Exit values

We can tell it was the rmdir command in the previous section that failed, because rmdir is the first word of the error message output. We can test the command in isolation, and look at the value of the special $? parameter with echo, to see its exit status:

$ rmdir ~/nonexistent
rmdir: failed to remove '/home/bashuser/nonexistent': No such file or directory
$ echo $?
1

The rmdir program returned an exit value of 1, because it could not delete a directory that didn't exist. If we create a directory first, and then remove it, both commands succeed, and the value of $? for both steps is 0:

$ mkdir ~/existent
$ echo $?
0
$ rmdir ~/existent
$ echo $?
0

Examining the exit values for the true and false built-in Bash commands is instructive; true always succeeds, and false always fails:

$ true ; echo $?
0
$ false ; echo $?
1

Bash will also raise an exit status of 127 for you if it can't find a way to run a command you request, such as notacommand:

$ notacommand ; echo $?
bash: notacommand: command not found
127

It's standard for programs to return 0 when they succeed, and something greater than 0 if they fail. Beyond that, programs vary in which exit values they choose for error conditions.