Shell by Example: Traps POSIX 

#!/bin/sh

The trap command lets you catch signals and execute cleanup code when your script exits or receives signals. It’s essential for robust scripts.

Basic trap syntax: trap ‘commands’ SIGNAL

Trap EXIT runs when the script exits (any reason):

 

demo_exit_trap() {
  (
    trap 'echo "  Goodbye! Cleaning up..."' EXIT
    echo "  Working..."
    echo "  Done working"
  )
}

echo "EXIT trap demo:"
demo_exit_trap

Common signals to trap: EXIT (0) - Script exit (success or failure) INT (2) - Interrupt (Ctrl+C) TERM (15) - Termination request HUP (1) - Hangup ERR - On error (bash only)

Trap for cleanup of temporary files:

 

cleanup_tempfile() {
  (
    tmpfile="/tmp/trap_demo_$$"
    trap 'rm -f "$tmpfile"; echo "  Cleaned up $tmpfile"' EXIT

    echo "test data" >"$tmpfile"
    echo "  Created $tmpfile"
    echo "  File exists: $(test -f "$tmpfile" && echo yes || echo no)"
    # Cleanup happens automatically on exit
  )
}

echo "Temp file cleanup demo:"
cleanup_tempfile

Multiple commands in trap:

 

multi_cmd_trap() {
  (
    trap 'echo "  Step 1: Save state"; echo "  Step 2: Close files"' EXIT
    echo "  Running..."
  )
}

echo "Multi-command trap:"
multi_cmd_trap

Reset or ignore a trap:

 

echo "Ignore and reset trap:"
(
  # Ignore SIGINT (Ctrl+C)
  trap '' INT
  echo "  SIGINT now ignored"

  # Reset to default behavior
  trap - INT
  echo "  SIGINT reset to default"
)

Display current traps:

 

echo "Current traps:"
trap 'echo trapped' EXIT
trap -p
trap - EXIT

Trap with signal number instead of name:

 

(
  trap 'echo "  Caught signal"' 2 # 2 = SIGINT
  echo "  Trap set for signal 2"
)

Common pattern: cleanup function

 

cleanup() {
  echo "  Performing cleanup..."
  # Remove temp files, kill background processes, etc.
}

robust_script() {
  (
    trap cleanup EXIT
    echo "  Script running"
    # Even if something fails, cleanup runs
  )
}

echo "Cleanup function pattern:"
robust_script

Trap preserves exit code:

 

exit_code_demo() {
  (
    trap 'echo "  Trap ran, exit code was: $?"' EXIT
    exit 42
  )
  echo "  Subshell exited with: $?"
}

echo "Exit code preservation:"
exit_code_demo

Combining multiple signals:

 

(
  trap 'echo "  Interrupted or terminated"' INT TERM
  echo "  Trapping both INT and TERM"
)
Bash

Bash provides ERR trap for errors:

 

set -e
trap 'echo "Error on line $LINENO"' ERR
This runs on any command failure

DEBUG trap runs before each command:

 

trap 'echo "Running: $BASH_COMMAND"' DEBUG

Practical example: Lock file with cleanup

 

demo_lockfile() {
  lockfile="/tmp/mylock_$$"

  (
    trap 'rm -f "$lockfile"' EXIT

    echo "  Creating lock: $lockfile"
    echo $$ >"$lockfile"

    echo "  Doing work..."
    sleep 1

    echo "  Lock will be removed on exit"
  )

  echo "  Lock removed: $(test -f "$lockfile" && echo no || echo yes)"
}

echo "Lock file pattern:"
demo_lockfile

Trap in a loop (handle Ctrl+C gracefully):

 

graceful_loop() {
  (
    running=true
    trap 'running=false; echo "  Stopping..."' INT TERM

    count=0
    while [ "$running" = true ] && [ "$count" -lt 3 ]; do
      count=$((count + 1))
      echo "  Iteration $count"
      sleep 1
    done
    echo "  Loop finished gracefully"
  )
}

echo "Graceful loop termination:"
graceful_loop

Best practice: Always trap EXIT for scripts that create temporary resources.

 

echo "Done with trap examples"

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by Jose Diaz-Gonzalez | inspired by Go by Example | source