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 (results are returned as an exit code):

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	[ -e /etc/passwd ]
-r	File is readable	[ -r /etc/hosts ]
-w	File is writable	[ -w /tmp ]
-x	File is executable	[ -x /usr/bin/ls ]
-f	File is a regular file	[ -f /dev/tty ]
-d	File is a directory	[ -d /dev/tty ]

These tests and comparisons are used with the test command or the synonym, [ 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.