Latin American School On Computational Materials Science

Exit codes

From the "Advanced Bash-Scripting Guide":

 Every command returns an exit status (sometimes referred to as a
 return status or exit code).  A successful command returns a 0, while
 an unsuccessful one returns a non-zero value that usually may be
 interpreted as an error code. Well-behaved UNIX commands, programs,
 and utilities return a 0 exit code upon successful completion, though
 there are some exceptions.

After the execution of a program, the BASH sets the variable ? to the exit status of the last process executed:

$ ls /proc/meminfo
/proc/meminfo
$ echo $?
0

or:

$ ls /proc/Meminfo
ls: cannot access /proc/Meminfo: No such file or directory
$ echo $?
2

In line with the content of the ls man page, which laconically recites:

Exit status is 0 if OK, 1 if minor problems, 2 if serious
trouble.

Keep in mind that although this 0 = success, non-0 = failure is a very widely used convention (that you should follow) it's not a rule and reading the manual of a program is always a good idea when relying on the exit status of a program.

The exit status of a program can be assigned to a variable, either directly (variable=program) or using the $? variable after the execution (program;variable=$?) to avoid losing it at the execution of the next command, if required.