Linux Process Management: Control Running Applications

A process is a running instance of a program. Linux runs dozens of processes simultaneously (services, daemons, user applications). Process management means viewing, controlling, prioritizing, and terminating processes. Essential for system administration, troubleshooting, and optimization.

Process Basics

Process anatomy:**

• PID (Process ID): Unique number identifying each process
• Parent PID (PPID): Process that started this process
• User: Who owns/runs the process
• State: Running, Sleeping, Stopped, Zombie
• Priority: How much CPU to allocate (nice value)

Daemons vs interactive processes:**

• Daemon: Runs in background, no terminal (Apache, MySQL, SSH)
• Interactive: Runs in foreground with terminal input (bash, vim, node)

View Running Processes

Simple process list:**

ps          # Simple list of current shell's processes
ps aux      # All processes, detailed (A=all, u=user-oriented, x=no-terminal)
ps axjf     # Tree view showing parent-child relationships

ps output explained:**

USER  PID  %CPU %MEM  VSZ    RSS STAT START TIME COMMAND
root  1    0.0  0.1   19352  4032 Ss   12:00 0:01 /sbin/init

USER:   Who owns the process
PID:    Process ID
%CPU:   CPU usage percentage
%MEM:   Memory usage percentage
STAT:   Status (S=sleeping, R=running, Z=zombie, T=stopped)
START:  When process started
TIME:   CPU time used
COMMAND: Process name and arguments

Monitor real-time:**

top     # Interactive process monitor (press 'q' to exit)
htop    # Better version of top (if installed)

Foreground and Background

Run command in background:**

# Start with &
./long-running-script.sh &
[1] 12345  # Job number [1], PID 12345

# View background jobs
jobs

# Bring job to foreground
fg %1    # Brings job number 1 to foreground
fg %12345  # Or use PID

Suspend and resume:**

# Press Ctrl+Z in terminal to suspend (SIGSTOP)
^Z
[1]+ Stopped      vim file.txt

# Resume in background
bg %1

# Resume in foreground
fg %1

Process Signals

Signals tell processes to do things:**

Kill a process:**

# Graceful stop (recommended first)
kill -15 12345

# Wait 5 seconds, then force kill if needed
sleep 5 && kill -9 12345

# Kill by name (all instances)
pkill -f "node app.js"

Process Priority

Nice values range -20 (highest priority) to +19 (lowest):**

# Start process with low priority (don't hog CPU)
nice -n 10 ./backup-script.sh

# Change priority of running process
renice -n 5 -p 12345     # Set to nice value 5
renice -n -5 -p 12345    # Set to nice value -5 (higher priority)

View nice values:**

ps aux --sort=-nice
# NI column shows nice value

Systemd Service Management

Start/stop/restart services:**

systemctl start apache2
systemctl stop apache2
systemctl restart apache2
systemctl reload apache2   # Reload config without full restart

# Auto-start on boot
systemctl enable apache2
systemctl disable apache2

# Check status
systemctl status apache2
systemctl is-active apache2

View systemd services:**

systemctl list-units --type=service
systemctl list-units --type=service --state=failed  # Show failed services

Resource Limits

Limit process resource usage:**

# Limit to 500MB memory
ulimit -v 512000

# Limit CPU time to 1 hour
ulimit -t 3600

# Limit open files
ulimit -n 1024

# Run command with limits
systemd-run --scope -p MemoryLimit=500M /usr/bin/program

Systemd unit resource limits:**

# In /etc/systemd/system/myapp.service
[Service]
MemoryLimit=500M
CPUQuota=50%
TasksMax=100

Troubleshooting

Process not responding (zombie):**

• Show zombie processes: ps aux | grep Z
• Kill parent process: kill -9 [PPID]
• Reboot as last resort

Process consuming too much CPU/memory:**

• Identify: ps aux --sort=-%cpu | head -5
• Check why: ps auxww | grep PID (see full command line)
• Kill if needed: kill -9 PID
• Investigate: journalctl -u service-name (for systemd services)

Service won't start:**

# Check service status
systemctl status apache2

# View detailed errors
journalctl -u apache2 -n 50

# Test config manually
apache2ctl -t   # For Apache

Related: Systemd services | Bash scripting | Resource limits | Process monitoring

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