BASH Exit Status
Exist Status
Each process that executes on a Linux system leaves a whole number exit code (or exit status, result code, or error code) when it terminates.
The value of the exit code is usually 0 if no errors were encountered, or non-zero if an error was encountered. The meaning of specific codes varies program-by-program; see the documentation (such as the man page) for a program to determine the meaning of those codes.
In a multi-command pipeline, the exit status of the last command is used as the exit status for the entire pipeline. (In BASH documentation, the term pipeline is used to refer to any single command or sequence of commands; see the BASH manpage for details).
Retrieving Exist Status
BASH places the exit status of the most recently-executed pipeline in the special variable $?. You can view the value of this variable using the echo command:
$ ls /tmp >/dev/null $ echo $? 0 $ ls /temp >/dev/null ls: cannot access /temp: No such file or directory $ echo $? 2
Setting the Exit Status when Exiting from a Shell Script
The BASH exit command can be used with a whole-number numeric positional argument to exit a script with a particular status code. For example:
exit 2
Will exit the script with an exit code of 2.
The test ([) Command
BASH has a built-in test command (similar to /bin/test) which can perform basic string and integer comparisons using these operators:
| Operator | Comparision type | Comparison | Example |
|---|---|---|---|
| -eq | Integer | Equal | $x -eq 4 |
| -ne | Integer | Not equal | $x -ne 4 |
| -gt | Integer | Greater than | $x -gt 0 |
| -lt | Integer | Less than | $x -lt 1000 |
| -ge | Integer | Greater than or equal to | $x -ge $y
|
| -le | Integer | Less than or equal to | $x -le 96 |
| = | String | Equal | "$x" = "Y" |
| != | String | Not equal | "$x" != "NEVER" |
There are also a number of unary file tests, including:
| Operator | Test | Example |
|---|---|---|
| -e | File exists | |
| -r | File is readable | |
| -w | File is writable | |
| -x | File is executable | |
| -f | File is a regular file | |
| -d | File is a directory |
These tests and comparisons are used with the test or [ command:
$ test 10 -gt 5 $ echo $? 0
$ test 10 -lt 5 $ echo $? 1
$ [ 10 -lt 5 ] $ echo $? 1
$ [ -f /etc/passwd ] $ echo $? 0
$ [ -w /etc/passwd ] $ echo $? 1
Tests can be combined with -o (or) and -a (and), and negated with !
$ a=500 $ [ "$a" -ge 100 -a "$a" -le 1000 ] $ echo $? 0
$ [ ! "a" = "b" ] $ echo $? 0
Exit Codes and Flow Control
Exit codes are used extensively with the BASH Flow Control operators.