File: autoconf.info,  Node: Assignments,  Next: Parentheses,  Prev: Shell Substitutions,  Up: Portable Shell

11.9 Assignments
================

When setting several variables in a row, be aware that the order of the
evaluation is undefined.  For instance ‘foo=1 foo=2; echo $foo’ gives
‘1’ with Solaris 10 ‘/bin/sh’, but ‘2’ with Bash.  You must use ‘;’ to
enforce the order: ‘foo=1; foo=2; echo $foo’.

   Don't rely on the following to find ‘subdir/program’:

     PATH=subdir$PATH_SEPARATOR$PATH program

as this does not work with Zsh 3.0.6.  Use something like this instead:

     (PATH=subdir$PATH_SEPARATOR$PATH; export PATH; exec program)

   Don't rely on the exit status of an assignment: Ash 0.2 does not
change the status and propagates that of the last statement:

     $ false || foo=bar; echo $?
     1
     $ false || foo=`:`; echo $?
     0

and to make things even worse, QNX 4.25 just sets the exit status to 0
in any case:

     $ foo=`exit 1`; echo $?
     0

   To assign default values, follow this algorithm:

  1. If the default value is a literal and does not contain any closing
     brace, use:

          : "${var='my literal'}"

  2. If the default value contains no closing brace, has to be expanded,
     and the variable being initialized is not intended to be IFS-split
     (i.e., it's not a list), then use:

          : ${var="$default"}

  3. If the default value contains no closing brace, has to be expanded,
     and the variable being initialized is intended to be IFS-split
     (i.e., it's a list), then use:

          var=${var="$default"}

  4. If the default value contains a closing brace, then use:

          test ${var+y} || var="has a '}'"

   In most cases ‘var=${var="$default"}’ is fine, but in case of doubt,
just use the last form.  *Note Shell Substitutions::, items
‘${VAR:-VALUE}’ and ‘${VAR=VALUE}’ for the rationale.