Node:Assignments, Next:Special Shell Variables, Previous:Shell Substitutions, Up:Portable Shell

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