Bash Reference Manual. Node: Bash POSIX Mode
The Restricted Shell
5.14: Bash POSIX Mode
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.2 standard by changing the behavior to match that
specified by POSIX.2 in areas where the Bash default differs.
The following list is what's changed when `POSIX mode' is in effect:
When a command in the hash table no longer exists, Bash will re-search
$PATH to find the new location. This is also available with
shopt -s checkhash'.
>&' redirection does not redirect stdout and stderr.
The message printed by the job control code and builtins when a job
exits with a non-zero status is `Done(status)'.
Reserved words may not be aliased.
PS2 expansions of `
the history number and `
!!' to `
!' are enabled,
and parameter expansion is performed on the values of
PS2 regardless of the setting of the
Interactive comments are enabled by default. (Bash has them on by
The POSIX.2 startup files are executed (
$ENV) rather than
the normal Bash files.
Tilde expansion is only performed on assignments preceding a command
name, rather than on all assignment statements on the line.
The default history file is `
~/.sh_history' (this is the
default value of
The output of `
kill -l' prints all the signal names on a single line,
separated by spaces.
Non-interactive shells exit if filename in
is not found.
Non-interactive shells exit if a syntax error in an arithmetic expansion
results in an invalid expression.
Redirection operators do not perform filename expansion on the word
in the redirection unless the shell is interactive.
Function names must be valid shell
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.
POSIX.2 `special' builtins are found before shell functions
during command lookup.
If a POSIX.2 special builtin returns an error status, a
non-interactive shell exits. The fatal errors are those listed in
the POSIX.2 standard, and include things like passing incorrect options,
redirection errors, variable assignment errors for assignments preceding
the command name, and so on.
cd builtin finds a directory to change to
value it assigns to the
PWD variable does not contain any
symbolic links, as if `
cd -P' had been executed.
$CDPATH is set, the
cd builtin will not implicitly
append the current directory to it. This means that
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
in the current directory.
A non-interactive shell exits with an error status if a variable
assignment error occurs when no command name follows the assignment
A variable assignment error occurs, for example, when trying to assign
a value to a readonly variable.
A non-interactive shell exits with an error status if the iteration
variable in a
for statement or the selection variable in a
select statement is a readonly variable.
Process substitution is not available.
Assignment statements preceding POSIX.2 special builtins
persist in the shell environment after the builtin completes.
readonly builtin commands display their
output in the format required by POSIX.2.
There is other POSIX.2 behavior that Bash does not implement.
The Restricted Shell
Assignment statements affect the execution environment of all
builtins, not just special ones.