# Bash Scripting - Exit Codes

# What is an exit code?

Every Linux or Unix command executed by the shell script or user has an exit status. Exit status is an integer number. 0 exit status means the command was successful without any errors. A non-zero (1-255 values) exit status means command was a failure.

# Getting an exit code

You need to use a particular shell variable: $? to get the exit status of the previously executed command. To print $? variable use the echo command or printf command:

date
echo $?
date-or-no-date
printf '%d\n' $?

# Using exit codes

Assign $? to a shell variable

command
status=$?
## run date command ##
cmd="date"
$cmd
## get status ##
status=$?
## take some decision ## 
[ $status -eq 0 ] && echo "$cmd command was successful" || echo "$cmd failed"

# Set an exit code for my own shell scripts?

The exit command cause normal shell script termination. It exits the shell with a status of N. The syntax is:

exit N
exit 1
exit 999

Example:

#!/bin/bash
/path/to/some/command
[ $? -eq 0 ]  || exit 1

# Script to get the exit code of a command

#!/bin/bash
#
# Sample shell script to demo exit code usage #
#
set -e
 
## find ip in the file ##
grep -q 192.168.2.254 /etc/resolv.conf
 
## Did we found IP address? Use exit status of the grep command ##
if [ $? -eq 0 ]
then
  echo "Success: I found IP address in file."
  exit 0
else
  echo "Failure: I did not found IP address in file. Script failed" >&2
  exit 1
fi

# Exit Codes With Special Meanings

Exit status	Description
1	Catchall for general errors
2	Misuse of shell builtins (according to Bash documentation)
126	Command invoked cannot execute
127	Command not found
128	Invalid argument to exit command
128+n	Fatal error signal “n”
130	Bash script terminated by Control-C
255*	Exit status out of range

# Exit codes of all piped commands

So to get the exit codes for something like command1 | command2 | commandN

or command1 | filter_data_command > output

or get_data_command | verify_data_command | process_data_command | format_data_command > output.data.file

# Pipes to connect programs

Use the vertical bar | between two commands. In this example, send netstat command output to grep command i.e. find out if nginx process exits or not in the system:

netstat -tulpn | grep nginx

# How to get exit status of process that’s piped to another

command1 | command2 
echo "${PIPESTATUS[@]}"

command1 | command2 
echo "${PIPESTATUS[0]} ${PIPESTATUS[1]}"

PIPESTATUS is an array variable containing a list of exit status values from the processes in the most-recently-executed foreground pipeline. Try the following commands:

netstat -tulpn | grep nginx
echo "${PIPESTATUS[@]}"
 
true | true
echo "The exit status of first command ${PIPESTATUS[0]}, and the second command ${PIPESTATUS[1]}"
 
true | false
echo "The exit status of first command ${PIPESTATUS[0]}, and the second command ${PIPESTATUS[1]}"
 
false | false | true 
echo "The exit status of first command ${PIPESTATUS[0]}, second command ${PIPESTATUS[1]}, and third command ${PIPESTATUS[2]}"

# Putting it all together

Here is a sample script that use ${PIPESTATUS[0]} to find out the exit status of mysqldump command in order to notify user on screen about database backup status:

#!/bin/bash
### Purpose: mysql.backup.sh : Backup database ###
### Author: Vivek Gite <https://www.cyberciti.biz>, under GPL v2.x+ or above. ###
### Change as per your needs ###
set -eo pipefail
MUSER='USERNAME-here'
MPASS='PASSWORD-here'
MHOST='10.0.3.100'  
DEST="/nfs42/backups/mysql"
NOWFORMAT="%m_%d_%Y_%H_%M_%S%P"
MYSQL="/usr/bin/mysql"
MYSQLDUMP="/usr/bin/mysqldump"
MKDIR="/bin/mkdir"
RM="/bin/rm"
GZIP="/bin/gzip"
DATE="/bin/date"
SED="/bin/sed"

# Failsafe? Create dir #
[  ! -d "$DEST" ] && $MKDIR -p "$DEST"
 
# Filter db names
DBS="$($MYSQL -u $MUSER -h $MHOST -p$MPASS -Bse 'show databases')"
DBS="$($SED -e 's/performance_schema//' -e 's/information_schema//' <<<$DBS)"
 
# Okay, let us go
for db in $DBS
do
    tTime=$(date +"${NOWFORMAT}")
    FILE="$DEST/${db}.${tTime}.gz"
    $MYSQLDUMP -u $MUSER -h $MHOST -p$MPASS $db | $GZIP -9 > $FILE
    if [ ${PIPESTATUS[0]} -ne "0" ];
    then
        echo "The command $MYSQLDUMP failed with error code ${PIPESTATUS[0]}."
        exit 1
    else
        echo "Database $db dump successfully."    
    fi
done

# Use the array called pipestatus as follows

true | true
echo "${pipestatus[1]} ${pipestatus[2]}"

## Outputs:
## 0 0