What is process management in Linux

Whenever you give a command in any operating system, the process starts. In Linux, when the process starts a five-digit number is created which is called process id or PID. This is a system-generated unique Id given for the process.

With the help of the PID, we can track the process and two processes won't have the same process IDs.

A process executes in two ways they are,

• Foreground Processes
• Background Processes

Foreground Processes

Every process by default runs in Foreground which means the output is printed on the screen.

The best example for the foreground process is the ls command which prints the output on the screen by listing the files and directories.

When a program is running in the foreground, we can not start another process without completing the previous process. It is because of this reason foreground process is considered a time-consuming process.

Background Processes

When a process starts running and it is not visible on the screen it is called a background process. You can simultaneously run the 'n' number of commands in the background process.

To enable the background process you have to provide the ampersand symbol (&) at the end of the command.

Example :

ls &

Whenever the process is completed and then you hit enter you will get a prompt like the below snippet, which means the background process is completed successfully.

[1] + Done ls & $

• Child process : A process that is created by some other process during run-time is called a child process. The child process is usually created to execute some binary form within an existing process.
• Daemon process : These are special processes that run in the background. These are system-related processes that have no associated terminals.
• Orphan process : When the parent process gets killed, the child process becomes the orphan process, and the child process is monitored by the init process. Though the init process adopts the orphan process, this is still called the orphan process as their parent process no longer exists.
• Zombie process : A zombie process is one that should have terminated but it is still active in the process table. This is usually caused when a parent process has not yet realized that it has completed or wants to create another process with the same process name but with using a different process id.
• Interactive process : This type of process interacts constantly with the users and therefore spends a lot of time waiting for keyboard and mouse inputs.
• Batch or automatic process : These do not need user interaction and hence they often run in the background.

The ps command is used to track or to list all the running processes.

Example :

$ ps

Output :

PID TTY TIME CMD 19 pts/1 00:00:00 ls 24 pts/1 00:00:00 ps

Options :

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

If you require more information about the process you can use the ps command along with the -f option.

$ ps –f

Output :

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

To get more information about a particular process, you can give the ps command along with the PID.

$ ps 19

Output :

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

The information provided by the ps command for the process are :

• 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: Time taken by the CPU for the process
• CMD: The command that started this process

When a process runs in the foreground, you can interrupt the process by hitting Ctrl+c. When a process runs in the background the kill command is used along with the process id to interrupt or to stop the running process.

Example:

$ kill 19

Output: The process which has the process id 19 is terminated.

Terminated

Sometimes a process cannot be terminated by using the kill command. To terminate such kind of process uses the below command.

$ kill -9 pid

-9 forces the termination of the process by blocking the signals.
Example :

$ kill -9 19

Output :

Terminated

The below command is used to get the list of jobs that are either running or stopped.

jobs

The bg command is used to resume the suspended jobs and runs them in the background.

Syntax :

bg [ %job ]

Example :

bg %19

The fg command is used to start the job which has already stopped in the foreground.

Syntax :

fg [ job_id ]

Example :

fg 19

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

Syntax :

top

Output :

Press 'q' on the keyboard to move out of the process display.

Terminologies of top command:

FieldDescription

PID	The process ID of each task
User	The username of the task owner
PR	Priority Can be -20(highest) or 19(lowest)
NI	The nice value of a task
VIRT	Virtual memory used (kb)
RES	Physical memory used (kb)
SHR	Shared memory used (kb)
S	Status of the process. The status is of five types: 'D' = uninterruptible sleep 'R' = running 'S' = sleeping 'T' = traced or stopped 'Z' = zombie
%CPU	% of CPU time
%MEM	Physical memory used
TIME+	Total CPU time
Command	Command name

The nice command starts a new process (job) and assigns it a priority (nice) value for the process. The nice value ranges from -20 to 19, where -20 is of the highest priority and 19 is of the lowest priority

Syntax :

nice [-nice value]

The renice command is used to change the priority of the running process.

Syntax :

renice [-nice value] [process id]

The df command shows the disk information of the Linux file systems.
Syntax :

df

Output :

Filesystem 1K-blocks Used Available Use% Mounted on /dev/loop0 1845236 15246876 2554440 86% / none 4 0 4 0% /sys/fs/task udev 452369 4 493808 1% /dev tmpfs 195263 1364 99308 2% /run none 2123 0 5120 0% /run/lock none 421980 1764 501588 1% /run/shm none 562831 20 102380 1% /run/user /dev/sda3 158297648 164417964 10348112 95% /host

Thefree command 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 :

total used free shared buffers cached Mem: 1006708 935872 70836 0 148244 346656 -/+ buffers/cache: 440972 565736 Swap: 262140 130084 132056

About Author :

I am Tharani N V, a Content writer, and SEO specialist. I have a year of experience in both technical and non-technical content writing.

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