Stop Syntax Errors in String Tests (Unix Power Tools, 3rd Edition) 37.4. Stop Syntax Errors in String Tests Using the test or [ (square bracket) command (Section 35.26) 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 Bourne shell 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} (Section 36.7) with just X and the first part of the case succeeds. Otherwise the default case, *), is used. See also Section 37.3 for a brief example of bash parameter expansion and dealing with unset or null values by reporting an error or by assigning default values. -- JP 37.3. Stop Syntax Errors in Numeric Tests37.5. Quoting and Command-Line Parameters Copyright © 2003 O'Reilly & Associates. All rights reserved.