Linux Process Management¶

Overview¶

Process management is essential for monitoring, controlling, and troubleshooting running applications and system services. Understanding how to view, manage, and terminate processes is crucial for system administration and DevOps work.

What is a Process?¶

A process is an instance of a running program. Each process has: - PID (Process ID): Unique identifier - PPID (Parent Process ID): ID of the parent process - User: Owner of the process - State: Running, sleeping, stopped, or zombie - Resources: CPU, memory, file descriptors

Viewing Running Processes¶

`ps` - Process Status¶

Display currently running processes.

Basic usage:

ps                    # Show processes for current user in current terminal
ps aux                # Show all processes with detailed info
ps -ef                # Show all processes in full format
ps -u username        # Show processes for specific user

Common options: - a - Show processes for all users - u - Display user-oriented format - x - Show processes not attached to a terminal - -e - Select all processes - -f - Full format listing

Example output:

ps aux
# USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
# root         1  0.0  0.1 169416 11680 ?        Ss   Jan01   0:05 /sbin/init
# www-data  1234  2.5  1.2 456789 98765 ?        S    10:30   1:23 nginx: worker

`top` - Real-time Process Monitor¶

Monitor system processes dynamically in real-time.

Usage:

top                   # Launch interactive process viewer
top -u username       # Show processes for specific user
top -p PID            # Monitor specific process

Interactive commands (while top is running): - k - Kill a process (prompts for PID) - r - Renice a process (change priority) - M - Sort by memory usage - P - Sort by CPU usage - q - Quit top - h - Help menu

Key metrics: - %CPU: CPU usage percentage - %MEM: Memory usage percentage - TIME+: Total CPU time used - COMMAND: Process name/command

`htop` - Enhanced Process Viewer¶

More user-friendly alternative to top (requires installation).

# Install htop
sudo apt install htop      # Debian/Ubuntu
sudo yum install htop      # RHEL/CentOS

# Run htop
htop

Features: - Color-coded display - Mouse support - Tree view of processes - Easy process killing and renicing

Process States¶

State	Symbol	Description
Running	R	Currently executing or ready to run
Sleeping	S	Waiting for an event (interruptible)
Uninterruptible Sleep	D	Waiting for I/O (cannot be interrupted)
Stopped	T	Stopped by job control signal
Zombie	Z	Terminated but not reaped by parent

Terminating Processes¶

`kill` - Send Signal to Process¶

Terminate or signal a process using its PID.

Basic syntax:

kill PID              # Send SIGTERM (graceful termination)
kill -9 PID           # Send SIGKILL (force kill)
kill -15 PID          # Send SIGTERM (same as default)
kill -STOP PID        # Pause process
kill -CONT PID        # Resume paused process

Common signals: | Signal | Number | Description | |--------|--------|-------------| | SIGTERM | 15 | Graceful termination (default) | | SIGKILL | 9 | Force kill (cannot be caught) | | SIGHUP | 1 | Hangup (reload config) | | SIGINT | 2 | Interrupt (Ctrl+C) | | SIGSTOP | 19 | Stop process | | SIGCONT | 18 | Continue stopped process |

Examples:

# Find and kill a process
ps aux | grep nginx
kill 1234

# Force kill unresponsive process
kill -9 1234

# Kill multiple processes
kill 1234 5678 9012

# Reload service configuration
kill -HUP $(cat /var/run/nginx.pid)

`killall` - Kill by Process Name¶

Terminate all processes matching a name.

killall process_name          # Kill all instances
killall -9 process_name       # Force kill all instances
killall -u username           # Kill all processes for user

Examples:

killall firefox               # Close all Firefox instances
killall -9 chrome             # Force kill all Chrome processes

`pkill` - Kill by Pattern¶

More flexible process killing using patterns.

pkill pattern                 # Kill processes matching pattern
pkill -u username             # Kill all processes for user
pkill -9 pattern              # Force kill matching processes

Examples:

pkill -f "python.*script.py"  # Kill Python processes running script.py
pkill -u testuser             # Kill all processes owned by testuser

Process Priority and Nice Values¶

Understanding Nice Values¶

Range: -20 (highest priority) to 19 (lowest priority)
Default: 0
Only root can set negative values

`nice` - Start Process with Priority¶

nice -n 10 command            # Start with lower priority
nice -n -5 command            # Start with higher priority (requires root)

`renice` - Change Running Process Priority¶

renice -n 5 -p PID            # Change priority of specific process
renice -n 10 -u username      # Change priority for all user processes

Examples:

# Start backup with low priority
nice -n 15 tar -czf backup.tar.gz /data

# Increase priority of critical process
sudo renice -n -10 -p 1234

Background and Foreground Jobs¶

Job Control¶

command &                     # Run command in background
jobs                          # List background jobs
fg %1                         # Bring job 1 to foreground
bg %1                         # Resume job 1 in background
Ctrl+Z                        # Suspend current foreground job

Examples:

# Start long-running process in background
./long_script.sh &

# List jobs
jobs
# [1]+  Running    ./long_script.sh &

# Bring to foreground
fg %1

# Suspend and resume
Ctrl+Z                        # Suspend
bg %1                         # Resume in background

Advanced Process Management¶

`pgrep` - Find Process IDs¶

pgrep process_name            # Get PIDs matching name
pgrep -u username             # Get PIDs for user
pgrep -l pattern              # List PIDs with names

`pidof` - Find PID of Running Program¶

pidof nginx                   # Get PID(s) of nginx
pidof -s nginx                # Get single PID

`/proc` Filesystem¶

Access process information directly.

# View process info
cat /proc/PID/status          # Process status
cat /proc/PID/cmdline         # Command line
cat /proc/PID/environ         # Environment variables
ls -l /proc/PID/fd            # Open file descriptors

Monitoring Process Resources¶

`pstree` - Display Process Tree¶

pstree                        # Show process hierarchy
pstree -p                     # Show with PIDs
pstree username               # Show for specific user

`lsof` - List Open Files¶

lsof                          # List all open files
lsof -p PID                   # Files opened by process
lsof -u username              # Files opened by user
lsof -i :80                   # Processes using port 80

Practical Examples¶

Find and Kill Hung Process¶

# Find the process
ps aux | grep hung_app
# or
pgrep -l hung_app

# Try graceful termination
kill 1234

# Wait a few seconds, then force kill if needed
kill -9 1234

Monitor Specific Application¶

# Watch process continuously
watch -n 1 'ps aux | grep myapp'

# Monitor with top
top -p $(pgrep myapp)

Kill All Processes for User¶

# List processes first
ps -u username

# Kill all user processes
pkill -u username
# or
killall -u username

Find Resource-Intensive Processes¶

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

# Top memory consumers
ps aux --sort=-%mem | head -10

Best Practices¶

Always try graceful termination first - Use kill (SIGTERM) before kill -9 (SIGKILL)
Verify before killing - Double-check PID to avoid killing wrong process
Monitor after changes - Ensure process terminated and didn't respawn
Use appropriate tools - killall for names, pkill for patterns, kill for specific PIDs
Check dependencies - Killing parent process may affect child processes
Document critical processes - Keep track of important PIDs and their purposes

Troubleshooting¶

Zombie Processes¶

# Find zombie processes
ps aux | grep 'Z'

# Zombies can't be killed directly - kill parent process
ps -o ppid= -p ZOMBIE_PID
kill PARENT_PID

Process Won't Die¶

# Check process state
ps aux | grep PID

# If in 'D' state (uninterruptible sleep), wait for I/O to complete
# If still stuck, may need system reboot

# Check what process is waiting for
lsof -p PID

Too Many Processes¶

# Check process limits
ulimit -u

# Count processes per user
ps aux | awk '{print $1}' | sort | uniq -c | sort -rn

# Increase limits (temporary)
ulimit -u 4096

System Monitoring - Resource usage and performance
Systemctl Services - Managing system services
Shell Scripting - Automating process management

Linux Process Management¶

Overview¶

What is a Process?¶

Viewing Running Processes¶

ps - Process Status¶

top - Real-time Process Monitor¶

htop - Enhanced Process Viewer¶

Process States¶

Terminating Processes¶

kill - Send Signal to Process¶

killall - Kill by Process Name¶

pkill - Kill by Pattern¶

Process Priority and Nice Values¶

Understanding Nice Values¶

nice - Start Process with Priority¶

renice - Change Running Process Priority¶

Background and Foreground Jobs¶

Job Control¶

Advanced Process Management¶

pgrep - Find Process IDs¶

pidof - Find PID of Running Program¶

/proc Filesystem¶

Monitoring Process Resources¶

pstree - Display Process Tree¶

lsof - List Open Files¶

Practical Examples¶

Find and Kill Hung Process¶

Monitor Specific Application¶

Kill All Processes for User¶

Find Resource-Intensive Processes¶

Best Practices¶

Troubleshooting¶

Zombie Processes¶

Process Won't Die¶

Too Many Processes¶

Related Topics¶

`ps` - Process Status¶

`top` - Real-time Process Monitor¶

`htop` - Enhanced Process Viewer¶

`kill` - Send Signal to Process¶

`killall` - Kill by Process Name¶

`pkill` - Kill by Pattern¶

`nice` - Start Process with Priority¶

`renice` - Change Running Process Priority¶

`pgrep` - Find Process IDs¶

`pidof` - Find PID of Running Program¶

`/proc` Filesystem¶

`pstree` - Display Process Tree¶

`lsof` - List Open Files¶