Understanding Linux Signals, kill Command, and trap Command

1. Linux Signal Types

Signals are software‑level simulations of hardware interrupts; sending a signal to a process triggers a corresponding handler.

Processes can respond to a signal by ignoring it (except SIGKILL and SIGSTOP, which cannot be ignored), catching it with a custom handler, or performing the default action defined by the kernel.

The way a signal is handled depends on the API function used to deliver it.

Linux defines many signals; the most common ones are highlighted in the table below.

Number

Signal Name

Default Action

Description

1

SIGHUP

Terminate

Terminate process, hang up

2

SIGINT

Terminate

Keyboard interrupt (Ctrl+C)

3

SIGQUIT

CoreDump

Keyboard quit (Ctrl+\)

9

SIGKILL

Terminate

Immediately stop process; cannot be caught or ignored

15

SIGTERM

Terminate

Graceful termination; can be caught

additional rows omitted for brevity

CoreDump means the kernel writes a core file for debugging when a process aborts unexpectedly.

Linux supports two categories of signals:

Standard signals (1‑31) are non‑real‑time, not queued, and may be lost if sent repeatedly.

Extended (real‑time) signals (32‑64) are reliable and queued, preserving the number of occurrences.

Understanding a few common signals is sufficient for most tasks.

2. kill Command

The kill command sends signals to processes.

Syntax:

kill [-s sigspec | -n signum | -sigspec] pid | jobspec ...

kill -l [sigspec]

Options:

-s – specify signal name

-n – specify signal number

-l – list signal names

Example:

# Send termination signal to a process
kill -s SIGTERM pid
# or using number
kill -n 15 pid
# or short form
kill -15 pid

3. trap Command

The trap command defines how a shell script should react to received signals.

Syntax:

trap [-lp] [[arg] signal_spec ...]

Common actions after catching a signal:

Remove temporary files

Ignore the signal

Prompt the user before terminating the script

Examples:

#!/bin/bash
trap "" 2    # Ignore SIGINT (Ctrl+C)
for i in {1..10}; do
echo $i
sleep 1
done

#!/bin/bash
trap "echo 'exit...'; exit" 2
for i in {1..10}; do
echo $i
sleep 1
done

#!/bin/bash
trap "func" 2
func() {
read -p "Terminate the process? (Y/N): " input
if [ "$input" == "Y" ]; then
exit
fi
}
for i in {1..10}; do
echo $i
sleep 1
done

These examples show how to ignore, handle, or prompt the user when a signal like SIGINT is received.