46.5 Stop Syntax Errors in String Tests

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" = X

something

 ]
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.