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. See Shell Substitutions, items ‘${var:-value}’ and ‘${var=value}’ for the rationale.