trap - Linux

Overview

The trap command in Linux is used to specify and handle signals and other system events during the execution of a shell script or a command. It defines actions to take when the script receives signals like INT (interrupt) or TERM (terminate). Utilizing trap can enhance script robustness by gracefully handling unexpected events or cleanup tasks.

Syntax

The basic syntax of the trap command is:

trap [OPTIONS] [ARG] [SIGNUM...]

Where ARG is the command to execute when one of the SIGNUM signals is received. SIGNUM can be a signal number or name (like SIGINT or 2). Multiple signals can be trapped by the same command by listing them with spaces.

Variations

To remove a trap, use an empty string as the argument:
```
trap "" SIGNAL
```
To reset the action for a signal to its default, use - as the argument:
```
trap - SIGNAL
```
To print the current trap settings, use:
```
trap -p
```

Options/Flags

-p: Display the trap commands associated with each signal.
-l: List signal names and their corresponding numbers.

These options help inspect current traps and understand signal specifications.

Examples

Basic trapping of a signal: Trap the SIGINT signal, usually triggered by pressing Ctrl+C:
```
trap "echo 'SIGINT received'" SIGINT
```
Trap multiple signals: Perform cleanup operations for both SIGINT and SIGTERM:
```
trap "echo 'Cleanup operations'; exit" SIGINT SIGTERM
```
Disable a trap: Disable the trap for a SIGINT:
```
trap - SIGINT
```
Trap on script exit: Execute a command when the script exits normally or is interrupted:
```
trap "echo 'Script is terminating'" EXIT
```

Common Issues

Trap not triggering: Ensure the script is running in a context where it can receive signals. Some subshells or script modes may not handle traps as expected.
Using variables in traps: Variables used in traps are evaluated when the trap is declared, not when triggered. Use single quotes to delay evaluation.

Integration

Combine trap with other commands for robust script management:

Creating a temporary file and ensuring its deletion:

temp_file=$(mktemp)
trap "rm -f $temp_file" EXIT
# Use temporary file in script

Integrating with sleep for interruptible waits:

trap "exit" INT
echo "Waiting for 5 seconds. Press Ctrl+C to interrupt."
sleep 5

kill: Send signals to other processes.
ps: Monitor or control active processes, useful for identifying process IDs to send signals with kill.

For further reading, consider the official GNU Bash documentation or use man trap on most Linux systems for more details.