# List processes
ps aux                      # All processes
ps -ef                      # Full format
ps --forest                 # Tree view

# Find processes
pgrep -f "pattern"          # PID by pattern
pidof nginx                 # PID by name
ps aux | grep '[n]ginx'     # Grep trick

# Resource usage
top                         # Real-time view
htop                        # Better interface

# Send signals
kill PID                    # SIGTERM (15)
kill -9 PID                 # SIGKILL (9)
kill -HUP PID               # SIGHUP (1)
killall nginx               # By name
pkill -f "pattern"          # By pattern

# Handle signals in scripts
trap 'cleanup' EXIT
trap 'echo "Interrupted"' INT

cleanup() {
    rm -f "$TEMP_FILE"
    exit 0
}

# Background execution
command &                   # Run in background
nohup command &             # Immune to hangup
nohup cmd > log.txt 2>&1 &  # With logging

# Job control
jobs                        # List jobs
fg %1                       # Foreground job 1
bg %1                       # Background job 1
disown                      # Detach from shell

# Cron format
# ┌─── minute (0-59)
# │ ┌─── hour (0-23)
# │ │ ┌─── day of month (1-31)
# │ │ │ ┌─── month (1-12)
# │ │ │ │ ┌─── day of week (0-6)
# * * * * * command

# Examples
0 * * * *     # Every hour
*/15 * * * *  # Every 15 minutes
0 0 * * *     # Daily at midnight
0 0 * * 0     # Weekly on Sunday

# Edit crontab
crontab -e
crontab -l

start_daemon() {
    nohup ./daemon.sh >> /var/log/daemon.log 2>&1 &
    echo "$!" > /var/run/daemon.pid
    disown
}

stop_daemon() {
    if [[ -f /var/run/daemon.pid ]]; then
        kill "$(cat /var/run/daemon.pid)"
        rm /var/run/daemon.pid
    fi
}

# Prevent overlapping runs
0 * * * * /usr/bin/flock -n /var/lock/job.lock /path/to/script.sh

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

cleanup() {
    echo "Cleaning up..."
    rm -f "$TEMP_FILE"
}

trap cleanup EXIT INT TERM

# Main logic
TEMP_FILE=$(mktemp)
# ... work with temp file

# Check if process exists
ps -p $PID

# Debug cron
grep CRON /var/log/syslog

# Trace process
strace -p $PID