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
startup files.
The following list is what's changed when `posix mode' is in effect:
SIGTSTP
.
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.
promptvars
option.
kill
builtin does not accept signal names with a ‘SIG’
prefix.
.
filename
is not found.
name
s. 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.
cd
builtin will not implicitly
append the current directory to it. This means that cd
will
fail if no valid directory name can be constructed from
any of the entries in $CDPATH, even if the a directory with
the same name as the name given as an argument to cd
exists
in the current directory.
for
statement or the selection variable in a
select
statement is a readonly variable.
export
and readonly
builtin commands display their
output in the format required by posix.
trap
builtin displays signal names without the leading
SIG
.
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
first argument.
.
and source
builtins do not search the current directory
for the filename argument if it is not found by searching PATH.
alias
builtin displays alias definitions, it does not
display them with a leading ‘alias ’ unless the -p option
is supplied.
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
constructed from $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.
pwd
builtin is supplied the -P option, it resets
$PWD
to a pathname containing no symlinks.
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
-P option.
fc
builtin does not include an
indication of whether or not a history entry has been modified.
fc
is ed
.
type
and 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 $PATH
.
vi
editing mode will invoke the vi
editor directly when
the ‘v’ command is run, instead of checking $VISUAL
and
$EDITOR
.
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 SIGCHLD
does
not interrupt the wait
builtin and cause it to return immediately.
The trap command is run once for each child that exits.
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
entries if FCEDIT
is unset, rather than defaulting directly to
ed
. fc
uses ed
if EDITOR
is unset.
xpg_echo
option to be enabled for
the 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).