e. ^X represents Control-X. Note that although only two prefix characters (usually ESC and ^X) are supported, some multi-character sequences can be supported.

The following default bindings show how the arrow keys, the home, end and delete key on a BSD wsvt25, xterm-xfree86 or GNU screen terminal are bound (of course some escape sequences won’t work out quite this nicely):

bind '^[['=prefix−2
bind '^XA'=up−history
bind '^XB'=down−history
bind '^XC'=forward−char
bind '^XD'=backward−char
bind '^X1~'=beginning−of−line
bind '^X7~'=beginning−of−line
bind '^XH'=beginning−of−line
bind '^X4~'=end−of−line
bind '^X8~'=end−of−line
bind '^XF'=end−of−line
bind '^X3~'=delete−char−forward

break [level]

Exit the levelth inner-most for, select, until, or while loop. level defaults to 1.

builtin command [arg ...]

Execute the built-in command command.

cd [−LP] [dir]

Set the working directory to dir. If the parameter CDPATH is set, it lists the search path for the directory containing dir. A NULL path means the current directory. If dir is found in any component of the CDPATH search path other than the NULL path, the name of the new working directory will be written to standard output. If dir is missing, the home directory HOME is used. If dir is ‘−’, the previous working directory is used (see the OLDPWD parameter).

If the −L option (logical path) is used or if the physical option isn’t set (see the set command below), references to ‘..’ in dir are relative to the path used to get to the directory. If the −P option (physical path) is used or if the physical option is set, ‘..’ is relative to the filesystem directory tree. The PWD and OLDPWD parameters are updated to reflect the current and old working directory, respectively.

cd [−LP] old new

The string new is substituted for old in the current directory, and the shell attempts to change to the new directory.

command [−pVv] cmd [arg ...]

If neither the −v nor −V option is given, cmd is executed exactly as if command had not been specified, with two exceptions: firstly, cmd cannot be a shell function; and secondly, special built-in commands lose their specialness (i.e. redirection and utility errors do not cause the shell to exit, and command assignments are not permanent).

If the −p option is given, a default search path is used instead of the current value of PATH, the actual value of which is system dependent.

If the −v option is given, instead of executing cmd, information about what would be executed is given (and the same is done for arg ...). For special and regular built-in commands and functions, their names are simply printed; for aliases, a command that defines them is printed; and for commands found by searching the PATH parameter, the full path of the command is printed. If no command is found (i.e. the path search fails), nothing is printed and command exits with a non-zero status. The −V option is like the −v option, except it is more verbose.

continue [level]

Jumps to the beginning of the levelth inner-most for, select, until, or while loop. level defaults to 1.

echo [−Een] [arg ...]

Prints its arguments (separated by spaces) followed by a newline, to the standard output. The newline is suppressed if any of the arguments contain the backslash sequence ‘\c’. See the print command below for a list of other backslash sequences that are recognised.

The options are provided for compatibility with BSD shell scripts. The −n option suppresses the trailing newline, −e enables backslash interpretation (a no-op, since this is normally done), and −E suppresses backslash interpretation.

If the posix option is set, only the first argument is treated as an option, and only if it is exactly ‘‘−n’’.

eval command ...

The arguments are concatenated (with spaces between them) to form a single string which the shell then parses and executes in the current environment.

exec [command [arg ...]]

The command is executed without forking, replacing the shell process.

If no command is given except for I/O redirection, the I/O redirection is permanent and the shell is not replaced. Any file descriptors greater than 2 which are opened or dup(2)’d in this way are not made available to other executed commands (i.e. commands that are not built-in to the shell). Note that the Bourne shell differs here; it does pass these file descriptors on.

exit [status]

The shell exits with the specified exit status. If status is not specified, the exit status is the current value of the $? parameter.

export [−p] [parameter[=value]]

Sets the export attribute of the named parameters. Exported parameters are passed in the environment to executed commands. If values are specified, the named parameters are also assigned.

If no parameters are specified, the names of all parameters with the export attribute are printed one per line, unless the −p option is used, in which case export commands defining all exported parameters, including their values, are printed.

false

A command that exits with a non-zero status.

fc [
−e
editor | −l [−n]] [−r] [first [last]]

first and last select commands from the history. Commands can be selected by history number or a string specifying the most recent command starting with that string. The −l option lists the command on standard output, and −n inhibits the default command numbers. The −r option reverses the order of the list. Without −l, the selected commands are edited by the editor specified with the −e option, or if no −e is specified, the editor specified by the FCEDIT parameter (if this parameter is not set, /bin/ed is used), and then executed by the shell.

fc −e − | −s [−g] [old=new] [prefix]

Re-execute the selected command (the previous command by default) after performing the optional substitution of old with new. If −g is specified, all occurrences of old are replaced with new. The meaning of −e − and −s is identical: re-execute the selected command without invoking an editor. This command is usually accessed with the predefined alias r='fc −e −'.

fg [job ...]

Resume the specified job(s) in the foreground. If no jobs are specified, %+ is assumed. See Job control below for more information.

getopts optstring name [arg ...]

Used by shell procedures to parse the specified arguments (or positional parameters, if no arguments are given) and to check for legal options. optstring contains the option letters that getopts is to recognise. If a letter is followed by a colon, the option is expected to have an argument. Options that do not take arguments may be grouped in a single argument. If an option takes an argument and the option character is not the last character of the argument it is found in, the remainder of the argument is taken to be the option’s argument; otherwise, the next argument is the option’s argument.

Each time getopts is invoked, it places the next option in the shell parameter name and the index of the argument to be processed by the next call to getopts in the shell parameter OPTIND. If the option was introduced with a ‘+’, the option placed in name is prefixed with a ‘+’. When an option requires an argument, getopts places it in the shell parameter OPTARG.

When an illegal option or a missing option argument is encountered, a question mark or a colon is placed in name (indicating an illegal option or missing argument, respectively) and OPTARG is set to the option character that caused the problem. Furthermore, if optstring does not begin with a colon, a question mark is placed in name, OPTARG is unset, and an error message is printed to standard error.

When the end of the options is encountered, getopts exits with a non-zero exit status. Options end at the first (non-option argument) argument that does not start with a ‘−’, or when a ‘−−’ argument is encountered.

Option parsing can be reset by setting OPTIND to 1 (this is done automatically whenever the shell or a shell procedure is invoked).

Warning: Changing the value of the shell parameter OPTIND to a value other than 1, or parsing different sets of arguments without resetting OPTIND, may lead to unexpected results.

hash [−r] [name ...]

Without arguments, any hashed executable command pathnames are listed. The −r option causes all hashed commands to be removed from the hash table. Each name is searched as if it were a command name and added to the hash table if it is an executable command.

jobs [−lnp] [job ...]

Display information about the specified job(s); if no jobs are specified, all jobs are displayed. The −n option causes information to be displayed only for jobs that have changed state since the last notification. If the −l option is used, the process ID of each process in a job is also listed. The −p option causes only the process group of each job to be printed. See Job control below for the format of job and the displayed job.

kill [
−s
signame | −signum | −signame] { job | pid | pgrp } ...

Send the specified signal to the specified jobs, process IDs, or process groups. If no signal is specified, the TERM signal is sent. If a job is specified, the signal is sent to the job’s process group. See Job control below for the format of job.

kill −l [exit-status ...]

Print the signal name corresponding to exit-status. If no arguments are specified, a list of all the signals, their numbers, and a short description of them are printed.

let [expression ...]

Each expression is evaluated (see Arithmetic expressions above). If all expressions are successfully evaluated, the exit status is 0 (1) if the last expression evaluated to non-zero (zero). If an error occurs during the parsing or evaluation of an expression, the exit status is greater than 1. Since expressions may need to be quoted, (( expr )) is syntactic sugar for let "expr".

mknod [−m mode] name [b | c] major minor
mknod
[−m mode] name p

Create a device special file. The file type may be b (block type device), c (character type device), or p (named pipe). The file created may be modified according to its mode (via the −m option), major (major device number), and minor (minor device number).

See mknod(8) for further information.

print [
−nprsu
[
n
] | −R [−en]] [argument ...]

print prints its arguments on the standard output, separated by spaces and terminated with a newline. The −n option suppresses the newline. By default, certain C escapes are translated. These include ‘\b’, ‘\f’, ‘\n’, ‘\r’, ‘\t’, ‘\u####’, ‘\v’, ‘\x##’, and ‘\0###’; ‘#’ is, in the case of \0###, an octal, or, in the case of \u#### or \x##, a hexadecimal digit, of which there may be 0 to 2/3/4. The \x## and \0### escapes translate to raw 8-bit octets; the \u#### escape translates a Unicode codepoint to UTF-8. ‘\c’ is equivalent to using the −n option. ‘\’ expansion may be inhibited with the −r option. The −s option prints to the history file instead of standard output; the −u option prints to file descriptor n (

n defaults to 1 if omitted ); and the −p option prints to the co-process (see Co-processes above).

The −R option is used to emulate, to some degree, the BSD echo(1) command which does not process ‘\’ sequences unless the −e option is given. As above, the −n option suppresses the trailing newline.

pwd [−LP]

Print the present working directory. If the −L option is used or if the physical option isn’t set (see the set command below), the logical path is printed (i.e. the path used to cd to the current directory). If the −P option (physical path) is used or if the physical option is set, the path determined from the filesystem (by following ‘..’ directories to the root directory) is printed.

read [−prsu[n]] [parameter ...]

Reads a line of input from the standard input, separates the line into fields using the IFS parameter (see Substitution above), and assigns each field to the specified parameters. If there are more parameters than fields, the extra parameters are set to NULL, or alternatively, if there are more fields than parameters, the last parameter is assigned the remaining fields (inclusive of any separating spaces). If no parameters are specified, the REPLY parameter is used. If the input line ends in a backslash and the −r option was not used, the backslash and the newline are stripped and more input is read. If no input is read, read exits with a non-zero status.

The first parameter may have a question mark and a string appended to it, in which case the string is used as a prompt (printed to standard error before any input is read) if the input is a tty(4) (e.g. read nfoo?'number of foos: ').

The −un and −p options cause input to be read from file descriptor n (n defaults to 0 if omitted) or the current co-process (see Co-processes above for comments on this), respectively. If the −s option is used, input is saved to the history file.

Another handy set of tricks: If read is run in a loop such as while read foo; do ...; done then leading whitespace will be removed (IFS) and backslashes processed (−r). You might want to use while IFS= read −r foo; do ...; done for pristine I/O (variable splitting, as in while IFS= read foo bar; do ...; done is not possible though).

The inner loop will be executed in a subshell and variable changes cannot be propagated if executed in a pipeline:

foo | bar | while read foo; do ...; done

Use co-processes instead:

foo | bar |&
while read −p foo; do ...; done
exec 3>&p; exec 3>&−

readonly [−p] [
parameter
[=value] ...]

Sets the read-only attribute of the named parameters. If values are given, parameters are set to them before setting the attribute. Once a parameter is made read-only, it cannot be unset and its value cannot be changed.

If no parameters are specified, the names of all parameters with the read-only attribute are printed one per line, unless the −p option is used, in which case readonly commands defining all read-only parameters, including their values, are printed.

realpath [−−] name

Prints the resolved absolute pathname corresponding to name.

rename from to

Renames the file from to to. Both must be complete pathnames and on the same device. This builtin is intended for emergency situations where /bin/mv becomes unusable, and directly calls rename(2).

return [status]

Returns from a function or . script, with exit status status. If no status is given, the exit status of the last executed command is used. If used outside of a function or . script, it has the same effect as exit. Note that mksh treats both profile and ENV files as . scripts, while the original Korn shell only treats profiles as . scripts.

set [+−abCefhkmnpsuvXx] [+−o option] [+−A name] [−−] [arg ...]

The set command can be used to set () or clear (+) shell options, set the positional parameters, or set an array parameter. Options can be changed using the +−o option syntax, where option is the long name of an option, or using the +−letter syntax, where letter is the option’s single letter name (not all options have a single letter name). The following table lists both option letters (if they exist) and long names along with a description of what the option does:

−A name

Sets the elements of the array parameter name to arg ... If −A is used, the array is reset (i.e. emptied) first; if +A is used, the first N elements are set (where N is the number of arguments); the rest are left untouched.

An alternative syntax for the command set −A foo −− a b c which is compatible to GNU bash and also supported by AT&T UNIX ksh93 is: foo=(a b c)

−a | allexport

All new parameters are created with the export attribute.

−b | notify

Print job notification messages asynchronously, instead of just before the prompt. Only used if job control is enabled (−m).

−C | noclobber

Prevent > redirection from overwriting existing files. Instead, >| must be used to force an overwrite.

−e | errexit

Exit (after executing the ERR trap) as soon as an error occurs or a command fails (i.e. exits with a non-zero status). This does not apply to commands whose exit status is explicitly tested by a shell construct such as if, until, while, &&, ||, or ! statements.

−f | noglob

Do not expand file name patterns.

−h | trackall

Create tracked aliases for all executed commands (see Aliases above). Enabled by default for non-interactive shells.

−k | keyword

Parameter assignments are recognised anywhere in a command.

−m | monitor

Enable job control (default for interactive shells).

−n | noexec

Do not execute any commands. Useful for checking the syntax of scripts (ignored if interactive).

−p | privileged

The shell is a privileged shell. It is set automatically if, when the shell starts, the real UID or GID does not match the effective UID (EUID) or GID (EGID), respectively. See above for a description of what this means.

−s | stdin

If used when the shell is invoked, commands are read from standard input. Set automatically if the shell is invoked with no arguments.

When −s is used with the set command it causes the specified arguments to be sorted before assigning them to the positional parameters (or to array name, if −A is used).

−U | utf8−mode

Enable UTF-8 support in the Emacs editing mode and internal string handling functions. This is enabled automatically for interactive shells if your system supports setlocale(LC_CTYPE, "") and optionally nl_langinfo(CODESET), or the LC_ALL, LC_CTYPE, or LANG environment variables, and at least one of these returns something that matches ‘‘UTF−8’’ or ‘‘utf8’’, or if the input begins with a UTF-8 Byte Order Mark.

−u | nounset

Referencing of an unset parameter is treated as an error, unless one of the ‘−’, ‘+’, or ‘=’ modifiers is used.

−v | verbose

Write shell input to standard error as it is read.

−X | markdirs

Mark directories with a trailing ‘/’ during file name generation.

−x | xtrace

Print commands and parameter assignments when they are executed, preceded by the value of PS4.

arc4random

Use arc4random(3) high-quality random numbers for the value of RANDOM if set (to either 1 or 2), or a semi-predictable sequence from rand(3) if unset. Setting this flag will change its value to 1; the default value is 2, which means it automatically switches to 0 if RANDOM is written to.

bgnice

Background jobs are run with lower priority.

braceexpand

Enable brace expansion (a.k.a. alternation). This is enabled by default. If disabled, tilde expansion after an equals sign is disabled as a side effect.

emacs

Enable BRL emacs-like command-line editing (interactive shells only); see Emacs editing mode.

gmacs

Enable gmacs-like command-line editing (interactive shells only). Currently identical to emacs editing except that transpose−chars (^T) acts slightly differently.

ignoreeof

The shell will not (easily) exit when end-of-file is read; exit must be used. To avoid infinite loops, the shell will exit if EOF is read 13 times in a row.

interactive

The shell is an interactive shell. This option can only be used when the shell is invoked. See above for a description of what this means.

login

The shell is a login shell. This option can only be used when the shell is invoked. See above for a description of what this means.

nohup

Do not kill running jobs with a SIGHUP signal when a login shell exits. Currently set by default, but this will change in the future to be compatible with AT&T UNIX ksh, which doesn’t have this option, but does send the SIGHUP signal.

nolog

No effect. In the original Korn shell, this prevents function definitions from being stored in the history file.

physical

Causes the cd and pwd commands to use ‘‘physical’’ (i.e. the filesystem’s) ‘..’ directories instead of ‘‘logical’’ directories (i.e. the shell handles ‘..’, which allows the user to be oblivious of symbolic links to directories). Clear by default. Note that setting this option does not affect the current value of the PWD parameter; only the cd command changes PWD. See the cd and pwd commands above for more details.

posix

Enable POSIX mode. Automatically enabled if the basename of the shell invocation begins with ‘‘sh’’ and this autodetection feature was requested at compilation time. As a side effect, setting this flag turns off braceexpand mode, which can be turned back on manually.

restricted

The shell is a restricted shell. This option can only be used when the shell is invoked. See above for a description of what this means.

vi

Enable vi(1)-like command-line editing (interactive shells only).

vi−esccomplete

In vi command-line editing, do command and file name completion when escape (^[) is entered in command mode.

vi−tabcomplete

In vi command-line editing, do command and file name completion when tab (^I) is entered in insert mode. This is the default.

viraw

No effect. In the original Korn shell, unless viraw was set, the vi command-line mode would let the tty(4) driver do the work until ESC (^[) was entered. mksh is always in viraw mode.

These options can also be used upon invocation of the shell. The current set of options (with single letter names) can be found in the parameter ‘$−’. set −o with no option name will list all the options and whether each is on or off; set +o will print the long names of all options that are currently on.

Remaining arguments, if any, are positional parameters and are assigned, in order, to the positional parameters (i.e. $1, $2, etc.). If options end with ‘−−’ and there are no remaining arguments, all positional parameters are cleared. If no options or arguments are given, the values of all names are printed. For unknown historical reasons, a lone ‘−’ option is treated specially – it clears both the −x and −v options.

shift [number]

The positional parameters number+1, number+2, etc. are renamed to ‘1’, ‘2’, etc. number defaults to 1.

source file [arg ...]

Like . (

‘‘
dot’’ ), except that the current working directory is appended to the PATH in GNU bash and mksh. In ksh93 and mksh, this is implemented as a shell alias instead of a builtin.

test expression
[
expression ]

test evaluates the expression and returns zero status if true, 1 if false, or greater than 1 if there was an error. It is normally used as the condition command of if and while statements. Symbolic links are followed for all file expressions except −h and −L.

The following basic expressions are available:

−a file

file exists.

−b file

file is a block special device.

−c file

file is a character special device.

−d file

file is a directory.

−e file

file exists.

−f file

file is a regular file.

−G file

file’s group is the shell’s effective group ID.

−g file

file’s mode has the setgid bit set.

−h file

file is a symbolic link.

−k file

file’s mode has the sticky(8) bit set.

−L file

file is a symbolic link.

−O file

file’s owner is the shell’s effective user ID.

−o option

Shell option is set (see the set command above for a list of options). As a non-standard extension, if the option starts with a ‘!’, the test is negated; the test always fails if option doesn’t exist (so [ −o foo −o −o !foo ] returns true if and only if option foo exists).

−p file

file is a named pipe.

−r file

file exists and is readable.

−S file

file is a unix(4)-domain socket.

−s file

file is not empty.

−t [fd]

File descriptor fd is a tty(4) device. fd may be left out, in which case it is taken to be 1.

−u file

file’s mode has the setuid bit set.

−w file

file exists and is writable.

−x file

file exists and is executable.

file1 −nt file2

file1 is newer than file2 or file1 exists and file2 does not.

file1 −ot file2

file1 is older than file2 or file2 exists and file1 does not.

file1 −ef file2

file1 is the same file as file2.

string

string has non-zero length.

−n string

string is not empty.

−z string

string is empty.

string = string

Strings are equal.

string == string

Strings are equal.

string > string

First string operand is greater than second string operand.

string < string

First string operand is less than second string operand.

string != string

Strings are not equal.

number −eq number

Numbers compare equal.

number −ne number

Numbers compare not equal.

number −ge number

Numbers compare greater than or equal.

number −gt number

Numbers compare greater than.

number −le number

Numbers compare less than or equal.

number −lt number

Numbers compare less than.

The above basic expressions, in which unary operators have precedence over binary operators, may be combined with the following operators (listed in increasing order of precedence):

expr −o expr Logical OR.

expr −a expr

Logical AND.

! expr

Logical NOT.

( expr )

Grouping.

Note that a number actually may be an arithmetic expression, such as a mathematical term or the name of an integer variable:

x=1; [ "x" −eq 1 ]

evaluates to true

Note that some special rules are applied (courtesy of POSIX) if the number of arguments to test or [ ... ] is less than five: if leading ‘!’ arguments can be stripped such that only one argument remains then a string length test is performed (again, even if the argument is a unary operator); if leading ‘!’ arguments can be stripped such that three arguments remain and the second argument is a binary operator, then the binary operation is performed (even if the first argument is a unary operator, including an unstripped ‘!’).

Note: A common mistake is to use ‘‘if [ $foo = bar ]’’ which fails if parameter ‘‘foo’’ is NULL or unset, if it has embedded spaces (i.e. IFS characters), or if it is a unary operator like ‘!’ or ‘−n’. Use tests like ‘‘if [ x"$foo" = x"bar" ]’’ instead, or the double-bracket operator: ‘‘if [[ $foo = bar ]]’’

time [−p] [pipeline]

If a pipeline is given, the times used to execute the pipeline are reported. If no pipeline is given, then the user and system time used by the shell itself, and all the commands it has run since it was started, are reported. The times reported are the real time (elapsed time from start to finish), the user CPU time (time spent running in user mode), and the system CPU time (time spent running in kernel mode). Times are reported to standard error; the format of the output is:

0m0.00s real 0m0.00s user 0m0.00s system

If the −p option is given the output is slightly longer:

real 0.00
user 0.00
sys 0.00

It is an error to specify the −p option unless pipeline is a simple command.

Simple redirections of standard error do not affect the output of the time command:

$ time sleep 1 2> afile
$ { time sleep 1; } 2> afile

Times for the first command do not go to ‘‘afile’’, but those of the second command do.

times

Print the accumulated user and system times used both by the shell and by processes that the shell started which have exited. The format of the output is:

0m0.00s 0m0.00s
0m0.00s 0m0.00s

trap [handler signal ...]

Sets a trap handler that is to be executed when any of the specified signals are received. handler is either a NULL string, indicating the signals are to be ignored, a minus sign (‘−’), indicating that the default action is to be taken for the signals (see signal(3)), or a string containing shell commands to be evaluated and executed at the first opportunity (i.e. when the current command completes, or before printing the next PS1 prompt) after receipt of one of the signals. signal is the name of a signal (e.g. PIPE or ALRM) or the number of the signal (see the kill −l command above).

There are two special signals: EXIT (also known as 0) which is executed when the shell is about to exit, and ERR, which is executed after an error occurs (an error is something that would cause the shell to exit if the −e or errexit option were set – see the set command above). EXIT handlers are executed in the environment of the last executed command. Note that for non-interactive shells, the trap handler cannot be changed for signals that were ignored when the shell started.

With no arguments, trap lists, as a series of trap commands, the current state of the traps that have been set since the shell started. Note that the output of trap cannot be usefully piped to another process (an artifact of the fact that traps are cleared when subprocesses are created).

The original Korn shell’s DEBUG trap and the handling of ERR and EXIT traps in functions are not yet implemented.

true

A command that exits with a zero value.

typeset [
[+−lprtUux] [−L[n]] [−R[n]] [−Z[n]] [−i[n]] | −f [−tux]] [
name
[=value] ...]

Display or set parameter attributes. With no name arguments, parameter attributes are displayed; if no options are used, the current attributes of all parameters are printed as typeset commands; if an option is given (or ‘−’ with no option letter), all parameters and their values with the specified attributes are printed; if options are introduced with ‘+’, parameter values are not printed.

If name arguments are given, the attributes of the named parameters are set () or cleared (+). Values for parameters may optionally be specified. If typeset is used inside a function, any newly created parameters are local to the function.

When −f is used, typeset operates on the attributes of functions. As with parameters, if no name arguments are given, functions are listed with their values (i.e. definitions) unless options are introduced with ‘+’, in which case only the function names are reported.

−f

Function mode. Display or set functions and their attributes, instead of parameters.

−i[n]

Integer attribute. n specifies the base to use when displaying the integer (if not specified, the base given in the first assignment is used). Parameters with this attribute may be assigned values containing arithmetic expressions.

−L[n]

Left justify attribute. n specifies the field width. If n is not specified, the current width of a parameter (or the width of its first assigned value) is used. Leading whitespace (and zeros, if used with the −Z option) is stripped. If necessary, values are either truncated or space padded to fit the field width.

−l

Lower case attribute. All upper case characters in values are converted to lower case. (In the original Korn shell, this parameter meant ‘‘long integer’’ when used with the −i option.)

−p

Print complete typeset commands that can be used to re-create the attributes and values of parameters.

−R[n]

Right justify attribute. n specifies the field width. If n is not specified, the current width of a parameter (or the width of its first assigned value) is used. Trailing whitespace is stripped. If necessary, values are either stripped of leading characters or space padded to make them fit the field width.

−r

Read-only attribute. Parameters with this attribute may not be assigned to or unset. Once this attribute is set, it cannot be turned off.

−t

Tag attribute. Has no meaning to the shell; provided for application use.

For functions, −t is the trace attribute. When functions with the trace attribute are executed, the xtrace (−x) shell option is temporarily turned on.

−U

Unsigned integer attribute. Integers are printed as unsigned values (combine with the −i option). This option is not in the original Korn shell.

−u

Upper case attribute. All lower case characters in values are converted to upper case. (In the original Korn shell, this parameter meant ‘‘unsigned integer’’ when used with the −i option which meant upper case letters would never be used for bases greater than 10. See the −U option.)

For functions, −u is the undefined attribute. See Functions above for the implications of this.

−x

Export attribute. Parameters (or functions) are placed in the environment of any executed commands. Exported functions are not yet implemented.

−Z[n]

Zero fill attribute. If not combined with −L, this is the same as −R, except zero padding is used instead of space padding. For integers, the number instead of the base is padded.

If any of the −i, −L, −l, −R, −U, −u, or −Z options are changed, all others from this set are cleared, unless they are also given on the same command line.

ulimit [−acdfHLlmnpSsTtvw] [value]

Display or set process limits. If no options are used, the file size limit (−f) is assumed. value, if specified, may be either an arithmetic expression or the word ‘‘unlimited’’. The limits affect the shell and any processes created by the shell after a limit is imposed. Note that some systems may not allow limits to be increased once they are set. Also note that the types of limits available are system dependent – some systems have only the −f limit.

−a

Display all limits; unless −H is used, soft limits are displayed.

−c n

Impose a size limit of n blocks on the size of core dumps.

−d n

Impose a size limit of n kibibytes on the size of the data area.

−f n

Impose a size limit of n blocks on files written by the shell and its child processes (files of any size may be read).

−H

Set the hard limit only (the default is to set both hard and soft limits).

−L n

Control flocks; documentation is missing.

−l n

Impose a limit of n kibibytes on the amount of locked (wired) physical memory.

−m n

Impose a limit of n kibibytes on the amount of physical memory used.

−n n

Impose a limit of n file descriptors that can be open at once.

−p n

Impose a limit of n processes that can be run by the user at any one time.

−S

Set the soft limit only (the default is to set both hard and soft limits).

−s n

Impose a size limit of n kibibytes on the size of the stack area.

−T n

Impose a time limit of n real seconds to be used by each process.

−t n

Impose a time limit of n CPU seconds spent in user mode to be used by each process.

−v n

Impose a limit of n kibibytes on the amount of virtual memory (address space) used.

−w n

Impose a limit of n kibibytes on the amount of swap space used.

As far as ulimit is concerned, a block is 512 bytes.

umask [−S] [mask]

Display or set the file permission creation mask, or umask (see umask(2)). If the −S option is used, the mask displayed or set is symbolic; otherwise, it is an octal number.

Symbolic masks are like those used by chmod(1). When used, they describe what permissions may be made available (as opposed to octal masks in which a set bit means the corresponding bit is to be cleared). For example, ‘‘ug=rwx,o=’’ sets the mask so files will not be readable, writable, or executable by ‘‘others’’, and is equivalent (on most systems) to the octal mask ‘‘007’’.

unalias [−adt] [name ...]

The aliases for the given names are removed. If the −a option is used, all aliases are removed. If the −t or −d options are used, the indicated operations are carried out on tracked or directory aliases, respectively.

unset [−fv] parameter ...

Unset the named parameters (

−v, the default ) or functions (−f).

wait [job ...]

Wait for the specified job(s) to finish. The exit status of wait is that of the last specified job; if the last job is killed by a signal, the exit status is 128 + the number of the signal (see kill −l exit-status above); if the last specified job can’t be found (because it never existed, or had already finished), the exit status of wait is 127. See Job control below for the format of job. wait will return if a signal for which a trap has been set is received, or if a SIGHUP, SIGINT, or SIGQUIT signal is received.

If no jobs are specified, wait waits for all currently running jobs (if any) to finish and exits with a zero status. If job monitoring is enabled, the completion status of jobs is printed (this is not the case when jobs are explicitly specified).

whence [−pv] [name ...]

For each name, the type of command is listed (reserved word, built-in, alias, function, tracked alias, or executable). If the −p option is used, a path search is performed even if name is a reserved word, alias, etc. Without the −v option, whence is similar to command −v except that whence will find reserved words and won’t print aliases as alias commands. With the −v option, whence is the same as command −V. Note that for whence, the −p option does not affect the search path used, as it does for command. If the type of one or more of the names could not be determined, the exit status is non-zero.

Job control
Job control refers to the shell’s ability to monitor and control jobs which are processes or groups of processes created for commands or pipelines. At a minimum, the shell keeps track of the status of the background (i.e. asynchronous) jobs that currently exist; this information can be displayed using the jobs commands. If job control is fully enabled (using set −m or set −o monitor), as it is for interactive shells, the processes of a job are placed in their own process group. Foreground jobs can be stopped by typing the suspend character from the terminal (normally ^Z), jobs can be restarted in either the foreground or background using the fg and bg commands, and the state of the terminal is saved or restored when a foreground job is stopped or restarted, respectively.

Note that only commands that create processes (e.g. asynchronous commands, subshell commands, and non-built-in, non-function commands) can be stopped; commands like read cannot be.

When a job is created, it is assigned a job number. For interactive shells, this number is printed inside ‘‘[..]’’, followed by the process IDs of the processes in the job when an asynchronous command is run. A job may be referred to in the bg, fg, jobs, kill, and wait commands either by the process ID of the last process in the command pipeline (as stored in the $! parameter) or by prefixing the job number with a percent sign (‘%’). Other percent sequences can also be used to refer to jobs:

%+ | %% | %

The most recently stopped job, or, if there are no stopped jobs, the oldest running job.

%−

The job that would be the %+ job if the latter did not exist.

%n

The job with job number n.

%?string

The job with its command containing the string string (an error occurs if multiple jobs are matched).

%string

The job with its command starting with the string string (an error occurs if multiple jobs are matched).

When a job changes state (e.g. a background job finishes or foreground job is stopped), the shell prints the following status information:

[number] flag status command

where...

number

is the job number of the job;

flag

is the ‘+’ or ‘−’ character if the job is the %+ or %− job, respectively, or space if it is neither;

status

indicates the current state of the job and can be:

Done [number]

The job exited. number is the exit status of the job which is omitted if the status is zero.

Running

The job has neither stopped nor exited (note that running does not necessarily mean consuming CPU time – the process could be blocked waiting for some event).

Stopped [signal]

The job was stopped by the indicated signal (if no signal is given, the job was stopped by SIGTSTP).

signal-description [‘‘core dumped’’]

The job was killed by a signal (e.g. memory fault, hangup); use kill −l for a list of signal descriptions. The ‘‘core dumped’’ message indicates the process created a core file.

command

is the command that created the process. If there are multiple processes in the job, each process will have a line showing its command and possibly its status, if it is different from the status of the previous process.

When an attempt is made to exit the shell while there are jobs in the stopped state, the shell warns the user that there are stopped jobs and does not exit. If another attempt is immediately made to exit the shell, the stopped jobs are sent a SIGHUP signal and the shell exits. Similarly, if the nohup option is not set and there are running jobs when an attempt is made to exit a login shell, the shell warns the user and does not exit. If another attempt is immediately made to exit the shell, the running jobs are sent a SIGHUP signal and the shell exits.

Interactive input line editing
The shell supports three modes of reading command lines from a tty(4) in an interactive session, controlled by the emacs, gmacs, and vi options (at most one of these can be set at once). The default is emacs. Editing modes can be set explicitly using the set built-in. If none of these options are enabled, the shell simply reads lines using the normal tty(4) driver. If the emacs or gmacs option is set, the shell allows emacs-like editing of the command; similarly, if the vi option is set, the shell allows vi-like editing of the command. These modes are described in detail in the following sections.

In these editing modes, if a line is longer than the screen width (see the COLUMNS parameter), a ‘>’, ‘+’, or ‘<’ character is displayed in the last column indicating that there are more characters after, before and after, or before the current position, respectively. The line is scrolled horizontally as necessary.

Completed lines are pushed into the history, unless they begin with an IFS character or IFS white space, or are the same as the previous line.

Emacs editing mode
When the emacs option is set, interactive input line editing is enabled. Warning: This mode is slightly different from the emacs mode in the original Korn shell. In this mode, various editing commands (typically bound to one or more control characters) cause immediate actions without waiting for a newline. Several editing commands are bound to particular control characters when the shell is invoked; these bindings can be changed using the bind command.

The following is a list of available editing commands. Each description starts with the name of the command, suffixed with a colon; an [n] (if the command can be prefixed with a count); and any keys the command is bound to by default, written using caret notation e.g. the ASCII ESC character is written as ^[. These control sequences are not case sensitive. A count prefix for a command is entered using the sequence ^[n, where n is a sequence of 1 or more digits. Unless otherwise specified, if a count is omitted, it defaults to 1.

Note that editing command names are used only with the bind command. Furthermore, many editing commands are useful only on terminals with a visible cursor. The default bindings were chosen to resemble corresponding Emacs key bindings. The user’s tty(4) characters (e.g. ERASE) are bound to reasonable substitutes and override the default bindings.

abort: ^C, ^G

Abort the current command, empty the line buffer and set the exit state to interrupted.

auto−insert: [n]

Simply causes the character to appear as literal input. Most ordinary characters are bound to this.

backward−char: [n] ^B, ^XD

Moves the cursor backward n characters.

backward−word: [n] ^[b

Moves the cursor backward to the beginning of the word; words consist of alphanumerics, underscore (‘_’), and dollar sign (‘$’) characters.

beginning−of−history: ^[<

Moves to the beginning of the history.

beginning−of−line: ^A

Moves the cursor to the beginning of the edited input line.

capitalise−word: [n] ^[C, ^[c

Uppercase the first character in the next n words, leaving the cursor past the end of the last word.

clear−screen: ^[^L

Prints a compile-time configurable sequence to clear the screen and home the cursor, redraws the entire prompt and the currently edited input line. The default sequence works for almost all standard terminals.

comment: ^[#

If the current line does not begin with a comment character, one is added at the beginning of the line and the line is entered (as if return had been pressed); otherwise, the existing comment characters are removed and the cursor is placed at the beginning of the line.

complete: ^[^[

Automatically completes as much as is unique of the command name or the file name containing the cursor. If the entire remaining command or file name is unique, a space is printed after its completion, unless it is a directory name in which case ‘/’ is appended. If there is no command or file name with the current partial word as its prefix, a bell character is output (usually causing a beep to be sounded).

complete−command: ^X^[

Automatically completes as much as is unique of the command name having the partial word up to the cursor as its prefix, as in the complete command above.

complete−file: ^[^X

Automatically completes as much as is unique of the file name having the partial word up to the cursor as its prefix, as in the complete command described above.

complete−list: ^I, ^[=

Complete as much as is possible of the current word, and list the possible completions for it. If only one completion is possible, match as in the complete command above. Note that ^I is usually generated by the TAB (tabulator) key.

delete−char−backward: [n] ERASE, ^?, ^H

Deletes n characters before the cursor.

delete−char−forward: [n]

Deletes n characters after the cursor.

delete−word−backward: [n] ERASE, ^[^?, ^[^H, ^[h

Deletes n words before the cursor.

delete−word−forward: [n] ^[d

Deletes characters after the cursor up to the end of n words.

down−history: [n] ^N, ^XB

Scrolls the history buffer forward n lines (later). Each input line originally starts just after the last entry in the history buffer, so down−history is not useful until either search−history or up−history has been performed.

downcase−word: [n] ^[L, ^[l

Lowercases the next n words.

edit−line: [n] ^Xe

Edit line n or the current line, if not specified, interactively. The actual command executed is fc −e ${VISUAL:−${EDITOR:−vi}} n.

end−of−history: ^[>

Moves to the end of the history.

end−of−line: ^E

Moves the cursor to the end of the input line.

eot: ^_

Acts as an end-of-file; this is useful because edit-mode input disables normal terminal input canonicalization.

eot−or−delete: [n] ^D

Acts as eot if alone on a line; otherwise acts as delete−char−forward.

error: (not bound)

Error (ring the bell).

exchange−point−and−mark: ^X^X

Places the cursor where the mark is and sets the mark to where the cursor was.

expand−file: ^[*

Appends a ‘*’ to the current word and replaces the word with the result of performing file globbing on the word. If no files match the pattern, the bell is rung.

forward−char: [n] ^F, ^XC

Moves the cursor forward n characters.

forward−word: [n] ^[f

Moves the cursor forward to the end of the nth word.

goto−history: [n] ^[g

Goes to history number n.

kill−line: KILL

Deletes the entire input line.

kill−region: ^W

Deletes the input between the cursor and the mark.

kill−to−eol: [n] ^K

Deletes the input from the cursor to the end of the line if n is not specified; otherwise deletes characters between the cursor and column n.

list: ^[?

Prints a sorted, columnated list of command names or file names (if any) that can complete the partial word containing the cursor. Directory names have ‘/’ appended to them.

list−command: ^X?

Prints a sorted, columnated list of command names (if any) that can complete the partial word containing the cursor.

list−file: ^X^Y

Prints a sorted, columnated list of file names (if any) that can complete the partial word containing the cursor. File type indicators are appended as described under list above.

newline: ^J, ^M

Causes the current input line to be processed by the shell. The current cursor position may be anywhere on the line.

newline−and−next: ^O

Causes the current input line to be processed by the shell, and the next line from history becomes the current line. This is only useful after an up−history or search−history.

no−op: QUIT

This does nothing.

prefix−1: ^[

Introduces a 2-character command sequence.

prefix−2: ^X, ^[[, ^[O

Introduces a 2-character command sequence.

prev−hist−word: [n] ^[., ^[_

The last (nth) word of the previous (on repeated execution, second-last, third-last, etc.) command is inserted at the cursor. Use of this editing command trashes the mark.

quote: ^^, ^V

The following character is taken literally rather than as an editing command.

redraw: ^L

Reprints the last line of the prompt string and the current input line on a new line.

search−character−backward: [n] ^[^]

Search backward in the current line for the nth occurrence of the next character typed.

search−character−forward: [n] ^]

Search forward in the current line for the nth occurrence of the next character typed.

search−history: ^R

Enter incremental search mode. The internal history list is searched backwards for commands matching the input. An initial ‘^’ in the search string anchors the search. The escape key will leave search mode. Other commands, including sequences of escape as prefix−1 followed by a prefix−1 or prefix−2 key will be executed after leaving search mode. The abort (^G) command will restore the input line before search started. Successive search−history commands continue searching backward to the next previous occurrence of the pattern. The history buffer retains only a finite number of lines; the oldest are discarded as necessary.

set−mark−command: ^[〈space〉

Set the mark at the cursor position.

transpose−chars: ^T

If at the end of line, or if the gmacs option is set, this exchanges the two previous characters; otherwise, it exchanges the previous and current characters and moves the cursor one character to the right.

up−history: [n] ^P, ^XA

Scrolls the history buffer backward n lines (earlier).

upcase−word: [n] ^[U, ^[u

Uppercase the next n words.

version: ^[^V

Display the version of mksh. The current edit buffer is restored as soon as a key is pressed. The restoring keypress is processed, unless it is a space.

yank: ^Y

Inserts the most recently killed text string at the current cursor position.

yank−pop: ^[y

Immediately after a yank, replaces the inserted text string with the next previously killed text string.

Vi editing mode
Note:
The vi command-line editing mode is orphaned, yet still functional.

The vi command-line editor in mksh has basically the same commands as the vi(1) editor with the following exceptions:

You start out in insert mode.

There are file name and command completion commands: =, \, *, ^X, ^E, ^F, and, optionally, 〈tab〉 and 〈esc〉.

The _ command is different (in mksh, it is the last argument command; in vi(1) it goes to the start of the current line).

The / and G commands move in the opposite direction to the j command.

Commands which don’t make sense in a single line editor are not available (e.g. screen movement commands and ex(1)-style colon (:) commands).

Like vi(1), there are two modes: ‘‘insert’’ mode and ‘‘command’’ mode. In insert mode, most characters are simply put in the buffer at the current cursor position as they are typed; however, some characters are treated specially. In particular, the following characters are taken from current tty(4) settings (see stty(1)) and have their usual meaning (normal values are in parentheses): kill (^U), erase (^?), werase (^W), eof (^D), intr (^C), and quit (^\). In addition to the above, the following characters are also treated specially in insert mode:

^E

Command and file name enumeration (see below).

^F

Command and file name completion (see below). If used twice in a row, the list of possible completions is displayed; if used a third time, the completion is undone.

^H

Erases previous character.

^J | ^M

End of line. The current line is read, parsed, and executed by the shell.

^V

Literal next. The next character typed is not treated specially (can be used to insert the characters being described here).

^X

Command and file name expansion (see below).

〈esc〉

Puts the editor in command mode (see below).

〈tab〉

Optional file name and command completion (see ^F above), enabled with set −o vi−tabcomplete.

In command mode, each character is interpreted as a command. Characters that don’t correspond to commands, are illegal combinations of commands, or are commands that can’t be carried out, all cause beeps. In the following command descriptions, an [n] indicates the command may be prefixed by a number (e.g. 10l moves right 10 characters); if no number prefix is used, n is assumed to be 1 unless otherwise specified. The term ‘‘current position’’ refers to the position between the cursor and the character preceding the cursor. A ‘‘word’’ is a sequence of letters, digits, and underscore characters or a sequence of non-letter, non-digit, non-underscore, and non-whitespace characters (e.g. ‘‘ab2*&^’’ contains two words) and a ‘‘big-word’’ is a sequence of non-whitespace characters.

Special mksh vi commands:

The following commands are not in, or are different from, the normal vi file editor:

[
n
]_

Insert a space followed by the nth big-word from the last command in the history at the current position and enter insert mode; if n is not specified, the last word is inserted.

#

Insert the comment character (‘#’) at the start of the current line and return the line to the shell (equivalent to I#^J).

[
n
]g

Like G, except if n is not specified, it goes to the most recent remembered line.

[
n
]v

Edit line n using the vi(1) editor; if n is not specified, the current line is edited. The actual command executed is fc −e ${VISUAL:−${EDITOR:−vi}} n.

* and ^X

Command or file name expansion is applied to the current big-word (with an appended ‘*’ if the word contains no file globbing characters) – the big-word is replaced with the resulting words. If the current big-word is the first on the line (or follows one of the following characters: ‘;’, ‘|’, ‘&’, ‘(’, or ‘)’ and does not contain a slash (‘/’) then command expansion is done; otherwise file name expansion is done. Command expansion will match the big-word against all aliases, functions, and built-in commands as well as any executable files found by searching the directories in the PATH parameter. File name expansion matches the big-word against the files in the current directory. After expansion, the cursor is placed just past the last word and the editor is in insert mode.

[
n
]\, [
n
]^F, [
n
]〈tab〉, and [
n
]〈esc〉

Command/file name completion. Replace the current big-word with the longest unique match obtained after performing command and file name expansion. 〈tab〉 is only recognised if the vi−tabcomplete option is set, while 〈esc〉 is only recognised if the vi−esccomplete option is set (see set −o). If n is specified, the nth possible completion is selected (as reported by the command/file name enumeration command).

= and ^E

Command/file name enumeration. List all the commands or files that match the current big-word.

^V

Display the version of mksh. The current edit buffer is restored as soon as a key is pressed. The restoring keypress is ignored.

@c

Macro expansion. Execute the commands found in the alias c.

Intra-line movement commands:

[
n
]h and [
n
]^H

Move left n characters.

[
n
]l and [
n
]〈space〉

Move right n characters.

0

Move to column 0.

^

Move to the first non-whitespace character.

[
n
]|

Move to column n.

$

Move to the last character.

[
n
]b

Move back n words.

[
n
]B

Move back n big-words.

[
n
]e

Move forward to the end of the word, n times.

[
n
]E

Move forward to the end of the big-word, n times.

[
n
]w

Move forward n words.

[
n
]W

Move forward n big-words.

%

Find match. The editor looks forward for the nearest parenthesis, bracket, or brace and then moves the cursor to the matching parenthesis, bracket, or brace.

[
n
]fc

Move forward to the nth occurrence of the character c.

[
n
]Fc

Move backward to the nth occurrence of the character c.

[
n
]tc

Move forward to just before the nth occurrence of the character c.

[
n
]Tc

Move backward to just before the nth occurrence of the character c.

[
n
];

Repeats the last f, F, t, or T command.

[
n
],

Repeats the last f, F, t, or T command, but moves in the opposite direction.

Inter-line movement commands:

[
n
]j, [
n
]+, and [
n
]^N

Move to the nth next line in the history.

[
n
]k, [
n
]−, and [
n
]^P

Move to the nth previous line in the history.

[
n
]G

Move to line n in the history; if n is not specified, the number of the first remembered line is used.

[
n
]g

Like G, except if n is not specified, it goes to the most recent remembered line.

[
n
]/string

Search backward through the history for the nth line containing string; if string starts with ‘^’, the remainder of the string must appear at the start of the history line for it to match.

[
n
]?string

Same as /, except it searches forward through the history.

[
n
]n

Search for the nth occurrence of the last search string; the direction of the search is the same as the last search.

[
n
]N

Search for the nth occurrence of the last search string; the direction of the search is the opposite of the last search.

Edit commands

[
n
]a

Append text n times; goes into insert mode just after the current position. The append is only replicated if command mode is re-entered i.e. 〈esc〉 is used.

[
n
]A

Same as a, except it appends at the end of the line.

[
n
]i

Insert text n times; goes into insert mode at the current position. The insertion is only replicated if command mode is re-entered i.e. 〈esc〉 is used.

[
n
]I

Same as i, except the insertion is done just before the first non-blank character.

[
n
]s

Substitute the next n characters (i.e. delete the characters and go into insert mode).

S

Substitute whole line. All characters from the first non-blank character to the end of the line are deleted and insert mode is entered.

[
n
]cmove-cmd

Change from the current position to the position resulting from n move-cmds (i.e. delete the indicated region and go into insert mode); if move-cmd is c, the line starting from the first non-blank character is changed.

C

Change from the current position to the end of the line (i.e. delete to the end of the line and go into insert mode).

[
n
]x

Delete the next n characters.

[
n
]X

Delete the previous n characters.

D

Delete to the end of the line.

[
n
]dmove-cmd

Delete from the current position to the position resulting from n move-cmds; move-cmd is a movement command (see above) or d, in which case the current line is deleted.

[
n
]rc

Replace the next n characters with the character c.

[
n
]R

Replace. Enter insert mode but overwrite existing characters instead of inserting before existing characters. The replacement is repeated n times.

[
n
]~

Change the case of the next n characters.

[
n
]ymove-cmd

Yank from the current position to the position resulting from n move-cmds into the yank buffer; if move-cmd is y, the whole line is yanked.

Y

Yank from the current position to the end of the line.

[
n
]p

Paste the contents of the yank buffer just after the current position, n times.

[
n
]P

Same as p, except the buffer is pasted at the current position.

Miscellaneous vi commands

^J and ^M

The current line is read, parsed, and executed by the shell.

^L and ^R

Redraw the current line.

[
n
].

Redo the last edit command n times.

u

Undo the last edit command.

U

Undo all changes that have been made to the current line.

intr and quit

The interrupt and quit terminal characters cause the current line to be deleted and a new prompt to be printed.

FILES
~/.mkshrc

User’s startup script (interactive shells). Used only if ENV is unset or empty.

~/.profile

User’s login profile.

/etc/profile

System login profile.

/etc/shells

Shell database.

/etc/suid_profile

Privileged shell profile.

SEE ALSO

awk(1), ed(1), getopt(1), sed(1), sh(1), stty(1), dup(2), execve(2), getgid(2), getuid(2), mknod(2), mkfifo(2), open(2), pipe(2), rename(2), wait(2), arc4random(3), getopt(3), nl_langinfo(3), rand(3), setlocale(3), signal(3), srand(3), system(3), tty(4), shells(5), environ(7), script(7), utf−8(7), mknod(8)

http://docsrv.sco.com:507/en/man/html.C/sh.C.html

Morris Bolsky

,

The KornShell Command and Programming Language ,
Prentice Hall PTR
,
xvi + 356 pages ,
1989 ,
ISBN 978−0−13−516972−8 (0−13−516972−0) .

Morris I. Bolsky

and

David G. Korn ,
The New KornShell Command and Programming Language (2nd Edition)
,
Prentice Hall PTR
,
xvi + 400 pages ,
1995 ,
ISBN 978−0−13−182700−4 (0−13−182700−6) .

Stephen G. Kochan

and

Patrick H. Wood ,
UNIX Shell Programming
,
Hayden
,
Revised Edition ,
xi + 490 pages ,
1990 ,
ISBN 978−0−672−48448−3 (0−672−48448−X) .

IEEE Inc.

,

IEEE Standard for Information Technology – Portable Operating System Interface (POSIX) ,
IEEE Press
,
Part 2: Shell and Utilities ,
xvii + 1195 pages ,
1993 ,
ISBN 978−1−55937−255−8 (1−55937−255−9) .

Bill Rosenblatt

,

Learning the Korn Shell ,
O’Reilly
,
360 pages ,
1993 ,
ISBN 978−1−56592−054−5 (1−56592−054−6) .

Bill Rosenblatt

and

Arnold Robbins ,
Learning the Korn Shell, Second Edition
,
O’Reilly
,
432 pages ,
2002 ,
ISBN 978−0−596−00195−7 (0−596−00195−9) .

Barry Rosenberg

,

KornShell Programming Tutorial ,
Addison-Wesley Professional
,
xxi + 324 pages ,
1991 ,
ISBN 978−0−201−56324−5 (0−201−56324−X) .

AUTHORS

This shell is based upon the Public Domain Korn SHell. It has been developed further by the MirOS project incorporating changes from the Debian and OpenBSD projects and patches and suggestions from users and many other persons, and is currently maintained by Thorsten Glaser 〈tg@mirbsd.de〉.

BUGS

This document attempts to describe mksh R37 and up, compiled without any options impacting functionality, such as MKSH_SMALL, for an operating environment supporting all of its advanced needs. Please report bugs in mksh to the 〈miros−discuss@mirbsd.org〉 mailing list or in the #!/bin/mksh (or #ksh) IRC channel at irc.freenode.net:6667.

MirBSD March 23, 2009 MirBSD