Processes (ps, top, kill, htop)

Processes are running instances of programs on your system. Understanding how to view, manage, and terminate processes is fundamental to working with Linux. This section covers essential commands for process management.

ps

  • Description: Displays information about currently running processes. ps by itself shows only processes associated with your current terminal session, which is usually not very informative. The real power of ps comes with its options.

  • Example Usage:

    ps aux  # The most common and useful combination of options

    Sample Output (truncated):

    USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  0.0  0.1  167748  9560 ?        Ss   Jul04   0:01 /sbin/init
root         2  0.0  0.0       0     0 ?        S    Jul04   0:00 [kthreadd]
user     12345  0.1  0.5  456789 20000 ?        S    09:30   0:05 /usr/bin/my_program
user     12346  0.0  0.1   12345  4000 pts/0    Ss   09:31   0:00 bash
user     12400  0.0  0.1   13456  4500 pts/0    R+   09:45   0:00 ps aux

    Explanation of Output Columns:

    • USER: The user who owns the process.
    • PID: The Process ID (a unique number assigned to each process). This is critical for kill.
    • %CPU: The percentage of CPU time the process is using.
    • %MEM: The percentage of system RAM the process is using.
    • VSZ: Virtual memory size (in KB).
    • RSS: Resident Set Size (physical memory in use, in KB).
    • TTY: The controlling terminal (often ? for daemons/system processes).

    • STAT: The process state (e.g., S = sleeping, R = running, Z = zombie).

      • S: Interruptible sleep (waiting for an event to complete)
      • R: Running or runnable (on run queue)
      • D: Uninterruptible sleep (usually IO)
      • T: Stopped, either by a job control signal or because it is being traced.
      • Z: “zombie” process, terminated but not reaped by its parent.
      • A trailing + indicates the process is in the foreground process group.
    • START: The time the process started.
    • TIME: The total CPU time the process has used.
    • COMMAND: The command that started the process.

    Key Options for ps:

    • ps aux: Shows all processes on the system, in a detailed format, including processes not associated with a terminal. This is generally the most useful way to use ps.
    • ps -ef: Another common way to view all processes, but with slightly different output format (includes parent process ID - PPID).
    • ps -u [username]: shows the processes which are related to a certain user.
    • ps -p <PID>: Shows information about a specific process ID. Useful after you’ve found a PID with ps aux or top.

top

  • Description: Displays a dynamic, real-time view of running processes. Think of it as a task manager for the command line.

  • Example Usage:

    top

    Sample Output (truncated - it’s a live, updating display):

    top - 10:01:23 up 1 day,  2:34,  1 user,  load average: 0.50, 0.45, 0.40
Tasks: 250 total,   1 running, 249 sleeping,   0 stopped,   0 zombie
%Cpu(s):  5.0 us,  1.7 sy,  0.0 ni, 93.3 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
MiB Mem :   7853.0 total,   4000.0 free,   2000.0 used,   1853.0 buff/cache
MiB Swap:   2048.0 total,   2048.0 free,      0.0 used.   5500.0 avail Mem

  PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND
12345 user      20   0  456789  20000  15000 S   5.0   0.3   0:05.00 my_program
    1 root      20   0  167748   9560   6000 S   0.0   0.1   0:01.00 init
...

    Key Interactive Commands within top:

    • q: Quit top.
    • k: Kill a process (you’ll be prompted for the PID).
    • u: Show processes for a specific user.
    • P: Sort by CPU usage (default).
    • M: Sort by memory usage.
    • h: Display help.
    • 1: show per-core CPU.
    • z: toggle colors.
    • space: Immediately refresh display.

kill

  • Description: Sends a signal to a process. The most common use is to terminate a process, but kill can send other signals as well.

  • Example Usage:

    kill 12345     # Sends the default signal (TERM - terminate) to process ID 12345
kill -9 12345  # Sends the SIGKILL signal (forcefully terminate) to process ID 12345
kill -l        #list all signal names.
kill -s HUP 123 #Sends the HUP signal to process with PID 123.

    Important Signals:

    • -1 (SIGHUP): “Hang up.” Some daemons reload their configuration when they receive this signal.
    • -2 (SIGINT): Interrupt. Usually same as pressing Ctrl+C.
    • -9 (SIGKILL): “Kill.” This forcefully terminates the process immediately. It should be used as a last resort, as the process doesn’t get a chance to clean up.
    • -15 (SIGTERM): “Terminate.” This is the default signal sent by kill. It allows the process to terminate gracefully (e.g., save files, close connections). This is generally the first signal you should try.

    Note: You can only kill processes that you own (or processes you have root privileges to kill).

htop (Suggested Addition)

  • Description: An enhanced, interactive process viewer, similar to top but with a more user-friendly interface, color-coding, and mouse support. It’s often not installed by default.
  • Installation (Debian/Ubuntu): 
    sudo apt install htop

    (Installation for other distributions will vary)

  • Installation (CentOS/Fedora):

    sudo yum install htop
#OR
sudo dnf install htop

  • Example Usage:

    htop

    Key Features of htop:

    • Color-coded display for easy readability.
    • Mouse support for scrolling and selecting processes.
    • Tree view of processes (showing parent-child relationships).
    • Easier process killing (select a process and press F9, or k).
    • Customizable display options.

    Key Interactive commands within htop:

    • F1,h,?: help.
    • F2, S: Setup options.
    • F3, /: Search processes.
    • F4, : Filter Processes
    • F5, t: Tree view.
    • F6, >: Sort processes.
    • F7, ]: Increase priority.
    • F8, [: Decrease priority.
    • F9, k: kill process.
    • F10, q: Quit.
    • Space: tag/untag a process.
    • U: Untag all processes.

    Sample Output (not representable in text, but it’s a colorful, interactive display)