Linux bash exit status and how to set exit status in bash nixCraft

Can you explain bash exit status code? How do I set bash exit status in my Linux shell scripts?

Each Linux or Unix command returns a status when it terminates normally or abnormally. You can use value of exit status in the shell script to display an error message or run commands. For example, if tar command is unsuccessful, it returns a code which tells the shell script to send an e-mail to sysadmins.

More on Linux bash shell exit status codes

1. Every Linux or Unix command executed by the shell script or user, has an exit status.
2. The exit status is an integer number.
3. For the bash shell’s purposes, a command which exits with a zero (0) exit status has succeeded.
4. A non-zero (1-255) exit status indicates failure.
5. If a command is not found, the child process created to execute it returns a status of 127. If a command is found but is not executable, the return status is 126.
6. All of the Bash builtins return exit status of zero if they succeed and a non-zero status on failure.

How do I display the exit status of shell command?

You can use special shell variable called $? to get the exit status of the previously executed command. To print $? variable use the echo command/printf command. The syntax is:
command
echo $?
OR
/path/to/script.sh
command
date
echo $?
printf "%d\n" $?
## run non-existence command ##
foobar13535
## display status code ##
echo $?

How to store the exit status of the command in a shell variable

Assign $? to a shell variable. The syntax is:

date
status=$?
echo "The date command exit status : ${status}"

Linux exit status and the conditional/list constructs

A simple shell script to locate host name (findhost.sh)

#!/bin/bash
# set var 
FILE="/etc/hosts" # get host name 
read -p "Enter a hostname : " hostname # try to locate hostname/computer name in $FILE
grep -q -w "${hostname}" "${FILE}" # store exit status of grep
# if found grep will return 0 exit status
# if not found, grep will return a nonzero exit status
status=$? if test $status -eq 0
then echo "Host '$hostname' found in $FILE file."
else echo "Host '$hostname' not found in $FILE file."
fi

How to use the && and || operators with exit codes

The syntax is:

command && echo "success"
command || echo "failed"
command && echo "success" || echo "failed"

If a dir named “/tmp/foo” not found create it:
[ ! -d "/tmp/foo" ] && mkdir -p "/tmp/foo"
For example, show usage syntax when filename not passed as the command line arg:

#!/bin/Bash
_files="$@" ## fail safe ##
[[ "$_files" == "" ]] && { echo "Usage: $0 file1.png file2.png"; exit 1; } ## continue below ##

Here is another shell script that shows usage:

#!/bin/bash
set -e
I=~/bin/tags.deleted.410
O="/tmp/https.www.cyberciti.biz.410.url.conf"
t="$1"
[ ! -f "$I" ] && { echo "$I file not found."; exit 10; }
[ "$t" == "" ] && { echo "Usage: $0 number-of-urls-to-purge-from-$I"; exit 11; } >$O cat "$I" | sort | uniq | while read -r u
do uu="${u##https://www.cyberciti.biz}" echo "~^$uu 1;" >>"${O}"
done
echo "* Config file created at ${O} ..."
echo "* Installing ${O} file on utls-wp-mg-www ..."
~/bin/install.py "${O}"
echo "* Send config to rest of cluster nodes ... "
~/bin/install.py --sync --cluster --reload cbz-www
echo "* Building list purge urls for Cloudflare CDN ..."
sleep 1
url=""
while IFS= read -r u
do url="$url $u"
done <<<"$(tail -${t} ${I})"
[ "$url" != "" ] && ~/bin/cloudflare.purge.urls.sh "$url"
[ -f "$O" ] && rm -f "$O"

List of common exit codes for GNU/Linux

The perror command explain error codes which is part of MySQL/MariaDB package:
perror 0
perror 1

Conclusion

This page explained bash exit status and related commands. For more info see bash shell man page here.