Traps, Cleanup and Temporary Files

Leave no mess behind

Scripts often create temporary files, lock files, mounted volumes or background processes that must be cleaned up even when something fails or the user presses Ctrl+C. The trap builtin runs a command when the shell receives a signal or reaches a special event. trap cleanup EXIT runs cleanup whenever the script exits, normally or because of an error under set -e, which covers most needs. You can also trap INT (Ctrl+C) and TERM (polite kill) to log and exit, and ERR to report the failing line, using $LINENO and $BASH_COMMAND (with set -E so the trap is inherited by functions). Create temporary files and directories with mktemp, which picks unique, unpredictable names in a safe location; never use fixed names like /tmp/data.txt, which collide between runs and can be abused by other users on shared machines.

A cleanup trap and an error reporter

Temporary files disappear and failures report where they happened.

#!/usr/bin/env bash
set -Eeuo pipefail

workdir=$(mktemp -d)
cleanup() {
    local status=$?
    rm -rf "${workdir:?}"
    exit "$status"
}
trap cleanup EXIT
trap 'echo "error on line $LINENO: $BASH_COMMAND" >&2' ERR
trap 'echo "interrupted" >&2; exit 130' INT TERM

curl -fsSL "https://example.com/data.tar.gz" -o "$workdir/data.tar.gz"
tar -xzf "$workdir/data.tar.gz" -C "$workdir"
./import.sh "$workdir"/*.csv
# however the script ends, $workdir is removed

Closing the shop at night

A trap on EXIT is the closing checklist pinned by the door: lights off, gas off, shutters down. It runs whether you close normally, rush out because of a problem, or are called away suddenly.

त्वरित जाँच: Which trap is the most general way to ensure cleanup runs when a script ends?

• trap cleanup INT
• trap cleanup EXIT
• trap cleanup HUP
• trap cleanup DEBUG