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

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