Error handling in Bash scripts

Scripting is one of the key tools for a sysadmin to manage a set of day-to-day activities such as running backups, adding users/groups, installing/updating packages, etc. While writing a script, error handling is one of the crucial things to manage.

This article shows some basic/intermediate techniques of dealing with error handling in Bash scripting. I discuss how to obtain the error codes, get verbose output while executing the script, deal with the debug function, and standard error redirection. Using these techniques, sysadmins can make their daily work easy.

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Exit status

In Bash scripting, $? prints the exit status. If it returns zero, it means there is no error. If it is non-zero, then you can conclude the earlier task has some issue.

A basic example is as follows:

$ cat myscript.sh
           #!/bin/bash
           mkdir learning
           echo $?

If you run the above script once, it will print 0 because the directory does not exist, therefore the script will create it. Naturally, you will get a non-zero value if you run the script a second time, as seen below:

$ sh myscript.sh
mkdir: cannot create directory 'learning': File exists
1

Best practices

It is always recommended to enable the debug mode by adding the -e option to your shell script as below:

$ cat test3.sh
!/bin/bash
set -x
echo "hello World"
mkdiir testing
 ./test3.sh
+ echo 'hello World'
hello World
+ mkdiir testing
./test3.sh: line 4: mkdiir: command not found

You can write a debug function as below, which helps to call it anytime, using the example below:

$ cat debug.sh
#!/bin/bash
_DEBUG="on"
function DEBUG()
{
 [ "$_DEBUG" == "on" ] && $@
}
DEBUG echo 'Testing Debudding'
DEBUG set -x
a=2
b=3
c=$(( $a + $b ))
DEBUG set +x
echo "$a + $b = $c"

Which prints:

$ ./debug.sh
Testing Debudding
+ a=2
+ b=3
+ c=5
+ DEBUG set +x
+ '[' on == on ']'
+ set +x
2 + 3 = 5

Standard error redirection

You can redirect all the system errors to a custom file using standard errors, which can be denoted by the number 2. Execute it in normal Bash commands, as demonstrated below:

$ mkdir users 2> errors.txt
$ cat errors.txt
mkdir: cannot create directory ‘users’: File exists

Most of the time, it is difficult to find the exact line number in scripts. To print the line number with the error, use the PS4 option (supported with Bash 4.1 or later). Example below:

$ cat test3.sh
#!/bin/bash
PS4='LINENO:'

set -x
echo "hello World"
mkdiir testing

You can easily see the line number while reading the errors:

$ /test3.sh
5: echo 'hello World'
hello World
6: mkdiir testing
./test3.sh: line 6: mkdiir: command not found

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Wrap up

Managing errors is a key skill for administrators when writing scripts. These tips should help make your life easier when troubleshooting Bash scripts or even general commands.