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Error Handling in bash

Exit Codes

Every process in a Linux system returns an exit code upon execution. In cases of a successful process the exit code is 0 (much like the HTTP 200 OK reponse code in the web HTTP protocol) and unsuccessful exit codes are represented by other integers. To determine the exit code returned for the last process in a bash shell, use:

$ date
Mon Feb 12 16:51:55 EST 2024

$ echo $?
0

To test for a non-zero exit code, you can invoke an error deliberately:

$ datez
bash: datez: command not found

$ echo $?
127

Here 127 represents not only an unsuccessful execution (i.e. non-zero) but specifically a "command not found" message since datez does not exist in the path.

The following are reserved codes determined by the Linux Documentation Project:

  • 1 - Catch-all for general errors
  • 2 - Misuse of shell built-ins
  • 126 - Command invoked cannot execute
  • 127 - “command not found”
  • 128 - Invalid argument to exit
  • 128+n - Fatal error signal “n”
  • 130 - Script terminated by Control-C
  • 255\* - Exit status out of range

Setting Error Codes

Shell scripts can return exit codes of your choosing. It would not make sense to return anything other than 0 upon successful completion, but any kind of error could return an integer you select.

#!/bin/bash

/usr/bin/<mycommand>
exit 14

Here

Evaluating Error Codes

Commands run within a portion of a shell script can be evaluated using normal if/then/else logic:

#!/bin/bash

date

if [ $? -eq 0 ]
then
  echo "Success"
  exit 0
else
  echo "Failure" >&2
  exit 1
fi

Notice the then and else stanzas each return their own exit codes.

Suppressing / Customizing Exit Codes

You can also write conditional exit codes within specific commands in your script.

Logging

Logging is a useful way of capturing errors and other informative output from your code. Broadly interpreted, logging can mean many things:

  • Appending a log message to a file
  • Sending a log message to a database
  • Sending a log message to a remote logging service
  • Notifying a user via email or other messaging service

Logging to files is made simple with basic output appended to a consistent log file. Here are some basic examples:

NOW=`date +'%Y-%m-%d-%H:%M:%S'`
STATUS="Warning"
MSG="This is a warning message in the log file"

echo $NOW - $STATUS - $MSG >> /var/log/app-error.log'

The log results from above:

2023-09-18 14:09:28,328 - WARNING - This is a warning message in the log file

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