Using the
test
or
[
(square bracket) command (
44.20
)
for a string test can cause errors if the variable starts with a dash (
-
). For example:
if [ "$var" =something
] then ...
If
$var
starts with
-r
, the
test
command may think that you want to test for a readable file.
One common fix (that doesn't always work; see below) is to put an extra character at the start of each side of the test. This means the first argument will never start with a dash; it won't look like an option:
if [ "X$var" = Xsomething
] then ...
That trick doesn't work if you want the test to fail when the variable is empty or not set. Here's a test that handles empty variables:
case "${var+X}" in X) ...do this if variable is set... ;; *) ...do this if variable is not set... ;; esac
If
$var
is set (even if it has an empty string), the shell replaces
${var+X}
(
45.12
)
with just
X
and the first part of the
case
succeeds. Otherwise the default case,
*)
, is used.
-