I'm using Bash in Linux and I found an example in which I compared the strings in this way but apparently it does not work.
In this case, $a is what the user writes to the console.
#!/bin/bash
a=$1
if [ "${a}"=="static" ]; then
(instrucciones)
else
(instrucciones)
fi
You can do help test for more information:
Empty string:
if [ -z "$VAR" ]; then ...
Equal chains
if [ "$VAR" = "CADENA" ]; then ...
Different chains
if [ "$VAR" != "CADENA" ]; then ...
In Bash you can compare the strings using the syntax:
[[ $variable == "valor" ]]
Note that here $variable do not need to have double quotes.
Here, both [[ and == are characteristic of Bash (what we call Bashisms ). The standard (that is, what POSIX defines) is to use [ and = :
[ "$variable" = "valor" ]
In which case the variable must be enclosed in quotation marks to protect us from word splitting .
Note that the syntax is of the form:
if [ condiciones ]
# ^ ^ ^
That is, the spaces around [ are necessary, because [ is a command itself. This command is called test and therefore you can use [ and test indistinctly, as well as search your instructions doing man test (as indicated by Trauma in your answer):
$ v="hola"
$ if test "$v" = "hola"; then echo "si"; fi
si
$ if [ "$v" = "hola" ]; then echo "si"; fi
si
[ and [[ . When we use [ , the variables expand causing errors if we do not put double quotes:
$ v="hola que tal"
With [ and quotation marks, 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 test instead of the maximum 3 accepted.
On the other hand, with the Bashismo [[ you can omit the quotes:
$ if [[ $v = "hola que tal" ]]; then echo "si"; fi
si
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
everyone indicates what they do, it's easy to understand.