linux - bash double bracket issue

Question

I'm very new to bash scripting and am running into an issue when using double brackets. I can't seem to get them to work at all in Ubuntu Server 11.10. My script below is in if_test.sh.

#!/bin/bash

if [[ "14"=="14" ]]; then 
    echo "FOO"
fi

When I run this simple shell script the output I get is: if_test.sh: 5: [[: not found

It seems that I'm running GNU bash version 4.2.10 after running bash --version from the terminal. Any help would be greatly appreciated. Thanks!

Answer 1

The problem lies in your script invocation. You're issuing:

$ sudo sh if_test.sh

On Ubuntu systems, /bin/sh is dash, not bash, and dash does not support the double bracket keyword (or didn't at the time of this posting, I haven't double-checked). You can solve your problem by explicitly invoking bash instead:

$ sudo bash if_test.sh

Alternatively, you can make your script executable and rely on the shebang line:

$ chmod +x if_test.sh
$ sudo ./if_test.sh

Also note that, when used between double square brackets, == is a pattern matching operator, not the equality operator. If you want to test for equality, you can either use -eq:

if [[ "14" -eq "14" ]]; then 
    echo "FOO"
fi

Or double parentheses:

if (( 14 == 14 )); then 
    echo "FOO"
fi