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?

Solution 1:

[[ is a bash-builtin. Your /bin/bash doesn't seem to be an actual bash.

From a comment:

Add #!/bin/bash at the top of file

Solution 2:

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.

Solution 3:

Is the first line in your script:

#!/bin/bash

or

#!/bin/sh

the sh shell produces this error messages, not bash

Solution 4:

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 =.