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. Note:Shell Substitutions, items
`${VAR:-VALUE}' and `${VAR=VALUE}' for the rationale.