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Limitations of Shell Builtins

No, no, we are serious: some shells do have limitations! :)

You should always keep in mind that any built-in or command may support options, and therefore have a very different behavior with arguments starting with a dash. For instance, the innocent echo "$word" can give unexpected results when word starts with a dash. It is often possible to avoid this problem using echo "x$word", taking the x into account later in the pipe.

Use . only with regular files (use test -f). Bash 2.03, for instance, chokes on . /dev/null. Also, remember that . uses PATH if its argument contains no slashes, so if you want to use . on a file foo in the current directory, you must use . ./foo.
You can't use !, you'll have to rewrite your code.
The use of break 2, etcetera, is safe.
You don't need to quote the argument; no splitting is performed.

You don't need the final ;;, but you should use it.

Because of a bug in its fnmatch, bash fails to properly handle backslashes in character classes:

bash-2.02$ case /tmp in [/\\]*) echo OK;; esac

This is extremely unfortunate, since you are likely to use this code to handle UNIX or MS-DOS absolute paths. To work around this bug, always put the backslash first:

bash-2.02$ case '\TMP' in [\\/]*) echo OK;; esac
bash-2.02$ case /tmp in [\\/]*) echo OK;; esac

Some shells, such as Ash 0.3.8, are confused by empty case/esac:

ash-0.3.8 $ case foo in esac;
error-->Syntax error: ";" unexpected (expecting ")")

Many shells still do not support parenthesized cases, which is a pity for those of us using tools that rely on balanced parentheses. For instance, Solaris 2.8's Bourne shell:

$ case foo in (foo) echo foo;; esac
error-->syntax error: `(' unexpected

The simple echo is probably the most surprising source of portability troubles. It is not possible to use echo portably unless both options and escape sequences are omitted. New applications which are not aiming at portability should use printf instead of echo.

Don't expect any option. See Preset Output Variables, ECHO_N etc. for a means to simulate -c.

Do not use backslashes in the arguments, as there is no consensus on their handling. On echo '\n' | wc -l, the sh of Digital Unix 4.0, MIPS RISC/OS 4.52, answer 2, but the Solaris' sh, Bash and Zsh (in sh emulation mode) report 1. Please note that the problem is truly echo: all the shells understand '\n' as the string composed of a backslash and an n.

Because of these problems, do not pass a string containing arbitrary characters to echo. For example, echo "$foo" is safe if you know that foo's value cannot contain backslashes and cannot start with -, but otherwise you should use a here-document like this:

cat <<EOF

The default value of exit is supposed to be $?; unfortunately, some shells, such as the DJGPP port of Bash 2.04, just perform exit 0.
bash-2.04$ foo=`exit 1` || echo fail
bash-2.04$ foo=`(exit 1)` || echo fail
bash-2.04$ foo=`(exit 1); exit` || echo fail

Using exit $? restores the expected behavior.

Some shell scripts, such as those generated by autoconf, use a trap to clean up before exiting. If the last shell command exited with nonzero status, the trap also exits with nonzero status so that the invoker can tell that an error occurred.

Unfortunately, in some shells, such as Solaris 8 sh, an exit trap ignores the exit command's status. In these shells, a trap cannot determine whether it was invoked by plain exit or by exit 1. Instead of calling exit directly, use the AC_MSG_ERROR macro that has a workaround for this problem.

The builtin export dubs environment variable a shell variable. Each update of exported variables corresponds to an update of the environment variables. Conversely, each environment variable received by the shell when it is launched should be imported as a shell variable marked as exported.

Alas, many shells, such as Solaris 2.5, IRIX 6.3, IRIX 5.2, AIX 4.1.5 and DU 4.0, forget to export the environment variables they receive. As a result, two variables are coexisting: the environment variable and the shell variable. The following code demonstrates this failure:

#! /bin/sh
echo $FOO
echo $FOO
exec /bin/sh $0

when run with FOO=foo in the environment, these shells will print alternately foo and bar, although it should only print foo and then a sequence of bars.

Therefore you should export again each environment variable that you update.

Don't expect false to exit with status 1: in the native Bourne shell of Solaris 8, it exits with status 255.
To loop over positional arguments, use:
for arg
  echo "$arg"

You may not leave the do on the same line as for, since some shells improperly grok:

for arg; do
  echo "$arg"

If you want to explicitly refer to the positional arguments, given the $@ bug (see Shell Substitutions), use:

for arg in ${1+"$@"}; do
  echo "$arg"

Using ! is not portable. Instead of:
if ! cmp -s file; then
  mv file


if cmp -s file; then :; else
  mv file

There are shells that do not reset the exit status from an if:

$ if (exit 42); then true; fi; echo $?

whereas a proper shell should have printed 0. This is especially bad in Makefiles since it produces false failures. This is why properly written Makefiles, such as Automake's, have such hairy constructs:

if test -f "$file"; then
  install "$file" "$dest"

This builtin faces the usual problem with arguments starting with a dash. Modern shells such as Bash or Zsh understand -- to specify the end of the options (any argument after -- is a parameters, even -x for instance), but most shells simply stop the option processing as soon as a non-option argument is found. Therefore, use dummy or simply x to end the option processing, and use shift to pop it out:
set x $my_list; shift

Not only is shifting a bad idea when there is nothing left to shift, but in addition it is not portable: the shell of MIPS RISC/OS 4.52 refuses to do it.
This command is not portable, as POSIX does not require it; use . instead.
The test program is the way to perform many file and string tests. It is often invoked by the alternate name [, but using that name in Autoconf code is asking for trouble since it is an M4 quote character.

If you need to make multiple checks using test, combine them with the shell operators && and || instead of using the test operators -a and -o. On System V, the precedence of -a and -o is wrong relative to the unary operators; consequently, POSIX does not specify them, so using them is nonportable. If you combine && and || in the same statement, keep in mind that they have equal precedence.

You may use ! with test, but not with if: test ! -r foo || exit 1.

test (files)
To enable configure scripts to support cross-compilation, they shouldn't do anything that tests features of the build system instead of the host system. But occasionally you may find it necessary to check whether some arbitrary file exists. To do so, use test -f or test -r. Do not use test -x, because 4.3BSD does not have it. Do not use test -e either, because Solaris 2.5 does not have it.
test (strings)
Avoid test "string", in particular if string might start with a dash, since test might interpret its argument as an option (e.g., string = "-n").

Contrary to a common belief, test -n string and test -z string are portable, nevertheless many shells (such as Solaris 2.5, AIX 3.2, UNICOS, Digital Unix 4 etc.) have bizarre precedence and may be confused if string looks like an operator:

$ test -n =
test: argument expected

If there are risks, use test "xstring" = x or test "xstring" != x instead.

It is frequent to find variations of the following idiom:

test -n "`echo $ac_feature | sed 's/[-a-zA-Z0-9_]//g'`" &&

to take an action when a token matches a given pattern. Such constructs should always be avoided by using:

echo "$ac_feature" | grep '[^-a-zA-Z0-9_]' >/dev/null 2>&1 &&

Use case where possible since it is faster, being a shell builtin:

case $ac_feature in
  *[!-a-zA-Z0-9_]*) action;;

Alas, negated character classes are probably not portable, although no shell is known to not support the POSIX.2 syntax [!...] (when in interactive mode, zsh is confused by the [!...] syntax and looks for an event in its history because of !). Many shells do not support the alternative syntax [^...] (Solaris, Digital Unix, etc.).

One solution can be:

expr "$ac_feature" : '.*[^-a-zA-Z0-9_]' >/dev/null &&

or better yet

expr "x$ac_feature" : '.*[^-a-zA-Z0-9_]' >/dev/null &&

expr "Xfoo" : "Xbar" is more robust than echo "Xfoo" | grep "^Xbar", because it avoids problems when foo contains backslashes.

It is safe to trap at least the signals 1, 2, 13 and 15. You can also trap 0, i.e., have the trap run when the script ends (either via an explicit exit, or the end of the script).

Although POSIX is not absolutely clear on this point, it is widely admitted that when entering the trap $? should be set to the exit status of the last command run before the trap. The ambiguity can be summarized as: "when the trap is launched by an exit, what is the last command run: that before exit, or exit itself?"

Bash considers exit to be the last command, while Zsh and Solaris 8 sh consider that when the trap is run it is still in the exit, hence it is the previous exit status that the trap receives:

$ cat
trap 'echo $?' 0
(exit 42); exit 0
$ zsh
$ bash

The portable solution is then simple: when you want to exit 42, run (exit 42); exit 42, the first exit being used to set the exit status to 42 for Zsh, and the second to trigger the trap and pass 42 as exit status for Bash.

The shell in FreeBSD 4.0 has the following bug: $? is reset to 0 by empty lines if the code is inside trap.

$ trap 'false

echo $?' 0
$ exit

Fortunately, this bug only affects trap.

Don't worry: as far as we know true is portable. Nevertheless, it's not always a builtin (e.g., Bash 1.x), and the portable shell community tends to prefer using :. This has a funny side effect: when asked whether false is more portable than true Alexandre Oliva answered:
In a sense, yes, because if it doesn't exist, the shell will produce an exit status of failure, which is correct for false, but not for true.

You cannot assume the support of unset, nevertheless, because it is extremely useful to disable embarrassing variables such as CDPATH, you can test for its existence and use it provided you give a neutralizing value when unset is not supported:
if (unset FOO) >/dev/null 2>&1; then
$unset CDPATH || CDPATH=:

See Special Shell Variables, for some neutralizing values. Also, see Limitations of Builtins, documentation of export, for the case of environment variables.