Sam Stelfox

Thoughts from a systems hacker and developer.

Fail Fast in Bash Scripts

I found myself writing another bash script that should exit should any of the few commands within it fail to run. As I began writing some error handling after each command, and isolating the sections into bash functions I figured there had to be a better way. After a little Googling and a trip through the bash manpages sure enough:


function error_handler() {
  echo "Error occurred in script at line: ${1}."
  echo "Line exited with status: ${2}"

trap 'error_handler ${LINENO} $?' ERR

set -o errexit
set -o errtrace
set -o nounset

echo "Everything is running fine..."

# A command outside of a conditional that will always return a exit code of 1
test 1 -eq 0

echo "This will never run, as a command has failed"
echo "Using unset variable ${TEST} will also cause this script to exit"

The first piece of that is setting up an error handler that will get run whenever an error condition occurs with the script. You can use this section to roll back any changes or cleanup your environment as well as give you some debug information about the failure.

I'm then setting a few bash options, The following is a description taken more or less directly from the bash man pages:

-o errexit: Exit immediately if a pipeline (which may consist of a single simple command), a subshell command enclosed in parentheses, or one of the commands executed as part of a command list enclosed by braces exits with a non-zero status.

-o errtrace: If set, any trap on ERR is inherited by shell functions, command substitutions, and commands execute in a subshell environment.

-o nounset: Treat unset variables and parameters other than the special parameters "@" and "*" as an error when performing parameter expansion.

If anything goes wrong in the script it will fail once, fail fast, and let you know where it died.