10.6 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 sh on Solaris, 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 will never 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 will 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 "${var+set}" = set || var='${indirection}'

In most cases `var=${var="$default"}' is fine, but in case of doubt, just use the latter. See section 10.5 Shell Substitutions, items `${var:-value}' and `${var=value}' for the rationale.