bash Cookbook

Telling If a Command Succeeded or Not

Problem

You need to know whether the command you ran succeeded.

Solution

The shell variable $? will be set with a non-zero value if the command fails—provided that the programmer who wrote that command or shell script followed the established convention:

$ somecommand
it works...
$ echo $?
0
$ badcommand
it fails...
$ echo $?
1
$

Discussion

The exit status of a command is kept in the shell variable referenced with $?. Its value can range from 0 to 255. When you write a shell script, it’s a good idea to have your script exit with a non-zero value if you encounter an error condition. (Just keep it below 255, or the numbers will wrap around.) You return an exit status with the exit statement (e.g., exit 1 or exit 0). But be aware that you only get one shot at reading the exit status:

$ badcommand
it fails...
$ echo $?
1
$ echo $?
0
$

Why does the second time give us 0 as a result? Because the second time is reporting on the status of the immediately preceding echo command. The first time we typed echo $? it returned a 1, which was the return value of bad command. But the echo command itself succeeds, therefore the new, most-recent status is success (i.e., a 0 value). So you only get one chance to check it. Therefore, many shell scripts will immediately assign the status to another shell variable, as in:

$ badcommand
it fails...
$ STAT=$?
$ echo $STAT
1
$ echo $STAT
1
$

We can keep the value around in the variable $STAT and check its value later on.

Although ...