Introduction to the Linux Command Line

1. What Is a Process in Linux?

Every process has a unique PID (Process ID). The very first process at boot is usually init with PID 1. Processes may create child processes, forming a parent-child hierarchy. The Linux kernel maintains process information such as memory usage, CPU state, and user ownership.

2. Viewing Processes with ps

ps         # Attached to current terminal
ps x       # User-owned processes, including daemons
ps aux     # All system processes with full details

Key Columns:

• PID — Process ID
• TTY — Associated terminal
• STAT — Process state (S = sleeping, R = running, Z = zombie)
• %CPU / %MEM — Resource usage

3. Live Monitoring with top

top

Displays real-time process data, sorted by CPU usage. Summary includes:

• System uptime and load average
• Number of tasks (running, sleeping, zombie)
• CPU and memory usage breakdown

Press h for help, q to quit.

4. Managing Processes Interactively

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

5. Sending Signals with kill and killall

kill 28401             # Send SIGTERM to PID
kill -9 28401          # Force kill with SIGKILL
killall xlogo          # Kill all processes named xlogo

Common Signals:

• SIGINT (2): Interrupt
• SIGTERM (15): Graceful termination
• SIGKILL (9): Immediate force termination
• SIGSTOP / SIGCONT: Pause / Continue

6. System Shutdown Commands

shutdown -r now    # Reboot immediately
poweroff           # Power down system
reboot             # Restart system

Proper shutdown prevents data loss and ensures filesystem integrity.

7. Advanced Monitoring Tools

Command	Description
pstree	Visual tree of process hierarchy
vmstat 5	Updates system stats every 5 seconds
tload	Text graph of system load
xload	Graphical CPU load monitor

Conclusion

Linux provides comprehensive tools to monitor and manage processes. From listing with ps and top to controlling jobs and sending signals via kill, users can fine-tune system performance and troubleshoot efficiently. Understanding process control boosts your confidence and skill as a Linux power user.