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_CORRECTvariable is set.
$PATHto find the new location. This is also available with ‘shopt -s checkhash’.
PS2expansions of ‘!’ to the history number and ‘!!’ to ‘!’ are enabled, and parameter expansion is performed on the values of
PS2regardless 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
PATHvariable are not expanded as described above under Tilde Expansion.
timereserved 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
TIMEFORMATvariable controls the format of the timing information.
timeas a reserved word if the next token begins with a ‘-’.
histexpandoption is enabled.
forstatement or the selection variable in a
selectstatement is a readonly variable.
.filename is not found.
sourcebuiltins, or in a string processed by the
$*as if it were double-quoted.
commandbuiltin 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
bgbuiltin 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.
killbuiltin does not accept signal names with a ‘SIG’ prefix.
readonlybuiltin commands display their output in the format required by POSIX.
trapbuiltin displays signal names without the leading
trapbuiltin 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.
trap -pdisplays signals whose dispositions are set to SIG_DFL and those that were ignored when the shell started.
sourcebuiltins do not search the current directory for the filename argument if it is not found by searching
inherit_errexitoption, so subshells spawned to execute command substitutions inherit the value of the -e option from the parent shell. When the
inherit_errexitoption is not enabled, Bash clears the -e option in such subshells.
shift_verboseoption, so numeric arguments to
shiftthat exceed the number of positional parameters will result in an error message.
aliasbuiltin displays alias definitions, it does not display them with a leading ‘alias ’ unless the -p option is supplied.
setbuiltin is invoked without options, it does not display shell function names and definitions.
setbuiltin is invoked without options, it displays variable values without quotes, unless they contain shell metacharacters, even if the result contains nonprinting characters.
cdbuiltin is invoked in logical mode, and the pathname constructed from
$PWDand the directory name supplied as an argument does not refer to an existing directory,
cdwill fail instead of falling back to physical mode.
cdbuiltin cannot change a directory because the length of the pathname constructed from
$PWDand the directory name supplied as an argument exceeds PATH_MAX when all symbolic links are expanded,
cdwill fail instead of attempting to use only the supplied directory name.
pwdbuiltin 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.
fcbuiltin does not include an indication of whether or not a history entry has been modified.
commandbuiltins 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
viediting mode will invoke the
vieditor directly when the ‘v’ command is run, instead of checking
xpg_echooption is enabled, Bash does not attempt to interpret any arguments to
echoas options. Each argument is displayed, after escape characters are converted.
ulimitbuiltin uses a block size of 512 bytes for the -c and -f options.
SIGCHLDwhen a trap is set on
SIGCHLDdoes not interrupt the
waitbuiltin and cause it to return immediately. The trap command is run once for each child that exits.
readbuiltin 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
readreturns an exit status greater than 128.
waitbuiltin is used to obtain it.
There is other POSIX behavior that Bash does not implement by default even when in POSIX mode. Specifically:
$EDITORas a program to edit history entries if
FCEDITis unset, rather than defaulting directly to
xpg_echooption to be enabled for the
echobuiltin 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).