Signals & Job Control

# Send SIGTERM (graceful shutdown)
kill 12345
kill -15 12345
kill -TERM 12345

# Send SIGKILL (force kill)
kill -9 12345
kill -KILL 12345

# Send SIGHUP (reload configuration)
kill -1 12345
kill -HUP 12345

# Send signal to process group
kill -9 -12345
# Negative PID = send to process group

# Send to all processes matching name
pkill -9 nginx
killall -9 nginx

# Send signal to process by name
pkill -HUP sshd

# Verify process received signal
kill -0 12345   # Check if process exists
echo $?          # 0 = exists, 1 = doesn't exist

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

# Trap SIGTERM and SIGINT
trap 'echo "Received SIGTERM, cleaning up..."; cleanup; exit 0' TERM INT
trap 'echo "Received SIGHUP, reloading config..."; reload_config' HUP
trap '' HUP    # Ignore SIGHUP

cleanup() {
    rm -f "$TEMP_FILE"
    kill "$BACKGROUND_PID" 2>/dev/null
    echo "Cleanup complete"
}

TEMP_FILE=$(mktemp)

# Start background process
sleep 1000 &
BACKGROUND_PID=$!

echo "PID: $$"
echo "Waiting for signal..."
wait $BACKGROUND_PID

# Start background job
sleep 1000 &
# [1] 12345

# List jobs
jobs
# [1]+  Running    sleep 1000 &

# Bring job to foreground
fg %1

# Send job to background (from foreground)
# Ctrl+Z to suspend, then:
bg %1

# Kill a job
kill %1

# Wait for background job to finish
wait

# Disown a job (remove from job table)
disown %1
# Job continues after terminal closes

# nohup: ignore SIGHUP (terminal close signal)
nohup long_running_task &

# Output goes to nohup.out
nohup long_running_task > output.log 2>&1 &

# Disown immediately
nohup long_running_task > output.log 2>&1 &
disown

# With screen/tmux (better approach)
screen -S mysession
# Run commands, then Ctrl+A D to detach
screen -r mysession

# With systemd (best for production)
systemd-run --unit=mytask --scope long_running_task

# Check if background job is still running
ps aux | grep long_running_task

# Login shell vs non-login shell
# Login: reads /etc/profile, ~/.bash_profile, ~/.bash_login, ~/.profile
# Non-login: reads ~/.bashrc

# SIGHUP propagation
# When terminal closes:
# 1. Shell receives SIGHUP
# 2. Shell sends SIGHUP to all children
# 3. Children terminate unless they ignore SIGHUP

# Start a new session
setsid command    # Creates new session, no terminal
nohup command &   # Ignores SIGHUP, runs in background

# Check terminal association
tty              # Shows current terminal
ps -p $$ -o tty  # Shows shell's terminal

# Screen session management
screen -ls        # List sessions
screen -r name    # Reattach
screen -d -r name # Detach and reattach