Lecture 8: Process Management

Core Definitions

• Program: Series of instructions telling the computer what to do.
• Process: A program in execution. A running instance of a program loaded into memory.
• Process Management: Tuning or controlling a process.
• Job: A process started from the shell.

Process Types / States

Based on Execution Mode

Processes in Linux can run in one of two modes, which determines their interaction with the user terminal.

Foreground Process

By default, all processes run in the foreground.
Runs on the terminal, receives input from keyboard (stdin) and sends output to screen (stdout).
Blocks the terminal until finished/killed.

Background Process

Runs independently without requiring keyboard input.
Allows other commands to run in the foreground simultaneously.
To run a command as a background process, append an ampersand (&) to the end of the command.

sleep 30 &

Based on Relationships & Status

1. Parent process
   Any process that creates another process.
   Every process in Linux has a parent process, except for the very first process started by the kernel.
2. Child process
   A process created by another process (its parent).
   Each child process always has exactly one parent at any given time.
3. Orphan process
   An orphan process occurs when a parent process terminates before its child finishes execution.
   The orphan's PPID becomes the PID of init (PID 0) or systemd (PID 1).
4. Zombie process
   A process that has finished execution (dead) but still has an entry in the process table.
   Zombie processes consume no CPU and minimal memory, but they remain visible in process listings until reaped by their parent.
5. Daemon process
   A long-running background process that typically starts during system boot or on demand and does not interact directly with a terminal.
   Daemons are usually detached from any controlling TTY, their TTY field often appears as ? in process listings.
   Examples include sshd, cron, and systemd services.

Key Commands

Job Control Commands

The following commands and keystrokes are used to manage jobs:

Command / Keystroke	Description
jobs	Lists background jobs and their Process IDs.
kill %N	Terminates the background job number N.
kill PID	Terminates process with the given Process ID.
fg %N	Brings background job N to the foreground.
Ctrl+Z	Suspends the current foreground job.
bg	Resumes the most recently suspended job and runs it in the background.
bg %N	Resumes the suspended job number N and runs it in the background.
Ctrl+C	Terminates the current foreground job.

Monitoring & Listing Processes

There are two primary commands available in Linux to track running processes: top and ps.

The top Command

The top command provides a real-time, dynamic view of the processes running on the system.

• Navigate: Up/Down arrows.
• Kill: Highlight process, press k.
• Quit: Press q.

The ps Command

The ps (process status) command is used to view currently running processes.

Basic Usage Running ps without options shows the processes for the current user and terminal.

To display all currently running processes in full format, use ps -ef.

• -e : show all processes
• -f : full-format listing (UID, PID, PPID, CPU usage, start time, TTY, and command)

Fields:

• UID: The User ID of the process owner.
• PID: The unique Process ID.
• PPID: The Parent Process ID.
• C: The CPU utilization of the process.
• STIME: The time the process started.
• TTY: The terminal type where the command is running.
• TIME: The cumulative CPU time used by the process.
• CMD: The command that initiated the process.

Alternatively, a more detailed output can be obtained with ps aux (can be written as ps -aux)

• a : Shows information about all users
• x : Shows information about processes without terminals (e.g. system daemons)
• u : Displays the output in a user-oriented format, which includes columns like %CPU and %MEM.

ps -u <username> (User-specific)

The -u option filters the process list to show only processes belonging to a specific user.
Multiple usernames can be specified, separated by a comma.

ps -fu <username>

ps -C <command> (By Command Name)

The -C option selects processes by their command name.

ps -C <command>

ps -p <PID> (By Process ID)

The -p option allows you to query a process directly using its unique Process ID (PID).
This is useful for checking the status of a specific application or service when you already know its PID.

ps --ppid <PPID> (By Parent Process ID)

The --ppid option is used to list all child processes that share the same Parent Process ID.

Terminating and Prioritizing Processes

Terminating Processes

There are three primary methods for ending a process:

1. CTRL+C
   Sends an interrupt signal (SIGINT) to a process running in the foreground, typically exits the command.
2. kill Command
   Terminates or signals a process, first find its PID using the ps command.
   Then, use the kill <PID> command to terminate it.
   This sends a terminate signal (SIGTERM) by default.
3. kill -9 Command
   If a process ignores the interrupt signal, you can force its termination using kill -9 <PID>.
   This sends a forceful kill signal (SIGKILL).

Prioritizing Processes (Niceness)

In Linux, process priority can be influenced by its 'Niceness' value.

• Niceness Value: Determines process priority. Range: -20 (highest) to 19 (lowest). Default is 0.
• Set for new process: nice -n [value] [process name]
• Change for running process: renice [value] -p 'PID'