04. String Tests =, !=

String tests can be useful for checking user's input.

Checking for string equality

To check if two strings are equal or not, simply use the = or != operators.

$ [ 'hi' = 'hi' ]; echo $?
0
$ [ 'hello' = 'hi' ]; echo $?
1

Checking if argument is empty or not

Two unary operators are used to check if a string is empty or not.

-z
Argument is empty
-n
Argument is not empty
$ [ -z "" ]; echo $?
0
$ [ -n "" ]; echo $?
1

Example script

Here's a simple script that reads an input, and depending on the string put in, returns a specific response.

#!/bin/bash
 
printf 'Please enter a word: '
 
read input
 
if [ $input = 'hi' ]; then
  printf 'Hi to you too!'
else
  printf 'You input %s.\n' $input
fi

We'll go over if-then-else statements soon, but notice how we can use tests and expressions such as these to control logic in our scripts.

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