How to Monitor Running Linux Processes

These examples demonstrate how to use top both interactively and non-interactively, enabling you to monitor system performance, filter by user, focus on specific processes, and customize the display for your needs.

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Basic top Usage

1. Start top
   Open the default system monitor:

   top

2. Update Interval
   Refresh the display every N seconds (e.g., 2 seconds):

   top -d 2

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Process Sorting

3. Sort by CPU Usage
   Press P (Shift + p) while top is running.
4. Sort by Memory Usage
   Press M (Shift + m).
5. Sort by Process ID (PID)
   Press N.

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Filtering Processes

6. Show Processes for a Specific User
   Press u, then enter the username.
   Example:

   top -u apache

7. Monitor a Single Process
   Track a process by PID:

   top -p 1234

8. Highlight Running Processes
   Press R to toggle highlighting.

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Batch Mode (Scripting)

9. Run top Once and Exit

   top -n 1 -b

10. Save Output to a File
    bash top -n 3 -b > top_log.txt

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Customizing the Display

11. Toggle Full Command Path
    Press c to show the full command line.
12. Add/Remove Columns
    Press f to select fields (e.g., PPID, priority).
13. Toggle Colors
    Press z.
14. Hide Idle Processes
    Press i to show only active tasks.

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Process Management

15. Kill a Process
    Press k, then enter the PID and signal (default: 15 for SIGTERM).
    Example: PID to kill: 1234 Signal: 9 # SIGKILL
16. Renice a Process
    Press r, then enter the PID and new priority (e.g., -15 to 19).

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Advanced Examples

17. Monitor Threads
    Show threads instead of processes: top -H
18. Show Cumulative CPU Time
    Press S to toggle cumulative mode.
19. Check Load Average
    Look at the load average in the header (1/5/15-minute averages).
20. Tree View
    Press V to show parent-child process hierarchy.

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Key Interactive Commands

Shortcut	Action
h	Help menu
q	Quit
W	Save current layout to ~/.toprc
l	Toggle load average display
t	Toggle task/cpu stats
1	Show individual CPU cores

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Common Use Cases

• Identify CPU Hogs: Sort by CPU (P) and check %CPU column.
• Find Memory Leaks: Sort by memory (M) and monitor %MEM.
• Kill Zombie Processes: Look for Z in the STAT column.
• Check I/O Wait: Look at %wa in the CPU summary line.

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Pro Tips

• Use shift + > or < to change the sort field dynamically.
• Combine with grep for quick filtering:

  top -b -n 1 | grep "nginx"

• For a modern alternative, try htop (sudo apt install htop).