bash double bracket issue

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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

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