Linux Processes and Services

# Every process traces back to PID 1 (systemd/init)
ps -ef | head -5

# Your processes in the current terminal
ps

# All processes on the system (full format)
ps -ef

# All processes (BSD style, with resource usage)
ps aux

# Filter by name
ps aux | grep nginx

# Show process tree
ps -ef --forest

USER   PID %CPU %MEM    VSZ   RSS TTY  STAT START  TIME COMMAND
mike  1234  0.5  1.2 123456 12345 pts/0 S  10:30  0:02 vim file.txt

top

htop

# Get PID of nginx processes
pgrep nginx

# With full command shown
pgrep -a nginx

# By user
pgrep -u mike

# Graceful stop (default SIGTERM)
kill 1234

# Force kill
kill -9 1234

# Reload configuration
kill -HUP 1234

# Kill by name
killall nginx

# Kill by name (with signal)
pkill -HUP nginx

# Kill all processes by a user
pkill -u baduser

# Start a command in the background
long-running-command &

# Check background jobs
jobs

# Bring a background job to the foreground
fg %1

# Send the current foreground job to the background
# First: Ctrl-z (suspends it)
# Then:
bg %1

# Start a download in the background
wget https://example.com/bigfile.tar.gz &

# Suspend a running editor (Ctrl-z), do something else, then resume
vim file.txt
# Press Ctrl-z
fg

nohup long-running-script.sh &

# Start a process with low priority
nice -n 10 heavy-computation

# Change priority of a running process
renice 10 -p 1234

# Only root can increase priority (lower nice value)
sudo renice -5 -p 1234

# Check if a service is running
systemctl status nginx

# Is it active? (returns 0 or 1)
systemctl is-active nginx

# Is it enabled to start at boot?
systemctl is-enabled nginx

# List all running services
systemctl list-units --type=service --state=running

# List all services (including inactive)
systemctl list-units --type=service --all

# Start a service
sudo systemctl start nginx

# Stop a service
sudo systemctl stop nginx

# Restart (stop + start)
sudo systemctl restart nginx

# Reload config without stopping
sudo systemctl reload nginx

# Enable at boot
sudo systemctl enable nginx

# Disable at boot
sudo systemctl disable nginx

# Enable and start in one command
sudo systemctl enable --now nginx

# Logs for a specific service
journalctl -u nginx

# Follow logs in real time (like tail -f)
journalctl -u nginx -f

# Logs since last boot
journalctl -b

# Logs since a specific time
journalctl --since "1 hour ago"
journalctl --since "2026-06-09 10:00:00"

# Only errors and above
journalctl -u nginx -p err

# Kernel messages
journalctl -k

[Unit]
Description=My Application
After=network.target

[Service]
Type=simple
User=myapp
ExecStart=/usr/local/bin/myapp --config /etc/myapp/config.toml
Restart=on-failure
RestartSec=5

[Install]
WantedBy=multi-user.target

# After creating or modifying a unit file
sudo systemctl daemon-reload

# Then start it
sudo systemctl start myapp

# System memory overview
free -h

# Detailed per-process memory
ps aux --sort=-%mem | head -10

# Per-CPU stats
mpstat

# Top CPU consumers
ps aux --sort=-%cpu | head -10

# Real-time I/O stats
iostat -x 1

# Per-process I/O
iotop

# Info about process 1234
ls /proc/1234/

# Command that started the process
cat /proc/1234/cmdline | tr '\0' ' '

# Environment variables
cat /proc/1234/environ | tr '\0' '\n'

# File descriptors (open files)
ls -l /proc/1234/fd/

# Memory maps
cat /proc/1234/maps

cat /proc/cpuinfo      # CPU details
cat /proc/meminfo      # Memory details
cat /proc/uptime       # System uptime
cat /proc/loadavg      # Load averages