An atomic file updater in shell.

atomcat.sh

#!/bin/sh

set -u

PROG="${0##*/}"
# Default temp file suffix
SUFFIX="${SUFFIX-.tmp$$}"

usage() {
	printf 'usage: echo foo | %s <file> [lockfile] [tempfile]\n' "$PROG" >&2
	printf '\n'
	printf 'This program performs the following:\n' >&2
	printf '  - relays STDIN to tempfile (Default file%s\n' "$SUFFIX" >&2
	printf '  - uses flock to ensure a single writer to file\n' >&2
	printf '  - moves tempfile onto file\n' >&2
	printf 'NOTE: Ensure file and tempfile are on the same filesystem\n' >&2
	printf '      for an atomic mv operation!\n' >&2
}

lock() {
	file="$1"
	lock_file="${2-${file}.lock}"

	# Create lock file with FD<10 for POSIX compatibility
	exec 9>"$lock_file"

	# Acquire lock
	flock -n 9 \
		&& return 0 \
		|| return 1
}

main() {
	if [ $# -lt 1 ]; then
		usage
		return 1
	fi

	file=$1
	lock_file="${2-${file}.lock}"
	temp_file="${3-${file}${SUFFIX}}"

	# Get the lock
	lock "$file" "$lock_file" \
		|| return 2

	# Deferred cleanup until program exit.
	# Remove lock file if process is interrupted.
	trap 'rm -f "$lock_file"' 1 2 3 6 15
	# Temp file is always removed.
	trap 'rm -f "$temp_file"' EXIT

	# Relay STDIN to a temp file
	cat > "$temp_file"
	if [ "$?" -ne 0 ]; then
		printf "Failed writing to %s\n" "$temp_file" >&2
		return 3
	fi

	# Move temp file into place, hopefully atomically
	mv "$temp_file" "$file"
	if [ "$?" -ne 0 ]; then
		printf "Failed moving %s to %s\n" "$temp_file" "$file" >&2
		return 4
	fi

	rm "$lock_file"
}

main "$@"