⚙️ Process Management: Controlling System Resources

Master process control, monitoring, and system resource management in Linux

📖 What You'll Learn

Process management is essential for system administration and troubleshooting. This chapter covers everything you need to know about controlling processes and system resources:

Understanding processes and process hierarchy
Monitoring running processes with ps, top, and htop
Starting, stopping, and controlling processes
Background and foreground job management
Process signals and communication
System resource monitoring
Service management with systemd
Performance optimization and troubleshooting

🌍 Why This Matters

Critical applications:

System Administration: Monitor and control system resources
Troubleshooting: Identify and resolve performance issues
Security: Detect suspicious processes and resource abuse
Automation: Manage services and background tasks
Performance: Optimize system resource utilization

🔍 Understanding Processes

Process Basics

bash

# Every running program is a process
# Each process has:
# - PID (Process ID): Unique identifier
# - PPID (Parent Process ID): Parent process
# - UID/GID: User and group ownership
# - State: Running, sleeping, stopped, zombie
# - Priority: CPU scheduling priority
# - Memory usage: RAM and virtual memory

# View your current shell process
$ echo $$
1234

# View parent process ID
$ echo $PPID
1000

# Process hierarchy example:
# systemd (PID 1)
# ├── sshd (PID 500)
# │   └── sshd (PID 1000) - your SSH session
# │       └── bash (PID 1234) - your shell
# │           └── ps (PID 1500) - command you're running

Process States

bash

# Process states in Linux:
# R - Running or runnable
# S - Interruptible sleep (waiting for event)
# D - Uninterruptible sleep (usually I/O)
# T - Stopped (by signal or debugger)
# Z - Zombie (finished but not cleaned up)
# X - Dead (should never be seen)

# View process states
$ ps aux | head -5
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  0.0  0.1 225316  9876 ?        Ss   10:00   0:01 /sbin/init
root         2  0.0  0.0      0     0 ?        S    10:00   0:00 [kthreadd]
root         3  0.0  0.0      0     0 ?        I<   10:00   0:00 [rcu_gp]
user      1234  0.0  0.2  21532  5432 pts/0    Ss   10:30   0:00 -bash

📊 Monitoring Processes

Using `ps` Command

bash

# Basic process listing
$ ps
  PID TTY          TIME CMD
 1234 pts/0    00:00:00 bash
 1567 pts/0    00:00:00 ps

# Show all processes
$ ps aux
$ ps -ef

# Show process tree
$ ps auxf
$ ps -ef --forest
$ pstree

# Show processes for specific user
$ ps -u username
$ ps aux | grep username

# Show processes by command name
$ ps aux | grep nginx
$ pgrep nginx
$ pgrep -l nginx  # Show PID and name

# Show process hierarchy
$ ps -ejH
$ ps axjf

# Custom output format
$ ps -eo pid,ppid,cmd,%mem,%cpu --sort=-%cpu
  PID  PPID CMD                         %MEM %CPU
 1500  1234 firefox                     15.2 25.3
 1600  1234 chrome                      12.1 18.7
 1234     1 bash                         0.2  0.1

Real-time Monitoring with `top`

bash

# Basic top usage
$ top

top - 10:30:45 up 2 days,  3:15,  2 users,  load average: 0.15, 0.25, 0.30
Tasks: 125 total,   1 running, 124 sleeping,   0 stopped,   0 zombie
%Cpu(s):  2.3 us,  1.2 sy,  0.0 ni, 96.2 id,  0.3 wa,  0.0 hi,  0.0 si,  0.0 st
MiB Mem :   8192.0 total,   2048.5 free,   3072.2 used,   3071.3 buff/cache
MiB Swap:   2048.0 total,   2048.0 free,      0.0 used.   4608.1 avail Mem

  PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND
 1500 user      20   0 2097152 524288  65536 S  25.3  15.2   5:23.45 firefox
 1600 user      20   0 1572864 393216  49152 S  18.7  12.1   3:45.67 chrome
 1234 user      20   0   21532   5432   3456 S   0.1   0.2   0:00.12 bash

# Top interactive commands:
# q - quit
# k - kill process (enter PID)
# r - renice process (change priority)
# u - show processes for specific user
# M - sort by memory usage
# P - sort by CPU usage
# T - sort by running time
# 1 - show individual CPU cores
# h - help

# Top with specific options
$ top -u username     # Show processes for specific user
$ top -p 1234,5678    # Monitor specific PIDs
$ top -n 1            # Run once and exit
$ top -b -n 1         # Batch mode (good for scripts)

Enhanced Monitoring with `htop`

bash

# Install htop (if not available)
$ sudo apt install htop  # Ubuntu/Debian
$ sudo yum install htop  # CentOS/RHEL

# Run htop
$ htop

# htop features:
# - Color-coded display
# - Mouse support
# - Tree view (F5)
# - Search (F3)
# - Filter (F4)
# - Kill process (F9)
# - Nice/renice (F7/F8)
# - Setup (F2)

# htop command line options
$ htop -u username    # Show processes for specific user
$ htop -p 1234,5678   # Monitor specific PIDs
$ htop -t             # Tree view
$ htop -s PERCENT_CPU # Sort by CPU usage

System Resource Monitoring

bash

# Memory usage
$ free -h
              total        used        free      shared  buff/cache   available
Mem:          8.0Gi       3.0Gi       2.0Gi       256Mi       3.0Gi       4.6Gi
Swap:         2.0Gi          0B       2.0Gi

# Detailed memory information
$ cat /proc/meminfo | head -10
MemTotal:        8388608 kB
MemFree:         2097152 kB
MemAvailable:    4718592 kB
Buffers:          524288 kB
Cached:          2621440 kB

# CPU information
$ lscpu
Architecture:        x86_64
CPU op-mode(s):      32-bit, 64-bit
Byte Order:          Little Endian
CPU(s):              4
On-line CPU(s) list: 0-3
Thread(s) per core:  2
Core(s) per socket:  2
Socket(s):           1

# Load average
$ uptime
 10:30:45 up 2 days,  3:15,  2 users,  load average: 0.15, 0.25, 0.30

$ cat /proc/loadavg
0.15 0.25 0.30 2/125 1567

# I/O statistics
$ iostat 1 3  # 1-second intervals, 3 times
Linux 5.4.0 (hostname)     12/15/2023      _x86_64_        (4 CPU)

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           2.50    0.00    1.25    0.25    0.00   96.00

Device             tps    kB_read/s    kB_wrtn/s    kB_read    kB_wrtn
sda               5.25        125.50        75.25     125500      75250

🎮 Process Control

Starting and Stopping Processes

bash

# Start process in foreground
$ long_running_command

# Start process in background
$ long_running_command &
[1] 1234

# Start with nohup (survives logout)
$ nohup long_running_command &
[1] 1234
nohup: ignoring input and appending output to 'nohup.out'

# Start with custom output redirection
$ nohup long_running_command > output.log 2>&1 &

# Disown process (remove from job control)
$ long_running_command &
$ disown %1

# Start process with specific priority
$ nice -n 10 cpu_intensive_task  # Lower priority
$ nice -n -5 important_task      # Higher priority (requires sudo)

Job Control

bash

# List active jobs
$ jobs
[1]+  Running                 long_running_command &
[2]-  Stopped                 vim file.txt

# Bring job to foreground
$ fg %1        # Bring job 1 to foreground
$ fg           # Bring most recent job to foreground

# Send job to background
$ bg %1        # Send job 1 to background
$ bg           # Send most recent job to background

# Suspend current foreground process
# Press Ctrl+Z
$ vim file.txt
^Z
[1]+  Stopped                 vim file.txt

# Resume suspended job in background
$ bg %1
[1]+ vim file.txt &

# Kill job
$ kill %1      # Kill job 1
$ kill %2      # Kill job 2

Process Signals

bash

# Common signals:
# SIGTERM (15) - Graceful termination (default)
# SIGKILL (9)  - Force kill (cannot be caught)
# SIGSTOP (19) - Stop process (cannot be caught)
# SIGCONT (18) - Continue stopped process
# SIGHUP (1)   - Hangup (often used to reload config)
# SIGINT (2)   - Interrupt (Ctrl+C)
# SIGQUIT (3)  - Quit (Ctrl+\)

# Send signals to processes
$ kill 1234           # Send SIGTERM to PID 1234
$ kill -9 1234        # Send SIGKILL to PID 1234
$ kill -TERM 1234     # Send SIGTERM (same as default)
$ kill -HUP 1234      # Send SIGHUP (reload config)
$ kill -STOP 1234     # Stop process
$ kill -CONT 1234     # Continue process

# Kill processes by name
$ killall firefox     # Kill all firefox processes
$ killall -9 firefox  # Force kill all firefox processes
$ pkill firefox       # Kill processes matching pattern
$ pkill -f "python script.py"  # Kill by full command line

# Kill processes by user
$ pkill -u username   # Kill all processes by user
$ sudo pkill -u username  # Kill as root

# Interactive process killing
$ top                 # Press 'k' and enter PID
$ htop                # Press F9 and select signal

Process Priority Management

bash

# View process priorities
$ ps -eo pid,ni,pri,pcpu,comm
  PID  NI PRI %CPU COMMAND
 1234   0  20  0.1 bash
 1500  10  30  5.2 backup_script
 1600 -10  10 15.3 important_app

# Change priority of running process
$ sudo renice 10 1234      # Lower priority (higher nice value)
$ sudo renice -5 1234      # Higher priority (lower nice value)
$ sudo renice 0 1234       # Normal priority

# Change priority by process name
$ sudo renice 10 $(pgrep firefox)

# Start process with specific priority
$ nice -n 19 backup_script.sh    # Lowest priority
$ nice -n 0 normal_script.sh     # Normal priority
$ sudo nice -n -20 critical_app  # Highest priority

# Real-time priority (use with caution)
$ sudo chrt -f 50 critical_realtime_app  # FIFO scheduling
$ sudo chrt -r 50 critical_realtime_app  # Round-robin scheduling

🔧 Service Management with systemd

Basic Service Operations

bash

# Check service status
$ sudo systemctl status nginx
● nginx.service - A high performance web server and a reverse proxy server
   Loaded: loaded (/lib/systemd/system/nginx.service; enabled; vendor preset: enabled)
   Active: active (running) since Mon 2023-12-15 10:00:00 UTC; 2h 30min ago
     Docs: man:nginx(8)
  Process: 1234 ExecStartPre=/usr/sbin/nginx -t -q -g daemon on; master_process on; (code=exited, status=0/SUCCESS)
  Process: 1235 ExecStart=/usr/sbin/nginx -g daemon on; master_process on; (code=exited, status=0/SUCCESS)
 Main PID: 1236 (nginx)
    Tasks: 5 (limit: 4915)
   Memory: 15.2M
   CGroup: /system.slice/nginx.service
           ├─1236 nginx: master process /usr/sbin/nginx -g daemon on; master_process on;
           ├─1237 nginx: worker process
           └─1238 nginx: worker process

# Start/stop/restart services
$ sudo systemctl start nginx
$ sudo systemctl stop nginx
$ sudo systemctl restart nginx
$ sudo systemctl reload nginx    # Reload config without restart

# Enable/disable services (auto-start at boot)
$ sudo systemctl enable nginx
$ sudo systemctl disable nginx

# Check if service is enabled
$ systemctl is-enabled nginx
enabled

# Check if service is active
$ systemctl is-active nginx
active

Service Management Commands

bash

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

# List all unit files
$ systemctl list-unit-files --type=service

# Show service dependencies
$ systemctl list-dependencies nginx

# View service logs
$ sudo journalctl -u nginx
$ sudo journalctl -u nginx -f        # Follow logs
$ sudo journalctl -u nginx --since today
$ sudo journalctl -u nginx --since "2023-12-15 10:00:00"

# Mask/unmask services (prevent starting)
$ sudo systemctl mask nginx
$ sudo systemctl unmask nginx

# Edit service configuration
$ sudo systemctl edit nginx          # Create override file
$ sudo systemctl edit --full nginx   # Edit full unit file

# Reload systemd configuration
$ sudo systemctl daemon-reload

Creating Custom Services

bash

# Create a simple service file
$ sudo nano /etc/systemd/system/myapp.service

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

[Service]
Type=simple
User=myuser
WorkingDirectory=/opt/myapp
ExecStart=/opt/myapp/start.sh
ExecReload=/bin/kill -HUP $MAINPID
Restart=always
RestartSec=10

[Install]
WantedBy=multi-user.target

# Enable and start the service
$ sudo systemctl daemon-reload
$ sudo systemctl enable myapp
$ sudo systemctl start myapp
$ sudo systemctl status myapp

# Service types:
# simple - Default, process doesn't fork
# forking - Process forks and parent exits
# oneshot - Process exits after completion
# notify - Process sends notification when ready
# idle - Waits for other jobs to complete

📈 Performance Monitoring and Optimization

System Performance Analysis

bash

# CPU usage over time
$ sar -u 1 10         # CPU usage every 1 second, 10 times
Linux 5.4.0 (hostname)     12/15/2023      _x86_64_        (4 CPU)

10:30:01 AM     CPU     %user     %nice   %system   %iowait    %steal     %idle
10:30:02 AM     all      2.50      0.00      1.25      0.25      0.00     96.00
10:30:03 AM     all      3.75      0.00      1.50      0.50      0.00     94.25

# Memory usage over time
$ sar -r 1 5          # Memory usage every 1 second, 5 times

# I/O statistics
$ sar -b 1 5          # I/O statistics
$ sar -d 1 5          # Disk statistics

# Network statistics
$ sar -n DEV 1 5      # Network device statistics

# Generate system activity report
$ sar -A > system_report.txt

Process Resource Usage

bash

# Detailed process information
$ cat /proc/1234/status
Name:	firefox
State:	S (sleeping)
Pid:	1234
PPid:	1000
VmPeak:	 2097152 kB
VmSize:	 1572864 kB
VmRSS:	  524288 kB

# Process memory maps
$ cat /proc/1234/maps | head -5
7f8b4c000000-7f8b4c021000 rw-p 00000000 00:00 0
7f8b4c021000-7f8b50000000 ---p 00000000 00:00 0
7f8b50000000-7f8b50021000 rw-p 00000000 00:00 0

# Process file descriptors
$ ls -l /proc/1234/fd/
total 0
lrwx------ 1 user user 64 Dec 15 10:30 0 -> /dev/pts/0
lrwx------ 1 user user 64 Dec 15 10:30 1 -> /dev/pts/0
lrwx------ 1 user user 64 Dec 15 10:30 2 -> /dev/pts/0

# Process environment
$ cat /proc/1234/environ | tr '\0' '\n' | head -5
PATH=/usr/local/bin:/usr/bin:/bin
HOME=/home/user
USER=user
SHELL=/bin/bash
TERM=xterm-256color

# Process command line
$ cat /proc/1234/cmdline
firefox--no-remote--profile

Resource Monitoring Scripts

bash

#!/bin/bash
# system-monitor.sh - Comprehensive system monitoring

echo "=== System Performance Monitor ==="
echo "Date: $(date)"
echo

# CPU usage
echo "CPU Usage:"
top -bn1 | grep "Cpu(s)" | awk '{print "User: " $2 ", System: " $4 ", Idle: " $8}'
echo

# Memory usage
echo "Memory Usage:"
free -h | grep -E "Mem|Swap"
echo

# Load average
echo "Load Average:"
uptime | awk -F'load average:' '{print $2}'
echo

# Top 5 CPU consumers
echo "Top 5 CPU Consumers:"
ps aux --sort=-%cpu | head -6 | awk '{print $11 " (" $3 "%)"}'  | tail -5
echo

# Top 5 Memory consumers
echo "Top 5 Memory Consumers:"
ps aux --sort=-%mem | head -6 | awk '{print $11 " (" $4 "%)"}'  | tail -5
echo

# Disk usage
echo "Disk Usage:"
df -h | grep -E "^/dev"
echo

echo "=== Monitor Complete ==="

Performance Optimization Tips

bash

# Find resource-intensive processes
$ ps aux --sort=-%cpu | head -10    # Top CPU users
$ ps aux --sort=-%mem | head -10    # Top memory users

# Find processes with many open files
$ lsof | awk '{print $2}' | sort | uniq -c | sort -nr | head -10

# Monitor process creation
$ sudo sysctl kernel.fork_rate
$ watch -n 1 'ps aux | wc -l'

# Check for zombie processes
$ ps aux | grep -E "<defunct>|Z"

# Monitor system calls
$ strace -p 1234          # Trace system calls for PID 1234
$ strace -c command       # Count system calls

# Profile CPU usage
$ perf top                # Real-time CPU profiling
$ perf record command     # Record performance data
$ perf report             # Analyze recorded data

🧠 Knowledge Check

Quick Quiz

What's the difference between kill and killall?

Answer

kill terminates a specific process by PID, while killall terminates all processes with a specific name.
How do you start a process that survives logout?
Answer
bash
```
nohup command &
# or
command &
disown
```
What does a load average of 2.0 mean on a 4-core system?

Answer

The system is 50% utilized. Load average represents the number of processes waiting for CPU time. 2.0 on a 4-core system means 2 cores are fully utilized on average.

How do you change the priority of a running process?

Answer

bash

sudo renice 10 PID    # Lower priority
sudo renice -5 PID    # Higher priority

Hands-On Challenges

Challenge 1: Process Monitoring Dashboard

bash

# Create a script that displays:
# - Top 5 CPU-consuming processes
# - Top 5 memory-consuming processes
# - Current load average
# - Available memory
# - Number of running processes

Challenge 2: Service Management

bash

# Create a custom systemd service for a simple application
# Configure it to:
# - Start automatically at boot
# - Restart on failure
# - Log to a specific file
# - Run as a non-root user

Challenge 3: Performance Troubleshooting

bash

# Simulate a high CPU load and:
# - Identify the problematic process
# - Reduce its priority
# - Monitor the impact
# - Create a script to automatically detect and handle such situations

🚀 Next Steps

Excellent! You've mastered Linux process management. You can now:

Monitor system processes and resource usage
Control process execution and priority
Manage background jobs effectively
Work with systemd services
Troubleshoot performance issues
Optimize system resource utilization

Ready for networking fundamentals? Continue to 05-networking-basics.md to learn about network concepts and protocols.

Pro Tip: Use htop for interactive process management, systemctl for service control, and always check system load before making changes. Remember that killing processes with -9 should be a last resort - try graceful termination first! ⚙️

⚙️ Process Management: Controlling System Resources

Master process control, monitoring, and system resource management in Linux

📖 What You'll Learn

Process management is essential for system administration and troubleshooting. This chapter covers everything you need to know about controlling processes and system resources:

Understanding processes and process hierarchy
Monitoring running processes with ps, top, and htop
Starting, stopping, and controlling processes
Background and foreground job management
Process signals and communication
System resource monitoring
Service management with systemd
Performance optimization and troubleshooting

🌍 Why This Matters

Critical applications:

System Administration: Monitor and control system resources
Troubleshooting: Identify and resolve performance issues
Security: Detect suspicious processes and resource abuse
Automation: Manage services and background tasks
Performance: Optimize system resource utilization

🔍 Understanding Processes

Process Basics

bash

# Every running program is a process
# Each process has:
# - PID (Process ID): Unique identifier
# - PPID (Parent Process ID): Parent process
# - UID/GID: User and group ownership
# - State: Running, sleeping, stopped, zombie
# - Priority: CPU scheduling priority
# - Memory usage: RAM and virtual memory

# View your current shell process
$ echo $$
1234

# View parent process ID
$ echo $PPID
1000

# Process hierarchy example:
# systemd (PID 1)
# ├── sshd (PID 500)
# │   └── sshd (PID 1000) - your SSH session
# │       └── bash (PID 1234) - your shell
# │           └── ps (PID 1500) - command you're running

Process States

bash

# Process states in Linux:
# R - Running or runnable
# S - Interruptible sleep (waiting for event)
# D - Uninterruptible sleep (usually I/O)
# T - Stopped (by signal or debugger)
# Z - Zombie (finished but not cleaned up)
# X - Dead (should never be seen)

# View process states
$ ps aux | head -5
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  0.0  0.1 225316  9876 ?        Ss   10:00   0:01 /sbin/init
root         2  0.0  0.0      0     0 ?        S    10:00   0:00 [kthreadd]
root         3  0.0  0.0      0     0 ?        I<   10:00   0:00 [rcu_gp]
user      1234  0.0  0.2  21532  5432 pts/0    Ss   10:30   0:00 -bash

📊 Monitoring Processes

Using `ps` Command

bash

# Basic process listing
$ ps
  PID TTY          TIME CMD
 1234 pts/0    00:00:00 bash
 1567 pts/0    00:00:00 ps

# Show all processes
$ ps aux
$ ps -ef

# Show process tree
$ ps auxf
$ ps -ef --forest
$ pstree

# Show processes for specific user
$ ps -u username
$ ps aux | grep username

# Show processes by command name
$ ps aux | grep nginx
$ pgrep nginx
$ pgrep -l nginx  # Show PID and name

# Show process hierarchy
$ ps -ejH
$ ps axjf

# Custom output format
$ ps -eo pid,ppid,cmd,%mem,%cpu --sort=-%cpu
  PID  PPID CMD                         %MEM %CPU
 1500  1234 firefox                     15.2 25.3
 1600  1234 chrome                      12.1 18.7
 1234     1 bash                         0.2  0.1

Real-time Monitoring with `top`

bash

# Basic top usage
$ top

top - 10:30:45 up 2 days,  3:15,  2 users,  load average: 0.15, 0.25, 0.30
Tasks: 125 total,   1 running, 124 sleeping,   0 stopped,   0 zombie
%Cpu(s):  2.3 us,  1.2 sy,  0.0 ni, 96.2 id,  0.3 wa,  0.0 hi,  0.0 si,  0.0 st
MiB Mem :   8192.0 total,   2048.5 free,   3072.2 used,   3071.3 buff/cache
MiB Swap:   2048.0 total,   2048.0 free,      0.0 used.   4608.1 avail Mem

  PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND
 1500 user      20   0 2097152 524288  65536 S  25.3  15.2   5:23.45 firefox
 1600 user      20   0 1572864 393216  49152 S  18.7  12.1   3:45.67 chrome
 1234 user      20   0   21532   5432   3456 S   0.1   0.2   0:00.12 bash

# Top interactive commands:
# q - quit
# k - kill process (enter PID)
# r - renice process (change priority)
# u - show processes for specific user
# M - sort by memory usage
# P - sort by CPU usage
# T - sort by running time
# 1 - show individual CPU cores
# h - help

# Top with specific options
$ top -u username     # Show processes for specific user
$ top -p 1234,5678    # Monitor specific PIDs
$ top -n 1            # Run once and exit
$ top -b -n 1         # Batch mode (good for scripts)

Enhanced Monitoring with `htop`

bash

# Install htop (if not available)
$ sudo apt install htop  # Ubuntu/Debian
$ sudo yum install htop  # CentOS/RHEL

# Run htop
$ htop

# htop features:
# - Color-coded display
# - Mouse support
# - Tree view (F5)
# - Search (F3)
# - Filter (F4)
# - Kill process (F9)
# - Nice/renice (F7/F8)
# - Setup (F2)

# htop command line options
$ htop -u username    # Show processes for specific user
$ htop -p 1234,5678   # Monitor specific PIDs
$ htop -t             # Tree view
$ htop -s PERCENT_CPU # Sort by CPU usage

System Resource Monitoring

bash

# Memory usage
$ free -h
              total        used        free      shared  buff/cache   available
Mem:          8.0Gi       3.0Gi       2.0Gi       256Mi       3.0Gi       4.6Gi
Swap:         2.0Gi          0B       2.0Gi

# Detailed memory information
$ cat /proc/meminfo | head -10
MemTotal:        8388608 kB
MemFree:         2097152 kB
MemAvailable:    4718592 kB
Buffers:          524288 kB
Cached:          2621440 kB

# CPU information
$ lscpu
Architecture:        x86_64
CPU op-mode(s):      32-bit, 64-bit
Byte Order:          Little Endian
CPU(s):              4
On-line CPU(s) list: 0-3
Thread(s) per core:  2
Core(s) per socket:  2
Socket(s):           1

# Load average
$ uptime
 10:30:45 up 2 days,  3:15,  2 users,  load average: 0.15, 0.25, 0.30

$ cat /proc/loadavg
0.15 0.25 0.30 2/125 1567

# I/O statistics
$ iostat 1 3  # 1-second intervals, 3 times
Linux 5.4.0 (hostname)     12/15/2023      _x86_64_        (4 CPU)

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           2.50    0.00    1.25    0.25    0.00   96.00

Device             tps    kB_read/s    kB_wrtn/s    kB_read    kB_wrtn
sda               5.25        125.50        75.25     125500      75250

🎮 Process Control

Starting and Stopping Processes

bash

# Start process in foreground
$ long_running_command

# Start process in background
$ long_running_command &
[1] 1234

# Start with nohup (survives logout)
$ nohup long_running_command &
[1] 1234
nohup: ignoring input and appending output to 'nohup.out'

# Start with custom output redirection
$ nohup long_running_command > output.log 2>&1 &

# Disown process (remove from job control)
$ long_running_command &
$ disown %1

# Start process with specific priority
$ nice -n 10 cpu_intensive_task  # Lower priority
$ nice -n -5 important_task      # Higher priority (requires sudo)

Job Control

bash

# List active jobs
$ jobs
[1]+  Running                 long_running_command &
[2]-  Stopped                 vim file.txt

# Bring job to foreground
$ fg %1        # Bring job 1 to foreground
$ fg           # Bring most recent job to foreground

# Send job to background
$ bg %1        # Send job 1 to background
$ bg           # Send most recent job to background

# Suspend current foreground process
# Press Ctrl+Z
$ vim file.txt
^Z
[1]+  Stopped                 vim file.txt

# Resume suspended job in background
$ bg %1
[1]+ vim file.txt &

# Kill job
$ kill %1      # Kill job 1
$ kill %2      # Kill job 2

Process Signals

bash

# Common signals:
# SIGTERM (15) - Graceful termination (default)
# SIGKILL (9)  - Force kill (cannot be caught)
# SIGSTOP (19) - Stop process (cannot be caught)
# SIGCONT (18) - Continue stopped process
# SIGHUP (1)   - Hangup (often used to reload config)
# SIGINT (2)   - Interrupt (Ctrl+C)
# SIGQUIT (3)  - Quit (Ctrl+\)

# Send signals to processes
$ kill 1234           # Send SIGTERM to PID 1234
$ kill -9 1234        # Send SIGKILL to PID 1234
$ kill -TERM 1234     # Send SIGTERM (same as default)
$ kill -HUP 1234      # Send SIGHUP (reload config)
$ kill -STOP 1234     # Stop process
$ kill -CONT 1234     # Continue process

# Kill processes by name
$ killall firefox     # Kill all firefox processes
$ killall -9 firefox  # Force kill all firefox processes
$ pkill firefox       # Kill processes matching pattern
$ pkill -f "python script.py"  # Kill by full command line

# Kill processes by user
$ pkill -u username   # Kill all processes by user
$ sudo pkill -u username  # Kill as root

# Interactive process killing
$ top                 # Press 'k' and enter PID
$ htop                # Press F9 and select signal

Process Priority Management

bash

# View process priorities
$ ps -eo pid,ni,pri,pcpu,comm
  PID  NI PRI %CPU COMMAND
 1234   0  20  0.1 bash
 1500  10  30  5.2 backup_script
 1600 -10  10 15.3 important_app

# Change priority of running process
$ sudo renice 10 1234      # Lower priority (higher nice value)
$ sudo renice -5 1234      # Higher priority (lower nice value)
$ sudo renice 0 1234       # Normal priority

# Change priority by process name
$ sudo renice 10 $(pgrep firefox)

# Start process with specific priority
$ nice -n 19 backup_script.sh    # Lowest priority
$ nice -n 0 normal_script.sh     # Normal priority
$ sudo nice -n -20 critical_app  # Highest priority

# Real-time priority (use with caution)
$ sudo chrt -f 50 critical_realtime_app  # FIFO scheduling
$ sudo chrt -r 50 critical_realtime_app  # Round-robin scheduling

🔧 Service Management with systemd

Basic Service Operations

bash

# Check service status
$ sudo systemctl status nginx
● nginx.service - A high performance web server and a reverse proxy server
   Loaded: loaded (/lib/systemd/system/nginx.service; enabled; vendor preset: enabled)
   Active: active (running) since Mon 2023-12-15 10:00:00 UTC; 2h 30min ago
     Docs: man:nginx(8)
  Process: 1234 ExecStartPre=/usr/sbin/nginx -t -q -g daemon on; master_process on; (code=exited, status=0/SUCCESS)
  Process: 1235 ExecStart=/usr/sbin/nginx -g daemon on; master_process on; (code=exited, status=0/SUCCESS)
 Main PID: 1236 (nginx)
    Tasks: 5 (limit: 4915)
   Memory: 15.2M
   CGroup: /system.slice/nginx.service
           ├─1236 nginx: master process /usr/sbin/nginx -g daemon on; master_process on;
           ├─1237 nginx: worker process
           └─1238 nginx: worker process

# Start/stop/restart services
$ sudo systemctl start nginx
$ sudo systemctl stop nginx
$ sudo systemctl restart nginx
$ sudo systemctl reload nginx    # Reload config without restart

# Enable/disable services (auto-start at boot)
$ sudo systemctl enable nginx
$ sudo systemctl disable nginx

# Check if service is enabled
$ systemctl is-enabled nginx
enabled

# Check if service is active
$ systemctl is-active nginx
active

Service Management Commands

bash

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

# List all unit files
$ systemctl list-unit-files --type=service

# Show service dependencies
$ systemctl list-dependencies nginx

# View service logs
$ sudo journalctl -u nginx
$ sudo journalctl -u nginx -f        # Follow logs
$ sudo journalctl -u nginx --since today
$ sudo journalctl -u nginx --since "2023-12-15 10:00:00"

# Mask/unmask services (prevent starting)
$ sudo systemctl mask nginx
$ sudo systemctl unmask nginx

# Edit service configuration
$ sudo systemctl edit nginx          # Create override file
$ sudo systemctl edit --full nginx   # Edit full unit file

# Reload systemd configuration
$ sudo systemctl daemon-reload

Creating Custom Services

bash

# Create a simple service file
$ sudo nano /etc/systemd/system/myapp.service

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

[Service]
Type=simple
User=myuser
WorkingDirectory=/opt/myapp
ExecStart=/opt/myapp/start.sh
ExecReload=/bin/kill -HUP $MAINPID
Restart=always
RestartSec=10

[Install]
WantedBy=multi-user.target

# Enable and start the service
$ sudo systemctl daemon-reload
$ sudo systemctl enable myapp
$ sudo systemctl start myapp
$ sudo systemctl status myapp

# Service types:
# simple - Default, process doesn't fork
# forking - Process forks and parent exits
# oneshot - Process exits after completion
# notify - Process sends notification when ready
# idle - Waits for other jobs to complete

📈 Performance Monitoring and Optimization

System Performance Analysis

bash

# CPU usage over time
$ sar -u 1 10         # CPU usage every 1 second, 10 times
Linux 5.4.0 (hostname)     12/15/2023      _x86_64_        (4 CPU)

10:30:01 AM     CPU     %user     %nice   %system   %iowait    %steal     %idle
10:30:02 AM     all      2.50      0.00      1.25      0.25      0.00     96.00
10:30:03 AM     all      3.75      0.00      1.50      0.50      0.00     94.25

# Memory usage over time
$ sar -r 1 5          # Memory usage every 1 second, 5 times

# I/O statistics
$ sar -b 1 5          # I/O statistics
$ sar -d 1 5          # Disk statistics

# Network statistics
$ sar -n DEV 1 5      # Network device statistics

# Generate system activity report
$ sar -A > system_report.txt

Process Resource Usage

bash

# Detailed process information
$ cat /proc/1234/status
Name:	firefox
State:	S (sleeping)
Pid:	1234
PPid:	1000
VmPeak:	 2097152 kB
VmSize:	 1572864 kB
VmRSS:	  524288 kB

# Process memory maps
$ cat /proc/1234/maps | head -5
7f8b4c000000-7f8b4c021000 rw-p 00000000 00:00 0
7f8b4c021000-7f8b50000000 ---p 00000000 00:00 0
7f8b50000000-7f8b50021000 rw-p 00000000 00:00 0

# Process file descriptors
$ ls -l /proc/1234/fd/
total 0
lrwx------ 1 user user 64 Dec 15 10:30 0 -> /dev/pts/0
lrwx------ 1 user user 64 Dec 15 10:30 1 -> /dev/pts/0
lrwx------ 1 user user 64 Dec 15 10:30 2 -> /dev/pts/0

# Process environment
$ cat /proc/1234/environ | tr '\0' '\n' | head -5
PATH=/usr/local/bin:/usr/bin:/bin
HOME=/home/user
USER=user
SHELL=/bin/bash
TERM=xterm-256color

# Process command line
$ cat /proc/1234/cmdline
firefox--no-remote--profile

Resource Monitoring Scripts

bash

#!/bin/bash
# system-monitor.sh - Comprehensive system monitoring

echo "=== System Performance Monitor ==="
echo "Date: $(date)"
echo

# CPU usage
echo "CPU Usage:"
top -bn1 | grep "Cpu(s)" | awk '{print "User: " $2 ", System: " $4 ", Idle: " $8}'
echo

# Memory usage
echo "Memory Usage:"
free -h | grep -E "Mem|Swap"
echo

# Load average
echo "Load Average:"
uptime | awk -F'load average:' '{print $2}'
echo

# Top 5 CPU consumers
echo "Top 5 CPU Consumers:"
ps aux --sort=-%cpu | head -6 | awk '{print $11 " (" $3 "%)"}'  | tail -5
echo

# Top 5 Memory consumers
echo "Top 5 Memory Consumers:"
ps aux --sort=-%mem | head -6 | awk '{print $11 " (" $4 "%)"}'  | tail -5
echo

# Disk usage
echo "Disk Usage:"
df -h | grep -E "^/dev"
echo

echo "=== Monitor Complete ==="

Performance Optimization Tips

bash

# Find resource-intensive processes
$ ps aux --sort=-%cpu | head -10    # Top CPU users
$ ps aux --sort=-%mem | head -10    # Top memory users

# Find processes with many open files
$ lsof | awk '{print $2}' | sort | uniq -c | sort -nr | head -10

# Monitor process creation
$ sudo sysctl kernel.fork_rate
$ watch -n 1 'ps aux | wc -l'

# Check for zombie processes
$ ps aux | grep -E "<defunct>|Z"

# Monitor system calls
$ strace -p 1234          # Trace system calls for PID 1234
$ strace -c command       # Count system calls

# Profile CPU usage
$ perf top                # Real-time CPU profiling
$ perf record command     # Record performance data
$ perf report             # Analyze recorded data

🧠 Knowledge Check

Quick Quiz

What's the difference between kill and killall?

Answer

kill terminates a specific process by PID, while killall terminates all processes with a specific name.
How do you start a process that survives logout?
Answer
bash
```
nohup command &
# or
command &
disown
```
What does a load average of 2.0 mean on a 4-core system?

Answer

The system is 50% utilized. Load average represents the number of processes waiting for CPU time. 2.0 on a 4-core system means 2 cores are fully utilized on average.

How do you change the priority of a running process?

Answer

bash

sudo renice 10 PID    # Lower priority
sudo renice -5 PID    # Higher priority

Hands-On Challenges

Challenge 1: Process Monitoring Dashboard

bash

# Create a script that displays:
# - Top 5 CPU-consuming processes
# - Top 5 memory-consuming processes
# - Current load average
# - Available memory
# - Number of running processes

Challenge 2: Service Management

bash

# Create a custom systemd service for a simple application
# Configure it to:
# - Start automatically at boot
# - Restart on failure
# - Log to a specific file
# - Run as a non-root user

Challenge 3: Performance Troubleshooting

bash

# Simulate a high CPU load and:
# - Identify the problematic process
# - Reduce its priority
# - Monitor the impact
# - Create a script to automatically detect and handle such situations

🚀 Next Steps

Excellent! You've mastered Linux process management. You can now:

Monitor system processes and resource usage
Control process execution and priority
Manage background jobs effectively
Work with systemd services
Troubleshoot performance issues
Optimize system resource utilization

Ready for networking fundamentals? Continue to 05-networking-basics.md to learn about network concepts and protocols.

Pro Tip: Use htop for interactive process management, systemctl for service control, and always check system load before making changes. Remember that killing processes with -9 should be a last resort - try graceful termination first! ⚙️

⚙️ Process Management: Controlling System Resources

📖 What You'll Learn

🌍 Why This Matters

🔍 Understanding Processes

Process Basics

Process States

📊 Monitoring Processes

Using ps Command

Real-time Monitoring with top

Enhanced Monitoring with htop

System Resource Monitoring

🎮 Process Control

Starting and Stopping Processes

Job Control

Process Signals

Process Priority Management

🔧 Service Management with systemd

Basic Service Operations

Service Management Commands

Creating Custom Services

📈 Performance Monitoring and Optimization

System Performance Analysis

Process Resource Usage

Resource Monitoring Scripts

Performance Optimization Tips

🧠 Knowledge Check

Quick Quiz

Hands-On Challenges

🚀 Next Steps

⚙️ Process Management: Controlling System Resources

📖 What You'll Learn

🌍 Why This Matters

🔍 Understanding Processes

Process Basics

Process States

📊 Monitoring Processes

Using ps Command

Real-time Monitoring with top

Enhanced Monitoring with htop

System Resource Monitoring

🎮 Process Control

Starting and Stopping Processes

Job Control

Process Signals

Process Priority Management

🔧 Service Management with systemd

Basic Service Operations

Service Management Commands

Creating Custom Services

📈 Performance Monitoring and Optimization

System Performance Analysis

Process Resource Usage

Resource Monitoring Scripts

Performance Optimization Tips

🧠 Knowledge Check

Quick Quiz

Hands-On Challenges

🚀 Next Steps

Using `ps` Command

Real-time Monitoring with `top`

Enhanced Monitoring with `htop`

Using `ps` Command

Real-time Monitoring with `top`

Enhanced Monitoring with `htop`