set - set or unset options and positional parameters
set -- [argument...]
If no options or arguments are specified, set shall write the names and values of all shell variables in the collation sequence of the current locale. Each name shall start on a separate line, using the format:
"%s=%s\n", <name>, <value>
The value string shall be written with appropriate quoting; see the description of shell quoting in Quoting . The output shall be suitable for reinput to the shell, setting or resetting, as far as possible, the variables that are currently set; read-only variables cannot be reset.
When options are specified, they shall set or unset attributes of the shell, as described below. When arguments are specified, they cause positional parameters to be set or unset, as described below. Setting or unsetting attributes and positional parameters are not necessarily related actions, but they can be combined in a single invocation of set.
The set special built-in shall support the Base Definitions volume of IEEE Std 1003.1-2001, Section 12.2, Utility Syntax Guidelines except that options can be specified with either a leading hyphen (meaning enable the option) or plus sign (meaning disable it) unless otherwise specified.
Implementations shall support the options in the following list in both their hyphen and plus-sign forms. These options can also be specified as options to sh.
"[%d]%c %s%s\n", <job-number>, <current>, <status>, <job-name>
where the fields shall be as follows:
When the shell notifies the user a job has been completed, it may remove the job's process ID from the list of those known in the current shell execution environment; see Asynchronous Lists . Asynchronous notification shall not be enabled by default.
This option is supported if the system supports the User Portability Utilities option. It shall set various options, many of which shall be equivalent to the single option letters. The following values of option shall be supported:
It need not be possible to set vi mode on for certain block-mode terminals.
The default for all these options shall be off (unset) unless stated otherwise in the description of the option or unless the shell was invoked with them on; see sh.
The remaining arguments shall be assigned in order to the positional parameters. The special parameter '#' shall be set to reflect the number of positional parameters. All positional parameters shall be unset before any new values are assigned.
The special argument "--" immediately following the set command name can be used to delimit the arguments if the first argument begins with '+' or '-' , or to prevent inadvertent listing of all shell variables when there are no arguments. The command set -- without argument shall unset all positional parameters and set the special parameter '#' to zero.
See the DESCRIPTION.
See the DESCRIPTION.
See the DESCRIPTION.
The standard error shall be used only for diagnostic messages.
The following sections are informative.
Write out all variables and their values:
Set $1, $2, and $3 and set "$#" to 3:
set c a b
Turn on the -x and -v options:
Unset all positional parameters:
Set $1 to the value of x, even if it begins with '-' or '+' :
set -- "$x"
Set the positional parameters to the expansion of x, even if x expands with a leading '-' or '+' :
set -- $x
The set -- form is listed specifically in the SYNOPSIS even though this usage is implied by the Utility Syntax Guidelines. The explanation of this feature removes any ambiguity about whether the set -- form might be misinterpreted as being equivalent to set without any options or arguments. The functionality of this form has been adopted from the KornShell. In System V, set -- only unsets parameters if there is at least one argument; the only way to unset all parameters is to use shift. Using the KornShell version should not affect System V scripts because there should be no reason to issue it without arguments deliberately; if it were issued as, for example:
set -- "$@"
and there were in fact no arguments resulting from "$@" , unsetting the parameters would have no result.
The set + form in early proposals was omitted as being an unnecessary duplication of set alone and not widespread historical practice.
The noclobber option was changed to allow set -C as well as the set -o noclobber option. The single-letter version was added so that the historical "$-" paradigm would not be broken; see Special Parameters .
The -h flag is related to command name hashing and is only required on XSI-conformant systems.
The following set flags were omitted intentionally with the following rationale:
set -k; echo name=value
set -k echo name=value
behave differently. The interaction with functions is even more complex. What is more, the -k flag is never needed, since the command line could have been reordered.
Consideration was given to rewriting set to simplify its confusing syntax. A specific suggestion was that the unset utility should be used to unset options instead of using the non- getopt() -able + option syntax. However, the conclusion was reached that the historical practice of using + option was satisfactory and that there was no compelling reason to modify such widespread historical practice.
The -o option was adopted from the KornShell to address user needs. In addition to its generally friendly interface, -o is needed to provide the vi command line editing mode, for which historical practice yields no single-letter option name. (Although it might have been possible to invent such a letter, it was recognized that other editing modes would be developed and -o provides ample name space for describing such extensions.)
Historical implementations are inconsistent in the format used for -o option status reporting. The +o format without an option-argument was added to allow portable access to the options that can be saved and then later restored using, for instance, a dot script.
Historically, sh did trace the command set +x, but ksh did not.
The ignoreeof setting prevents accidental logouts when the end-of-file character (typically <control>-D) is entered. A user shall explicitly exit to leave the interactive shell.
The set -m option was added to apply only to the UPE because it applies primarily to interactive use, not shell script applications.
The ability to do asynchronous notification became available in the 1988 version of the KornShell. To have it occur, the user had to issue the command:
trap "jobs -n" CLD
The C shell provides two different levels of an asynchronous notification capability. The environment variable notify is analogous to what is done in set -b or set -o notify. When set, it notifies the user immediately of background job completions. When unset, this capability is turned off.
The other notification ability comes through the built-in utility notify. The syntax is:
notify [%job ... ]
By issuing notify with no operands, it causes the C shell to notify the user asynchronously when the state of the current job changes. If given operands, notify asynchronously informs the user of changes in the states of the specified jobs.
To add asynchronous notification to the POSIX shell, neither the KornShell extensions to trap, nor the C shell notify environment variable seemed appropriate ( notify is not a proper POSIX environment variable name).
The set -b option was selected as a compromise.
The notify built-in was considered to have more functionality than was required for simple asynchronous notification.
Special Built-In Utilities
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Created 1996-2021 by Maxim Chirkov
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