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AgileSoul
Asked: 2022-04-27 10:47:55 +0800 CST 2022-04-27 10:47:55 +0800 CST

What is the difference between using = or == in a text comparison in Bash? Which one should you use?

  • 772

These two codes work:

if [ $animal = "perro" ]; then
    echo "El animal es un perro"
else
    echo "Se desconoce el animal"
fi

Y:

if [ $animal == "perro" ]; then
    ...

In one I used =and in the other ==to do a comparison in a Bash script.

Which one should you use? The normal equal ' = ' or the double equal ' == ' ?

bash

2 Answers

  1. If you're using Bash, both are valid: POSIX has it defined =as the standard. Bash follows this standard, but also incorporates ==.

    So if you want to use the most versatile code possible, use =. If instead you know that it will only run in Bash, you can use ==.

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  2. Best Answer

    Emphasizing @fedorqui's answer about == : It's a bashism .

    That is to say, it is a slang that is very typical of bash and of another shell that wants to adopt it; but there is no consensus . It is not in the POSIX standard .

    Which says that it may not be compatible between shells that follow the POSIX standard .

    Complementing the answer of @fedorqui I put an example.

    Let's use bash, zsh, or tcsh:

    $ bash -c '[ "a" == "a" ] && echo si || echo no'
si

    Everything looks good.

    Now let's use another shell like dash:

    $ dash -c '[ "a" == "a" ] && echo si || echo no'
dash: 1: [: a: unexpected operator
no

    This happens because even though bash supports the == token , it doesn't dash(which is usually the default alias of sh!).

    And this does not stop. Let's continue with the exotic shellfish :

    $ fish -c '[ "a" == "a" ] && echo si || echo no'
[: unexpected argument at index 2: '=='
Standard input (line 1):
[ "a" == "a" ] && echo si || echo no
^
no

    And we have an error.

    Although it is very exotic anyway fish, ie, little POSIX.

    Lastly, let's try ash:

    $ ash -c '[ "a" == "a" ] && echo si || echo no'
ash: 1: [: a: unexpected operator
no

    As we could see, we use 3 shells where the == token is not recognized.

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