ps

1. ps (Process Status)

ps is a command used to show information about the currently running processes.

Syntax:

ps [options]

Common Options:

• -e or -A: Show all processes.
• -f: Full format listing.
• -u username: Show processes for a specific user.
• -aux: Show all processes, including those from other users.

Example:

ps aux

This shows a full list of processes running on the system.

Sample Output:

USER       PID  %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root        1  0.0  0.1 169448  6092 ?        Ss   10:15   0:03 /sbin/init
user      1234  1.2  0.3 254548 12576 pts/0    Ss+  10:45   0:10 bash
user      1300  0.0  0.1 123456  6789 pts/0    S+   10:47   0:00 vim myfile.txt

• Explanation:
  • PID: Process ID.
  • %CPU: CPU usage percentage.
  • %MEM: Memory usage percentage.
  • VSZ: Virtual memory size.
  • RSS: Resident Set Size (physical memory).
  • TTY: Terminal associated with the process.
  • STAT: Process status (e.g., S for sleeping, R for running).
  • START: Start time of the process.
  • TIME: Cumulative CPU time used by the process.
  • COMMAND: Command that started the process.

2. top (Task Manager)

top provides a dynamic, real-time view of the system's processes.

Syntax:

top

Sample Output:

top - 10:47:09 up 5 days,  3:22,  2 users,  load average: 0.15, 0.13, 0.11
Tasks: 130 total,   1 running, 129 sleeping,   0 stopped,   0 zombie
%Cpu(s):  3.8 us,  1.1 sy,  0.0 ni, 95.0 id,  0.1 wa,  0.0 hi,  0.0 si,  0.0 st
MiB Mem :   3921.7 total,   1286.0 free,   1227.3 used,   1408.4 buff/cache
MiB Swap:   2048.0 total,   2048.0 free,      0.0 used.   2435.2 avail Mem

  PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND
  123 root      20   0  169448   6092   4120 S   1.2  0.2   0:03.12 init
  1300 user      20   0  254548  12576   1048 S   0.0  0.3   0:10.34 bash

• Explanation:
  • Shows the top processes by CPU and memory usage.
  • %CPU: CPU usage by the process.
  • %MEM: Memory usage by the process.
  • TIME+: Total time the process has been running.

Press q to exit top.

3. htop (Interactive Process Viewer)

htop is a more user-friendly, interactive version of top that allows you to scroll and search processes.

Install htop (if not already installed):

sudo apt install htop  # For Debian/Ubuntu
sudo yum install htop  # For CentOS/RedHat

Syntax:

htop

Sample Output:

  PID USER      PRI  NI  VIRT   RES   SHR S CPU% MEM% TIME+  COMMAND
  1234 user      20   0 254548 12576  1048 S  0.0  0.3  0:10 bash
  5678 root      20   0  169448  6092  4120 S  1.2  0.2  0:03 init

• Explanation:
  • Similar to top, but with colorful and more customizable output.
  • You can scroll up/down to see all processes, search for processes, and even kill processes directly from the interface.

Press F10 to exit htop.

4. pgrep (Process Grep)

pgrep searches for processes based on name or other criteria.

Syntax:

pgrep [options] pattern

Example:

To find all processes with "python" in the name:

pgrep python

Sample Output:

1234
5678

• Explanation:
  • pgrep returns the PID of processes matching the search term (python in this case).

5. kill (Terminate Process)

The kill command is used to send signals to processes, typically to terminate them.

Syntax:

kill [signal] PID

Example:

To kill a process with PID 1234:

kill 1234

To Force Kill:

If a process doesn't respond to a normal kill, use:

kill -9 1234

Sample Output:

No output on success, but if the process is killed successfully, it will stop running.

6. nice (Start a Process with Modified Scheduling Priority)

nice is used to run a command with a specified priority (niceness).

Syntax:

nice -n [priority] command

• Priority: The range is from -20 (highest priority) to 19 (lowest priority).

Example:

To run a command with lower priority:

nice -n 10 my_script.sh

7. renice (Change Process Priority)

renice allows you to change the priority of an already running process.

Syntax:

renice -n [priority] -p PID

Example:

To change the priority of PID 1234 to 10:

sudo renice -n 10 -p 1234

Sample Output:

1234 (process PID) old priority 0, new priority 10

8. pstree (Process Tree)

pstree displays processes in a tree-like format, showing the parent-child relationships between processes.

Syntax:

pstree

Example:

pstree

Sample Output:

init─┬─systemd─┬─sshd───bash─┬─vim
      │        └─sshd───bash───pstree

• Explanation:
  • Shows a tree of processes, starting from the init process at the top.
  • Processes are grouped by their parent-child relationship.

9. time (Measure Execution Time)

The time command is used to measure the execution time of a command.

Syntax:

time command

Example:

time ls

Sample Output:

real    0m0.004s
user    0m0.001s
sys     0m0.002s

• Explanation:
  • real: Actual elapsed time.
  • user: Time spent in user mode.
  • sys: Time spent in kernel mode.

10. watch (Run Command Repeatedly)

The watch command is useful for monitoring changes in real time, such as process updates.

Syntax:

watch [options] command

Example:

To monitor CPU and memory usage every 2 seconds:

watch -n 2 "ps aux"

Sample Output:

Every 2.0s: ps aux                                                            Tue Nov 12 10:50:00 2019

USER       PID  %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root        1  0.0  0.1 169448  6092 ?        Ss   10:15   0:03 /sbin/init