Processes in Linux/Unix

A program/command when executed, a special instance is provided by the system to the process. This instance consists of all the services/resources that may be utilized by the process under execution. 

• Whenever a command is issued in Unix/Linux, it creates/starts a new process. For example, pwd when issued which is used to list the current directory location the user is in, a process starts.
• Through a 5 digit ID number Unix/Linux keeps an account of the processes, this number is called process ID or PID. Each process in the system has a unique PID.
• Used up pid’s can be used in again for a newer process since all the possible combinations are used.
• At any point of time, no two processes with the same pid exist in the system because it is the pid that Unix uses to track each process.

Initializing a process

A process can be run in two ways:  

Method 1: Foreground Process

Every process when started runs in foreground by default, receives input from the keyboard, and sends output to the screen.  When issuing pwd command 

pwd

Output: 

• When a command or process runs in the foreground, it occupies the terminal until it finishes executing.
• This means no other commands or processes can be started during that time.
• The terminal prompt becomes available only after the current process completes.

Method 2: Background processes

It run independently of the terminal, allowing you to perform other tasks simultaneously.

• They do not require keyboard input while running.
• To start a process in the background, add & at the end of the command.

$ pwd &

Since pwd doesn’t need user input, it runs in the background and then stops until brought to the foreground (if needed). After pressing Enter, you see

• The first line shows the job number and process ID (PID) of the background job, indicating it completed successfully.
• The $ symbol indicates that the terminal is now ready for another command.

Tracking ongoing processes

ps (Process status) can be used to see/list all the running processes. 

ps

• For more information -f (full) can be used along with ps  

$ ps –f

Output:

• For single-process information, ps along with process id is used  

Command:

$ ps 19

Output:

PID       TTY      TIME        CMD
19        pts/1    00:00:00    sh

For a running program (named process) Pidof finds the process id’s (pids) 
Fields described by ps are described as: 

• UID: User ID that this process belongs to (the person running it)
• PID: Process ID
• PPID: Parent process ID (the ID of the process that started it)
• C: CPU utilization of process
• STIME: Process start time
• TTY: Terminal type associated with the process
• TIME: CPU time is taken by the process
• CMD: The command that started this process

There are other options which can be used along with ps command : 

• -a: Shows information about all users
• -x: Shows information about processes without terminals
• -u: Shows additional information like -f option
• -e: Displays extended information

Stopping a process:

When running in foreground, hitting Ctrl + c (interrupt character) will exit the command. For processes running in background kill command can be used if it’s pid is known. 

Command:

$ ps –f

Output:

UID      PID  PPID C STIME    TTY        TIME CMD
52471     19     1 0 07:20    pts/1  00:00:00      sh
52471     25    19 0 08:04    pts/1  00:00:00      ps –f

Command:

$ kill 19

Output:

Terminated

If a process ignores a regular kill command, you can use kill -9 followed by the process ID.

Command: 

$ kill -9 19

Output:

Terminated

Other process commands:

1. bg: A job control command that resumes suspended jobs while keeping them running in the background 
Syntax:

bg [ job ]

For example: 

bg %19

2. fg: It continues a stopped job by running it in the foreground. 

Syntax:  

fg [ %job_id ]

For example  

fg 19

3. top: This command is used to show all the running processes within the working environment of Linux. 

Syntax:  

top

Output:

4. nice: It starts a new process (job) and assigns it a priority (nice) value at the same time. 

Syntax:  

nice [-nice value] 

nice value ranges from -20 to 19, where -20 is of the highest priority. 

5. renice : To change the priority of an already running process renice is used. 

Syntax:  

renice [-nice value] [process id]

6. df: It shows the amount of available disk space being used by file systems 
Syntax:  

df

Output:  

7. free: It shows the total amount of free and used physical and swap memory in the system, as well as the buffers used by the kernel 
Syntax:  

free

Output:  

Types of Processes

There are three types of Processes

1. Parent and Child process :

• Every process in Linux is created by another process, known as the parent process.
• The newly created process is called the child process.
• The ps -f command displays the Process ID (PID) and the Parent Process ID (PPID) in its 2nd and 3rd columns.
• Most user processes have the shell as their parent process.

2. Zombie and Orphan process :

• When a child process completes its execution, it sends a SIGCHLD signal to the parent process, informing it of the termination.
• If the parent process is terminated before the child finishes, the child becomes an orphan process.
• Orphan processes are then adopted by the init (PID 1) process.
• A zombie process is a process that has finished execution but still remains in the process table because its parent has not yet read its exit status.
• Zombie processes are essentially dead processes that occupy minimal system resources.

3. Daemon process :

Daemon processes are background system processes that run continuously to provide services. 

• They usually run with root privileges and handle requests from other processes.
• Common examples include the print daemon or web server daemons.
• Daemons do not have an associated terminal (TTY), when you run ps -ef, processes with a ‘?’ in the TTY field are daemon processes.