Unix Process Commands : ps, nice, kill, nohup, time

We will learn about processes in Unix and some essential Unix process commands that allow us to view ongoing processes in our terminal like ps Unix command, nice Unix command, kill Unix command, nohup Unix command, time Unix command and top.

Below topics are covered in this article:

What is a Process?
Types of process: Foreground process and background process
Top command in Unix
PS command in Unix
Kill command in Unix
NICE command in Unix
NOHUP command in Unix
TIME command in Unix

What is a Process?

Any running program or a command given to a system is called a process. A process could run in foreground or background.
A process is a program in execution.

- Each process is allocated a process identifier or PID.
- In general , each process is started by another process, known as parent process.

Types of processes in Unix

There are two types of unix process:

Foreground Processes Unix:

- It runs on the screen and needs user input, hereafter you have to wait for its completion before being able to enter a new command.
- It is also referred as interactive process.
- For example, MS Office power point, Word etc.

Background Processes Unix:

- It runs in the background and usually do not need user input. Unlike foreground processes, when a background process has been executed you don’t have to wait for its completion to execute some other command.
- It is also referred as non-interactive process.
- For example ,system scanning using antivirus.

Any command can be run as a background process by typing a space and ‘&’ after the command.

SYNTAX:
$ <command> &

Example for foreground and background process:

$ ls pwd

During the execution o above command, no other processes can be run or started as the prompt would not be available.
Press Ctrl-c which will send a kill signal to any process running in foreground, terminating it immediately or press Ctrl-z to send a suspend signal to any process running in the foreground, pausing it immediately.

Top command in Unix

It is used to show all the running processes within the working environment along with their memory and CPU usage.

Syntax:

$ top

Output fields are:

PID: Unique Process ID given to each process.
User: Username of the process owner.
PR: Priority given to a process while scheduling.
NI: ‘nice’ value of a process.
VIRT: Amount of virtual memory used by a process.
RES: Amount of physical memory used by a process.
SHR: Amount of memory shared with other processes.
S: state of the process
‘D’ = uninterruptible sleep
‘R’ = running
‘S’ = sleeping
‘T’ = traced or stopped
‘Z’ = zombie
%CPU: Percentage of CPU used by the process.
%MEM :Percentage of RAM used by the process.
TIME+: Total CPU time consumed by the process.
Command: Command used to activate the process.

ps command in Unix

ps stands for ‘Process Status’ and displays information about a selection of the active processes.

If you want a repetitive update of the selection and the displayed information, use top instead.

- gives the details about the processes
- Option –f gives full listing
- Option –e or –A displays all processes

To see every process on the system using standard syntax:
          ps -e
          ps -ef
          ps -eF
          ps -ely

The output fields are:

PID :process ID TTY :terminal type TIME: total time the process has been running CMD :name of the command that launches the process

nice command in Unix

nice command in unix is used to execute utility or shell script with a particular CPU priority, thus giving the process more or less CPU time than other processes.

A niceness of -20 is the highest priority and 19 is the lowest priority.
Default value is 0
Execute commands with lower priority.

SYNTAX:
nice [option] command [arguments]

You can change the nice value of a process that is already running by using renice command

SYNTAX:
renice [value] -p [PID]

kill command in Unix

Unix has no stop command instead it has kill command. Kill is used to send a signal to a process. The most commonly used signal is “terminate” (SIGTERM) or “kill” (SIGKILL). However, there are many more. The full list can be shown with kill -l.

Send signals to processes.
The default signal is 15, which is SIGTERM.

SYNTAX:
kill -[option] pid

the process with particular pid will be terminated.

nohup command in Unix

Nohup means no hang up . It is a command that keep processes running even after exiting the shell or terminal.
Nohup prevents the processes or jobs from receiving the SIGHUP (Signal Hang UP) signal.
This is a signal that is sent to a process upon closing or exiting the terminal

- Continue execution even after logout.

SYNTAX:
nohup command [arguments] 
      or
nohup options

Checking the version of Nohup

You can begin by checking the version of Nohup using the syntax below:

SYNTAX: nohup --version

Starting a process in the background using Nohup

To start a process in the background use the & symbol at the end of the command.

SYNTAX: nohup command &

Example: nohup ping google.com & —–we are pinging google.com and sending it to the background.

Killing a Nohup process running in background

When a command is put in the background for running using nohup , the background operator (&) will give you the PID at the command prompt.
If your plan is to manually manage the process, you can save that PID and use it later to kill the process if needed, via kill PID or kill -9 PID (if you need to force kill).
You can also get PID later on by ps -ef | grep "command name" and then use kill PID or kill -9 PID (if you need to force kill).

time command in Unix

The time command is used to determine how long a given command takes to run. It is useful for testing the performance of your scripts and commands.

- Displays time usage of a command
  Syntax:
```
time [option] [COMMAND]
```

for more details refer here

Tutorials Hut

Unix For Testers