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Linux Full Tutorial -- Part 5: Process Management

By Suraj Ahir 2025-12-01 11 min read

Every running program in Linux is a process. Managing processes -- understanding what is running, how much resources each uses, how to control them, and how to make services start automatically -- is core Linux sysadmin and DevOps work. This part covers everything from the ps command to systemd service management.

Viewing Processes

ps aux                    # All processes with details
ps aux | grep nginx       # Find nginx processes
ps -ef                    # All processes, full format
pgrep nginx               # Get PID(s) of nginx

top                       # Live process monitor
# In top: q=quit k=kill r=renice Shift+M=sort by memory

htop                      # Better top (install first: apt install htop)
# In htop: F3=search F4=filter F9=kill F10=quit

watch -n 2 "ps aux | grep myapp"  # Repeat command every 2 seconds

Killing Processes

kill 1234               # SIGTERM (15) -- graceful shutdown
kill -9 1234            # SIGKILL -- force kill (cannot be caught)
kill -HUP 1234          # SIGHUP -- reload config
pkill nginx             # Kill by name
pkill -f "python app.py"  # Kill by full command
killall python3         # Kill all processes named python3

# Find and kill process using a port
lsof -i :8080
fuser -k 8080/tcp       # Kill process using TCP port 8080

Background Processes

./script.sh &            # Run in background
nohup ./script.sh &      # Immune to hangup (survives SSH disconnect)
screen -S mysession      # Create a named screen session
screen -r mysession      # Resume session
tmux new -s work         # Create tmux session
tmux attach -t work      # Attach to session

jobs                     # List background jobs in current shell
bg 1                     # Resume job 1 in background
fg 1                     # Bring job 1 to foreground
Ctrl+Z                   # Suspend current process

systemd Service Management

systemctl status nginx             # Check service status
systemctl start nginx              # Start service
systemctl stop nginx               # Stop service
systemctl restart nginx            # Stop and start
systemctl reload nginx             # Reload config (no downtime)
systemctl enable nginx             # Start on boot
systemctl disable nginx            # Do not start on boot
systemctl is-active nginx          # Is it running?
systemctl list-units --type=service  # All services

# View service logs
journalctl -u nginx                # All logs for nginx
journalctl -u nginx --since "1 hour ago"  # Recent logs
journalctl -f -u nginx             # Follow live logs

Creating a systemd Service

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

[Service]
Type=simple
User=ubuntu
WorkingDirectory=/home/ubuntu/myapp
ExecStart=/home/ubuntu/myapp/venv/bin/python app.py
Restart=always
RestartSec=5
Environment=PORT=8000
Environment=LOG_LEVEL=INFO

[Install]
WantedBy=multi-user.target

# After creating the file:
systemctl daemon-reload
systemctl enable myapp
systemctl start myapp

Frequently Asked Questions

In Part 6, we cover Linux networking commands for configuring and debugging network connectivity.

Written by

Suraj Ahir

Cloud & DevOps engineer running four live production services on my own AWS infrastructure. I write everything on this site myself — no ghostwriters, no AI filler.

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Linux OOM Killer and Memory Management

# OOM (Out Of Memory) killer: Linux kills processes when RAM is exhausted
# Check if OOM killer has run recently
dmesg | grep -i "oom\|killed process"
grep "Out of memory" /var/log/syslog | tail -10

# See OOM score of processes (higher = more likely to be killed)
cat /proc/$(pgrep nginx)/oom_score
cat /proc/$(pgrep postgres)/oom_score

# Protect critical processes from OOM killer
echo -1000 | sudo tee /proc/$(pgrep nginx)/oom_score_adj   # Never kill
echo 500  | sudo tee /proc/$(pgrep chrome)/oom_score_adj   # Kill this first

# Memory usage by process
ps aux --sort=-%mem | head -10
cat /proc/meminfo              # Detailed memory information

ulimit: Resource Limits Per Process

ulimit -a                     # Show all limits for current shell

# Common limits
ulimit -n 65536               # Open files limit (critical for web servers)
ulimit -u 4096                # Max processes
ulimit -m 1048576             # Max memory (KB)

# Permanent limits: /etc/security/limits.conf
*    soft nofile 65536
*    hard nofile 65536
nginx soft nofile 131072
nginx hard nofile 131072

# Per-service in systemd:
# [Service]
# LimitNOFILE=65536
# LimitNPROC=4096

strace: Tracing System Calls

# Trace a command
strace ls /tmp 2>&1 | head -30

# Trace a running process
strace -p 1234

# Only show file-related calls
strace -e trace=file ls /tmp

# Trace with timestamps
strace -t ls /tmp

# Find why a process is slow (show time per syscall)
strace -c -p 1234           # Summary after Ctrl+C

# Find what files a process is reading
strace -e openat ls /tmp 2>&1 | grep "openat"