Starting Bash with the --posix command-line option or executing ‘set -o posix’ while Bash is running will cause Bash to conform more closely to the POSIX standard by changing the behavior to match that specified by POSIX in areas where the Bash default differs.
When invoked as
sh, Bash enters POSIX mode after reading the
The following list is what’s changed when ‘POSIX mode’ is in effect:
POSIXLY_CORRECT variable is set.
$PATH to find the new location. This is also available with
‘shopt -s checkhash’.
PS2 expansions of ‘!’ to
the history number and ‘!!’ to ‘!’ are enabled,
and parameter expansion is performed on the values of
PS2 regardless of the setting of the
$ENV) rather than
the normal Bash files.
names. That is, they may not
contain characters other than letters, digits, and underscores, and
may not start with a digit. Declaring a function with an invalid name
causes a fatal syntax error in non-interactive shells.
type), Bash does
not print the
PATH variable are not expanded as described above
under Tilde Expansion.
time reserved word may be used by itself as a command. When
used in this way, it displays timing statistics for the shell and its
completed children. The
TIMEFORMAT variable controls the format
of the timing information.
time as a reserved word if the next
token begins with a ‘-’.
histexpand option is enabled.
for statement or the selection variable in a
select statement is a readonly variable.
is not found.
source builtins, or in a string processed by
$* as if it were
command builtin does not prevent builtins that take assignment
statements as arguments from expanding them as assignment statements;
when not in POSIX mode, assignment builtins lose their assignment
statement expansion properties when preceded by
bg builtin uses the required format to describe each job placed
in the background, which does not include an indication of whether the job
is the current or previous job.
kill builtin does not accept signal names with a ‘SIG’
readonly builtin commands display their
output in the format required by POSIX.
trap builtin displays signal names without the leading
trap builtin doesn’t check the first argument for a possible
signal specification and revert the signal handling to the original
disposition if it is, unless that argument consists solely of digits and
is a valid signal number. If users want to reset the handler for a given
signal to the original disposition, they should use ‘-’ as the
trap -p displays signals whose dispositions are set to SIG_DFL and
those that were ignored when the shell started.
source builtins do not search the current directory
for the filename argument if it is not found by searching
inherit_errexit option, so
subshells spawned to execute command substitutions inherit the value of
the -e option from the parent shell.
inherit_errexit option is not enabled,
Bash clears the -e option in such subshells.
shift_verbose option, so numeric arguments to
that exceed the number of positional parameters will result in an
alias builtin displays alias definitions, it does not
display them with a leading ‘alias ’ unless the -p option
set builtin is invoked without options, it does not display
shell function names and definitions.
set builtin is invoked without options, it displays
variable values without quotes, unless they contain shell metacharacters,
even if the result contains nonprinting characters.
cd builtin is invoked in logical mode, and the pathname
$PWD and the directory name supplied as an argument
does not refer to an existing directory,
cd will fail instead of
falling back to physical mode.
cd builtin cannot change a directory because the
length of the pathname
$PWD and the directory name supplied as an argument
PATH_MAX when all symbolic links are expanded,
fail instead of attempting to use only the supplied directory name.
pwd builtin verifies that the value it prints is the same as the
current directory, even if it is not asked to check the file system with the
fc builtin does not include an
indication of whether or not a history entry has been modified.
command builtins will not report a non-executable
file as having been found, though the shell will attempt to execute such a
file if it is the only so-named file found in
vi editing mode will invoke the
vi editor directly when
the ‘v’ command is run, instead of checking
xpg_echo option is enabled, Bash does not attempt to interpret
any arguments to
echo as options. Each argument is displayed, after
escape characters are converted.
ulimit builtin uses a block size of 512 bytes for the -c
and -f options.
SIGCHLD when a trap is set on
not interrupt the
wait builtin and cause it to return immediately.
The trap command is run once for each child that exits.
read builtin may be interrupted by a signal for which a trap
has been set.
If Bash receives a trapped signal while executing
read, the trap
handler executes and
read returns an exit status greater than 128.
printf builtin uses
strtod) to convert
arguments corresponding to floating point conversion specifiers, instead of
long double if it’s available. The ‘L’ length modifier forces
printf to use
long double if it’s available.
wait builtin is used to obtain it.
There is other POSIX behavior that Bash does not implement by default even when in POSIX mode. Specifically:
fc builtin checks
$EDITOR as a program to edit history
FCEDIT is unset, rather than defaulting directly to
EDITOR is unset.
xpg_echo option to be enabled for
echo builtin to be fully conformant.
Bash can be configured to be POSIX-conformant by default, by specifying
the --enable-strict-posix-default to
configure when building
(see Optional Features).