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Shell Script

Table of Contents

Robust Shell Scripts

First of all, if your script is over 100 lines, it should probably be rewritten in Python or Go.

Bash Options

Always do set -o errexit -o pipefail -o nounset at the start of the script.

  • -o errexit: Abort when a command exits with non-zero status (except in until, while, if)
  • -o pipefail: Causes a pipeline to return the exit status of the last command in the pipe that returned a non-zero return value.
  • -o nounset: Attempt to use undefined variable causes an error and forces an exit

Trapping Deferred Calls

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trap '{
  rm -f '"${TEMP_FILE}" "${LOCK_FILE}"'
}' INT TERM EXIT
  • INT: capturing SIGINT (Interrupt). ctrl-c sends such a signal.
  • TERM: capturing SIGTERM (Terminate).
  • EXIT is a pseudo-signal triggered when the script exits, either through reaching the end of the script, an exit command, or a failing command when set -o errexit.

Just Quote Everything

Variable expansion:

  • Good: "$my_var"
  • Bad: $my_var

Command substitution:

  • Good: "$(cmd)"
  • Bad: $(cmd)

Linter and Formatter

  • Use shellcheck, a shell script static analysis tool, to improve your script.
  • Use shfmt to format your script

Further Reading

Bash Pitfalls: a compilation of common mistakes made by bash users.

Snippets

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# log interpolates and writes message to stderr with timestamp.
log() {
  echo "[$(date +'%Y-%m-%dT%H:%M:%S%z')]: $*" >&2
}

# Retry a command up to a specific numer of times until it exits successfully,
# with exponential back off.
#
#  $ retry 5 echo Hello
#  Hello
#
#  $ retry 5 false
#  Retry 1/5 "false" exited 1, retrying in 1 seconds...
#  Retry 2/5 "false" exited 1, retrying in 2 seconds...
#  Retry 3/5 "false" exited 1, retrying in 4 seconds...
#  Retry 4/5 "false" exited 1, retrying in 8 seconds...
#  Retry 5/5 "false" exited 1, no more retries left.
#
retry() {
	local retries=$1
	shift

	local count=0
	until "$@"; do
		exit=$?
		wait=$((2 ** count))
		count=$((count + 1))
		if [ $count -lt "$retries" ]; then
			log "Retry $count/$retries \"$*\" exited $exit, retrying in $wait seconds..."
			sleep $wait
		else
			log "Retry $count/$retries \"$*\" exited $exit, no more retries left."
			return $exit
		fi
	done
	return 0
}