File: autoconf.info,  Node: Shell Functions,  Next: Limitations of Builtins,  Prev: Special Shell Variables,  Up: Portable Shell

11.13 Shell Functions
=====================

Nowadays, it is difficult to find a shell that does not support shell
functions at all.  However, some differences should be expected.

   When declaring a shell function, you must include whitespace between
the ‘)’ after the function name and the start of the compound
expression, to avoid upsetting ‘ksh’.  While it is possible to use any
compound command, most scripts use ‘{...}’.

     $ /bin/sh -c 'a(){ echo hi;}; a'
     hi
     $ ksh -c 'a(){ echo hi;}; a'
     ksh: syntax error at line 1: `}' unexpected
     $ ksh -c 'a() { echo hi;}; a'
     hi

   Inside a shell function, you should not rely on the error status of a
subshell if the last command of that subshell was ‘exit’ or ‘trap’, as
this triggers bugs in zsh 4.x; while Autoconf tries to find a shell that
does not exhibit the bug, zsh might be the only shell present on the
user's machine.

   Likewise, the state of ‘$?’ is not reliable when entering a shell
function.  This has the effect that using a function as the first
command in a ‘trap’ handler can cause problems.

     $ bash -c 'foo() { echo $?; }; trap foo 0; (exit 2); exit 2'; echo $?
     2
     2
     $ ash -c 'foo() { echo $?; }; trap foo 0; (exit 2); exit 2'; echo $?
     0
     2

   DJGPP bash 2.04 has a bug in that ‘return’ from a shell function
which also used a command substitution causes a segmentation fault.  To
work around the issue, you can use ‘return’ from a subshell, or
‘AS_SET_STATUS’ as last command in the execution flow of the function
(*note Common Shell Constructs::).

   Not all shells treat shell functions as simple commands impacted by
‘set -e’, for example with Solaris 10 ‘/bin/sh’:

     $ bash -c 'f() { return 1; }; set -e; f; echo oops'
     $ /bin/sh -c 'f() { return 1; }; set -e; f; echo oops'
     oops

   Shell variables and functions may share the same namespace, for
example with Solaris 10 ‘/bin/sh’:

     $ f () { :; }; f=; f
     f: not found

For this reason, Autoconf (actually M4sh, *note Programming in M4sh::)
uses the prefix ‘as_fn_’ for its functions.

   Handling of positional parameters and shell options varies among
shells.  For example, Korn shells reset and restore trace output (‘set
-x’) and other options upon function entry and exit.  Inside a function,
IRIX sh sets ‘$0’ to the function name.

   It is not portable to pass temporary environment variables to shell
functions.  Solaris 10 ‘/bin/sh’ does not see the variable.  Meanwhile,
not all shells follow the Posix rule that the assignment must affect the
current environment in the same manner as special built-ins.

     $ /bin/sh -c 'func() { echo $a;}; a=1 func; echo $a'
     ⇒
     ⇒
     $ ash -c 'func() { echo $a;}; a=1 func; echo $a'
     ⇒1
     ⇒
     $ bash -c 'set -o posix; func() { echo $a;}; a=1 func; echo $a'
     ⇒1
     ⇒1

   Some ancient Bourne shell variants with function support did not
reset ‘$I, I >= 0’, upon function exit, so effectively the arguments of
the script were lost after the first function invocation.  It is
probably not worth worrying about these shells any more.

   With AIX sh, a ‘trap’ on 0 installed in a shell function triggers at
function exit rather than at script exit.  *Note Limitations of Shell
Builtins: trap.