This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.
set — set or unset options and positional parameters
set [−abCefhmnuvx] [−o option] [argument...]
set [+abCefhmnuvx] [+o option] [argument...]
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:
The value string shall be written with appropriate quoting; see the description of shell quoting in Section 2.2, 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 POSIX.1‐2008, 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.
"%s=%s\n", <name>, <value>
- When this option is on, the export attribute shall be set for each variable to which an assignment is performed; see the Base Definitions volume of POSIX.1‐2008, Section 4.22, Variable Assignment. If the assignment precedes a utility name in a command, the export attribute shall not persist in the current execution environment after the utility completes, with the exception that preceding one of the special built-in utilities causes the export attribute to persist after the built-in has completed. If the assignment does not precede a utility name in the command, or if the assignment is a result of the operation of the getopts or read utilities, the export attribute shall persist until the variable is unset.
- This option shall be supported if the implementation supports the User Portability Utilities option. It shall cause the shell to notify the user asynchronously of background job completions. The following message is written to standard error:
"[%d]%c %s%s\n", <job-number>, <current>, <status>, <job-name>
where the fields shall be as follows:
- The character '+' identifies the job that would be used as a default for the fg or bg utilities; this job can also be specified using the job_id "%+" or "%%". The character '−' identifies the job that would become the default if the current default job were to exit; this job can also be specified using the job_id "%−". For other jobs, this field is a <space>. At most one job can be identified with '+' and at most one job can be identified with '−'. If there is any suspended job, then the current job shall be a suspended job. If there are at least two suspended jobs, then the previous job also shall be a suspended job.
- A number that can be used to identify the process group to the wait, fg, bg, and kill utilities. Using these utilities, the job can be identified by prefixing the job number with '%'.
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 Section 18.104.22.168
. Asynchronous notification shall not be enabled by default.
- (Uppercase C.) Prevent existing files from being overwritten by the shell's '>' redirection operator (see Section 2.7.2, Redirecting Output); the ">|" redirection operator shall override this noclobber option for an individual file.
- When this option is on, when any command fails (for any of the reasons listed in Section 2.8.1, Consequences of Shell Errors or by returning an exit status greater than zero), the shell immediately shall exit with the following exceptions:
- The failure of any individual command in a multi-command pipeline shall not cause the shell to exit. Only the failure of the pipeline itself shall be considered.
- The −e setting shall be ignored when executing the compound list following the while, until, if, or elif reserved word, a pipeline beginning with the ! reserved word, or any command of an AND-OR list other than the last.
- If the exit status of a compound command other than a subshell command was the result of a failure while −e was being ignored, then −e shall not apply to this command.
This requirement applies to the shell environment and each subshell environment separately. For example, in:
set -e; (false; echo one) | cat; echo two
command causes the subshell to exit without executing echo one
; however, echo two
is executed because the exit status of the pipeline (false; echo one) | cat
- The shell shall disable pathname expansion.
- Locate and remember utilities invoked by functions as those functions are defined (the utilities are normally located when the function is executed).
- This option shall be supported if the implementation supports the User Portability Utilities option. All jobs shall be run in their own process groups. Immediately before the shell issues a prompt after completion of the background job, a message reporting the exit status of the background job shall be written to standard error. If a foreground job stops, the shell shall write a message to standard error to that effect, formatted as described by the jobs utility. In addition, if a job changes status other than exiting (for example, if it stops for input or output or is stopped by a SIGSTOP signal), the shell shall write a similar message immediately prior to writing the next prompt. This option is enabled by default for interactive shells.
- The shell shall read commands but does not execute them; this can be used to check for shell script syntax errors. An interactive shell may ignore this option.
- Write the current settings of the options to standard output in an unspecified format.
- Write the current option settings to standard output in a format that is suitable for reinput to the shell as commands that achieve the same options settings.
- −o option
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:
- Equivalent to −a.
- Equivalent to −e.
- Prevent an interactive shell from exiting on end-of-file. This setting prevents accidental logouts when <control>‐D is entered. A user shall explicitly exit to leave the interactive shell.
- Equivalent to −m. This option is supported if the system supports the User Portability Utilities option.
- Equivalent to −C (uppercase C).
- Equivalent to −f.
- Equivalent to −n.
- Prevent the entry of function definitions into the command history; see Command History List.
- Equivalent to −b.
- Equivalent to −u.
- Equivalent to −v.
- Allow shell command line editing using the built-in vi editor. Enabling vi mode shall disable any other command line editing mode provided as an implementation extension.
It need not be possible to set vi
mode on for certain block-mode terminals.
- Equivalent to −x.
- When the shell tries to expand an unset parameter other than the '@' and '*' special parameters, it shall write a message to standard error and shall not execute the command containing the expansion, but for the purposes of setting the '?' special parameter and the exit status of the shell the command shall be treated as having been executed and returned an exit status of between 1 and 125 inclusive. A non-interactive shell shall immediately exit. An interactive shell shall not exit.
- The shell shall write its input to standard error as it is read.
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.
If the first argument is '−', the results are unspecified.
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.
The standard error shall be used only for diagnostic messages.
The following sections are informative.
Application writers should avoid relying on set −e within functions. For example, in the following script:
- The shell shall write to standard error a trace for each command after it expands the command and before it executes it. It is unspecified whether the command that turns tracing off is traced.
the −e setting is ignored within the function body (because the function is a command in an AND-OR list other than the last). Therefore, if some_server fails, the function carries on to echo "some_serverstartedsuccessfully", and the exit status of the function is zero (which means "some_serverfailed" is not output).
Write out all variables and their values:
echo some_server started successfully
start || echo >&2 some_server failed
Set $1, $2, and $3 and set "$#" to 3:
Turn on the −x and −v options:
Unset all positional parameters:
Set $1 to the value of x, even if it begins with '−' or '+':
Set the positional parameters to the expansion of x, even if x expands with a leading '−' or '+':
The set −− form is listed specifically in the SYNOPSIS even though this usage is implied by the Utility Syntax Guidelines. The explanation of this feature removes any ambiguity about whether the set −− form might be misinterpreted as being equivalent to set without any options or arguments. The functionality of this form has been adopted from the KornShell. In System V, set −− only unsets parameters if there is at least one argument; the only way to unset all parameters is to use shift. Using the KornShell version should not affect System V scripts because there should be no reason to issue it without arguments deliberately; if it were issued as, for example:
and there were in fact no arguments resulting from "$@", 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 Section 2.5.2, Special Parameters.
The description of the −e option is intended to match the behavior of the 1988 version of the KornShell.
The −h flag is related to command name hashing. See hash.
The following set flags were omitted intentionally with the following rationale:
- The −k flag was originally added by the author of the Bourne shell to make it easier for users of pre-release versions of the shell. In early versions of the Bourne shell the construct set name=value had to be used to assign values to shell variables. The problem with −k is that the behavior affects parsing, virtually precluding writing any compilers. To explain the behavior of −k, it is necessary to describe the parsing algorithm, which is implementation-defined. For example:
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:
- The −t flag is hard to specify and almost never used. The only known use could be done with here-documents. Moreover, the behavior with ksh and sh differs. The reference page says that it exits after reading and executing one command. What is one command? If the input is date;date, sh executes both date commands while ksh does only the first.
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:
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.
Historically, some shells applied the −u option to all parameters including $@ and $*. The standard developers felt that this was a misfeature since it is normal and common for $@ and $* to be used in shell scripts regardless of whether they were passed any arguments. Treating these uses as an error when no arguments are passed reduces the value of −u for its intended purpose of finding spelling mistakes in variable names and uses of unset positional parameters.
Section 2.14, Special Built-In Utilities, hash
The Base Definitions volume of POSIX.1‐2008, Section 4.22, Variable Assignment, Section 12.2, Utility Syntax Guidelines
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.unix.org/online.html .
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