SET(1P) POSIX Programmer's Manual SET(1P)PROLOG
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NAMEset — set or unset options and positional parameters
SYNOPSISset [−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:
"%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
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 speci‐
Implementations shall support the options in the following list in both
their <hyphen> and <plus-sign> forms. These options can also be speci‐
fied 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 com‐
mand, the export attribute shall not persist in the current exe‐
cution environment after the utility completes, with the excep‐
tion 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
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 prefix‐
ing 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 220.127.116.11, Exam‐
ples. 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, Redi‐
recting 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 immedi‐
ately 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 sub‐
shell 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 sub‐
shell 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 func‐
tions 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.
This option is supported if the system supports the User Porta‐
bility Utilities option. It shall set various options, many of
which shall be equivalent to the single option letters. The fol‐
lowing 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 sys‐
tem 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 com‐
mand 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 com‐
mand line editing mode provided as an implementation
It need not be possible to set vi mode on for certain
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 stan‐
dard 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 imme‐
diately 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 num‐
ber 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
See the DESCRIPTION.
See the DESCRIPTION.
See the DESCRIPTION.
The standard error shall be used only for diagnostic messages.
CONSEQUENCES OF ERRORS
The following sections are informative.
Application writers should avoid relying on set −e within functions.
For example, in the following script:
echo some_server started successfully
start || echo >&2 some_server failed
the −e setting is ignored within the function body (because the func‐
tion is a command in an AND-OR list other than the last). Therefore, if
some_server fails, the function carries on to echo "some_serverstarted‐
successfully", and the exit status of the function is zero (which means
"some_serverfailed" is not output).
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 −− $x
The set −− form is listed specifically in the SYNOPSIS even though this
usage is implied by the Utility Syntax Guidelines. The explanation of
this feature removes any ambiguity about whether the set −− form might
be misinterpreted as being equivalent to set without any options or
arguments. The functionality of this form has been adopted from the
KornShell. In System V, set −− only unsets parameters if there is at
least one argument; the only way to unset all parameters is to use
shift. Using the KornShell version should not affect System V scripts
because there should be no reason to issue it without arguments delib‐
erately; if it were issued as, for example:
set −− "$@"
and there were in fact no arguments resulting from "$@", unsetting the
parameters would have no result.
The set + form in early proposals was omitted as being an unnecessary
duplication of set alone and not widespread historical practice.
The noclobber option was changed to allow set −C as well as the set −o
noclobber option. The single-letter version was added so that the his‐
torical "$−" paradigm would not be broken; see Section 2.5.2, Special
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
−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, virtu‐
ally 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
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 behav‐
ior with ksh and sh differs. The reference page says that it
exits after reading and executing one command. What is one com‐
mand? 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 syn‐
tax. 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
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
trap "jobs −n" CLD
The C shell provides two different levels of an asynchronous notifica‐
tion 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 Korn‐
Shell extensions to trap, nor the C shell notify environment variable
seemed appropriate (notify is not a proper POSIX environment variable
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 mis‐
feature 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 Electri‐
cal 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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