To write robust Bash scripts, I recommend you to:
1. Activate the Bash options: errexit, nounset
Add these two options at the top of your Bash scripts:
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#!/usr/bin/env bash set -o errexit set -o nounset |
- errexit will stop your Bash script each time a command in your script returns an exit code different from “0”. It does not show any message when it quits, this is why it is important to show a traceback (check the second advice below “show a Bash traceback”).
- nounset will stop your script and output an error if you attempt to use undefined variables. nounset is very important because it can avoid you to run dangerous commands like: rm –fr “/$undefinedvariable”
2. Show a Bash traceback
Because the option “errexit” does not show any message when it stops your Bash script in some cases (for example var=$(yourcommand) will exit without any message, even when yourcommand returns an exit code different from zero), I recommend you to add the code below to show a traceback each time “errexit” forces your Bash script to stop:
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#!/usr/bin/env bash set -o errexit # stop the script each time a command fails set -o nounset # stop if you attempt to use an undef variable function bash_traceback() { local lasterr="$?" set +o xtrace local code="-1" local bash_command=${BASH_COMMAND} echo "Error in ${BASH_SOURCE[1]}:${BASH_LINENO[0]} ('$bash_command' exited with status $lasterr)" if [ ${#FUNCNAME[@]} -gt 2 ]; then # Print out the stack trace described by $function_stack echo "Traceback of ${BASH_SOURCE[1]} (most recent call last):" for ((i=0; i < ${#FUNCNAME[@]} - 1; i++)); do local funcname="${FUNCNAME[$i]}" [ "$i" -eq "0" ] && funcname=$bash_command echo -e " $i: ${BASH_SOURCE[$i+1]}:${BASH_LINENO[$i]}\t$funcname" done fi echo "Exiting with status ${code}" exit "${code}" } # provide an error handler whenever a command exits nonzero trap 'bash_traceback' ERR # propagate ERR trap handler functions, expansions and subshells set -o errtrace |
The traceback will help you to know which command in your Bash script failed with a return code different from “0”:
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Error in ./test.sh:43 ('false' exited with status 1) Traceback of ./test.sh (most recent call last): 0: ./test.sh:43 false 1: ./test.sh:49 main Exiting with status -1 |
3. Check your Bash scripts with shellcheck and follow the recommendations of Advanced Bash-Scripting Guide.
