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SET(1P) POSIX Programmer's Manual SET(1P)
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...]
set −o
set +o
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 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.
−a 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.
−b 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:
<current> 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.
<job-number>
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 '%'.
<status> Unspecified.
<job-name> Unspecified.
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 2.9.3.1,
Examples. Asynchronous notification shall not be enabled by
default.
−C (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.
−e 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:
1. 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.
2. 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.
3. 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
the false 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 is zero.
−f The shell shall disable pathname expansion.
−h Locate and remember utilities invoked by functions as those
functions are defined (the utilities are normally located when
the function is executed).
−m 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.
−n 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.
−o Write the current settings of the options to standard output in
an unspecified format.
+o 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:
allexport Equivalent to −a.
errexit Equivalent to −e.
ignoreeof 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.
monitor Equivalent to −m. This option is supported if the
system supports the User Portability Utilities
option.
noclobber Equivalent to −C (uppercase C).
noglob Equivalent to −f.
noexec Equivalent to −n.
nolog Prevent the entry of function definitions into the
command history; see Command History List.
notify Equivalent to −b.
nounset Equivalent to −u.
verbose Equivalent to −v.
vi 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.
xtrace Equivalent to −x.
−u 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.
−v The shell shall write its input to standard error as it is
read.
−x 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 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.
See the DESCRIPTION.
See the DESCRIPTION.
Not used.
None.
None.
Default.
See the DESCRIPTION.
The standard error shall be used only for diagnostic messages.
None.
None.
Zero.
Default.
The following sections are informative.
Application writers should avoid relying on set −e within functions.
For example, in the following script:
set -e
start() {
some_server
echo some_server started successfully
}
start || echo >&2 some_server failed
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:
set
Set $1, $2, and $3 and set "$#" to 3:
set c a b
Turn on the −x and −v options:
set −xv
Unset all positional parameters:
set −−
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 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(1p).
The following set flags were omitted intentionally with the following
rationale:
−k 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:
set −k; echo name=value
and:
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.
−t 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.
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.
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.
None.
Section 2.14, Special Built-In Utilities, hash(1p)
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 .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 SET(1P)
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