Every command in bash returns an exit status, which is an integer between 0 and 255 that represents whether the command succeeded or failed, and if it failed, what kind of error occurred. Functions also can return an exit status via the return keyword. The exit status can be checked using bash's special $? variable, which always stores the exit status of the last command run.
To check the exit status of a command, we use the special question ? mark variable that bash sets for us.
$ echo $? ## 0
See what it looks like if we do something that intentionally won't work. If you ls on a folder that doesn't exist, it will return an error.
$ ls noexist ls: noexist: No such file or directory
Let's see what the exit status was.
$ echo $? 1
If we don't set an exit status explicitly, it's assumed to be 0 or it's the results of the last command that was run.
Let's go in here. Let's see how exit statuses work with functions. I am going to create an ok function. Instead of using exit here, because exit will cause entire script to stop and exit out, use the return keyword. For ok, I am going to return 0, and then for my fail function here, I am going to return 1.
ok(){ return 0 } fail(){ return 1 } fail echo $? ok echo $?
This question mark variable is set for every command or function that's run.
$ ./script.sh 1 0