If output of bash command is empty, do something

Question

Hi I am new to bash so please excuse me if I have a really silly/easy question. I am writing a script which allows the user to change their region (for wireless). What I am wanting to do is put a check in place, so if they type in an incorrect value, it brings up the prompt again to input the region. I want to do this by checking if the output of the command sudo iw reg set $reg, if it is a correct input, there is no output. But if it is a wrong input, it gives an error message. I tried to do this but im getting an error:

#!/bin/bash
echo "Please set a region: " 
read reg

if [(sudo iw reg set $reg) -ne 0]; then
    echo "Please set a valid region: "  
    read reg

else    
    echo "Setting reg as $reg"
    sudo iw reg set $reg 
fi

Thanks in advance

Answer 1

You can use the -z test, type help test in Bash to learn more (test is the same as the [ command).

You should only call iw reg set once, unless it fails.

echo "Please set a region: "

while true # infinite loop
do

    # read in the region:
    read reg

    # try the command, and catch its output:
    output=$( sudo iw reg set "$reg" 2>&1 )

    if [ -z "$output" ]
    then
        # output is empty - success - leave the loop:
        break
    else
        # output is non-empty - continue:
        echo "Please set a valid region. "
    fi

done

This snippet checks the success condition you gave in your question (empty output), but it should be noted that usually exit codes should be used if possible.

Note the 2>&1 operator redirecting stderr to stdout so any output on either file descriptor will be considered a failure.