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FrEqDe
Asked: 2020-02-17 21:13:07 +0800 CST 2020-02-17 21:13:07 +0800 CST

How do you compare strings in Bash?

  • 772

I'm using Bash on Linux and found an example where I compared strings this way but it doesn't seem to work.

In this case, $ait is what the user types into the console.

#!/bin/bash
a=$1
if [ "${a}"=="static" ]; then
(instrucciones)
else
(instrucciones)
fi
linux

3 Answers

  1. Best Answer

    You can do help testfor more information:

    Empty string:

    if [ -z "$VAR" ]; then ...

    equal strings

    if [ "$VAR" = "CADENA" ]; then ...

    different chains

    if [ "$VAR" != "CADENA" ]; then ...
    • 7

  2. In Bash you can compare strings using the syntax:

    [[ $variable == "valor" ]]

    Note that you $variabledon't need to use double quotes here.

    Here, as well [[as ==Bash's own features (what we call Bashisms ). The standard (that is, what POSIX defines) is to use [and =:

    [ "$variable" = "valor" ]

    In which case the variable must be enclosed in quotes to protect us from word splitting .

    Note that the syntax is of the form:

    if [ condiciones ]
# ^ ^           ^

    That is, the spaces around [are necessary, since it [is a command in itself. This command is called testand therefore you can use [and testinterchangeably, as well as search for its instructions by doing man test(as indicated by Trauma in his answer):

    $ v="hola"
$ if test "$v" = "hola"; then echo "si"; fi
si
$ if [ "$v" = "hola" ]; then echo "si"; fi
si

    Additional explanation regarding the use of double quotes within [and [[.

    When we use [, the variables are expanded causing errors if we don't put double quotes:

    $ v="hola que tal"

    With [and quotes, the expansion is correct:

    $ if [ "$v" = "hola que tal" ]; then echo "si"; fi
si

    If we remove the quotes, it gives an error:

    $ if [ $v = "hola que tal" ]; then echo "si"; fi
bash: [: too many arguments

    Well, it's expanding to:

    $ if [ hola que tal = "hola que tal" ]; then echo "si"; fi

    giving 5 arguments to testinstead of the maximum 3 accepted.

    On the other hand, with Bashism [[you can omit the quotes:

    $ if [[ $v = "hola que tal" ]]; then echo "si"; fi
si
    • 7

  3. Examples:

    test="test"
if [[ $test == "test" ]]; then
    echo "Son iguales las cadenas."
elif [[ ! $test ]]; then
    echo "La cadena está vacía entonces."
elif [[ $test =~ "^t" ]]; then
    echo "El regex indica que la cadena ${test} empieza con t"
elif [[ ${test:1:1} == "e" ]]; then
    echo "Aquí estamos comparando el caracter 1 con la letra e, es verdadero"
fi

    each indicates what it does, it is easy to understand.

    • 2