Examples of 'if' and test in bash


I have a hard time keeping the various syntaxes used for testing variales used in shell scripting. You'd think that with all the books available on shell scripting, ONE of them would have a chapter or appendix that just listed a bunch of examples of when to use [[]] instead of [] or (()), and so on. If there is such a book, I haven't found it. So, this page is my own little set of examples so that I can quickly lookup what syntax to use depending on what exactly I'm testing.

Some preface is in order:

The syntax used in a script would be

if [[ condition ]]; then

but I've left out the 'if', the semicolon and the 'then' just for brevity. For each example given below, if the example works for a shell, that shell is listed in green and if it doesn't work for a shell, the shell name is showin in red.

String comparison

Summary: Use double brackets [[]] and single equal sign =

For the examples below, we'll define x=hello4you. Let's start with string comparison which uses = and ==. (This always seems backwards to me, to use math signs for strings and letters like -eq for number comparison.)

  • [[ $x = hello4you ]]
    • bash ksh
    This is the preferred syntax and is shown in the O'Reilly books.

    Negative: [[ $x != hello4you ]]

  • [ $x = hello4you ]
    • bash ksh
    This works in bash and ksh and is the syntax I've seen in many scripts--grep for 'if' in /etc/init.d/ or /sbin/init.d/ and you'll see most scripts use single brackets. But since it won't handle wildcards (see below) and it's not the syntax given in books, I'm not sure why its use is so common.

  • [[ $x == hello4you ]]
    • bash ksh
    This works in bash but fails in ksh. ksh doesn't like the == although the QA book shows this exact syntax as a working example for ksh. Using single or double brackets makes no difference, nor does quoting. It does seem odd that = works and == does not since we're evaluating $x, not setting it.

    Wildcards

  • [[ $x = h*you ]]
    • bash ksh
    This works in bash and ksh; wildcards REQUIRE [[]], and fail with []. The bottom line is use double bracket and single equal sign and you'll be okay in all cases. Wildcards don't work inside quotes; for example, if x="John Brown", then

    [[ $x = John* ]]

    will work, but

    [[ $x = "John*" ]] will not.

    Negative: [[ $x != *youX ]]
    The negative example will be true because of the X at the end of the string which means $x does not match it.

    Number comparison (not calculation)

    Summary: Use (()) and == for simple true/false test

    For these examples, we set x=57. O'Reilly says [[]] will also work, but that (()) is "considerably more efficient", so both are shown here.

  • (( $x == 57 ))
    • bash ksh
    This works in bash and ksh. Note the use of the double-equal sign. This is probably where confusion is created as to whether single or double equals should be used, since [[]] uses single and (()) uses double on ksh. Also note that we're not doing any calculations here; for that the $(()) must be used (see below). The difference is that (()) just produces a result code, 0 if true, 1 if not, whereas $(()) produces a 'textual result' (the answer). O'Reilly calls these "arithmetic conditionals".

    Negative: (( $x != 58 ))

  • [[ $x -eq 57 ]]
    • bash ksh
    This works in bash and ksh. It produces just a result code of 0 or 1, similar to (()). O'Reilly calls these "integer conditionals".

    Negative: [[ $x -ne 58 ]]

  • [ $x = 57 ]
    • bash ksh
    This works in bash and ksh but 57 is being treated as a string not a number, which probably isn't what you want.

  • [ $x -gt 47 ]
    • bash ksh
    Single brackets works in bash and ksh. This is just an example showing how numbers can be compared.

    File tests

    For these examples, we set x to a valid file name.

    02/28/2006