[bash] String comparison in bash. [[: not found

I am trying to compare strings in bash. I already found an answer on how to do it on stackoverflow. In script I am trying, I am using the code submitted by Adam in the mentioned question:

#!/bin/bash
string='My string';

if [[ "$string" == *My* ]]
then
  echo "It's there!";
fi

needle='y s'
if [[ "$string" == *"$needle"* ]]; then
  echo "haystack '$string' contains needle '$needle'"
fi

I also tried approach from ubuntuforums that you can find in 2nd post

if [[ $var =~ regexp ]]; then
  #do something
fi

In both cases I receive error:

[[: not found

What am I doing wrong?

I had this problem when installing Heroku Toolbelt

This is how I solved the problem

$ ls -l /bin/sh
lrwxrwxrwx 1 root root 4 ago 15  2012 /bin/sh -> dash

As you can see, /bin/sh is a link to "dash" (not bash), and [[ is bash syntactic sugarness. So I just replaced the link to /bin/bash. Careful using rm like this in your system!

$ sudo rm /bin/sh
$ sudo ln -s /bin/bash /bin/sh

How you are running your script? If you did with

$ sh myscript

you should try:

$ bash myscript

or, if the script is executable:

$ ./myscript

sh and bash are two different shells. While in the first case you are passing your script as an argument to the sh interpreter, in the second case you decide on the very first line which interpreter will be used.

Specify bash instead of sh when running the script. I personally noticed they are different under ubuntu 12.10:

bash script.sh arg0 ... argn

As @Ansgar mentioned, [[ is a bashism, ie built into Bash and not available for other shells. If you want your script to be portable, use [. Comparisons will also need a different syntax: change == to =.

If you know you're on bash, and still get this error, make sure you write the if with spaces.

[[1==1]] # This outputs error

[[ 1==1 ]] # OK

Is the first line in your script:

#!/bin/bash

or

#!/bin/sh

the sh shell produces this error messages, not bash