When a command finishes execution, it returns an exit code. The exit code is not displayed on the screen by default. To examine the exit code, you need to examine a special variable, "$?"
Say, you are searching for a string in a text file.
$ grep x1y2z3 somefile.txt $
The standard output of the command returns null, which is a pretty good indication that the string cannot be found in the file.
But what if you embed the grep command in a script? How can you tell if the string is found or not?
Checking the exit code will tell you. Let's first try it out interactively.
$ grep x1y2z3 somefile.txt $ echo $? 1
Note that in bash, the exit status is 0 if the command succeeded, and 1 if failed. For grep, 0 means that the string was found, and 1 (or higher), otherwise.
To check the exit status in a script, you may use the following pattern:
somecommand argument1 argument2 RETVAL=$? [ $RETVAL -eq 0 ] && echo Success [ $RETVAL -ne 0 ] && echo Failure