(autoconf.info)Assignments


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10.7 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 `/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+set}" = set || 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.


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