Process Management Skill

Master process control, signals, scheduling, and monitoring

Learning Objectives

After completing this skill, you will be able to:

• List and inspect running processes
• Send and handle signals properly
• Run background jobs and daemons
• Schedule tasks with cron and systemd
• Monitor system resources

Prerequisites

• Bash basics
• Linux system fundamentals
• User permissions understanding

Core Concepts

1. Process Inspection

# 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

2. Signal Handling

# 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
}

3. Background Jobs

# 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

4. Cron Scheduling

# 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

Common Patterns

Daemon Pattern

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
}

Cron with Locking

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

Signal Handler

#!/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

Signal Reference

Signal	Number	Default	Common Use
SIGHUP	1	Terminate	Reload config
SIGINT	2	Terminate	Ctrl+C
SIGQUIT	3	Core dump	Ctrl+\
SIGKILL	9	Terminate	Force kill
SIGTERM	15	Terminate	Graceful stop
SIGSTOP	19	Stop	Pause
SIGCONT	18	Continue	Resume

Anti-Patterns

Don't	Do	Why
kill -9 first	kill -TERM first	Allow cleanup
Kill PID 1	Never	Crashes system
Cron without logs	Log all output	Debug issues

Practice Exercises

1. Process Monitor: Script to monitor a process
2. Daemon Script: Create a proper daemon
3. Cron Job: Schedule a backup job
4. Signal Handler: Graceful shutdown script

Troubleshooting

Common Errors

Error	Cause	Fix
No such process	Already dead	Check with ps
Operation not permitted	Wrong owner	Use sudo
Cron not running	PATH issues	Use full paths

Debug Techniques

# Check if process exists
ps -p $PID

# Debug cron
grep CRON /var/log/syslog

# Trace process
strace -p $PID

Resources

• GNU Coreutils - Process
• Crontab Guru
• systemd Timers