Subprocess Module in Python

What is a Process?

Every program will have its respective system state, i.e., the state in which it is running. A program that is in a running state or in a state of being executed is known as a Process. The word process itself defines that the functioning takes place in sequential order. In other words, the statements in the process are executed in a sequential control flow. This can be defined as the main thread process in some of the relevant and particular situations.

Whenever you work with a PC, you'll constantly be involved and work with the programs. A process is the working framework's reflection of a running project. Thus, working with PC generally include processes. When you think of performing any kind of operation within the PC, many processes will be involved in order to complete that task. A normal working framework will report hundreds or even a huge number of running cycles. Many instructions can be executed simultaneously within the CPU to make things faster and optimized. Thus, you might consider how a large number of cycles can seem to run simultaneously.

So, the working framework is a magnificent multitasker, as it must be. The CPU is the mind of a PC. However, it works within a fraction of a second. Performing multiple tasks keeps the CPU occupied. The working framework monitors processes in a cycle or interaction control block.

What is a Subprocess?

Our PC runs subprocesses constantly. Any activity we do on our PC includes conjuring a subprocess. That stays genuine regardless of whether we are composing a basic script or program in python. As a matter of fact, by simply perusing this article, you're running a great deal of cycles like an organization director or the web program.

The idea of the subprocess may appear to be dark regardless of whether you've been getting the hang of programming for some time. To be more precise, a subprocess is a computerized process that is, in turn, created by another process. This article will investigate the primary idea of the subprocess and how to utilize the Python subprocess standard library.

It is just a chain of actions where the work is divided into fragments. On the whole, if the process is supposed to do some work or a task that consumes 10 minutes of time, the computerized process divides the work into fragments where the fragmented work is distributed among the subprocesses. This type of fragmentation is done in order to perform the given task within less amount of time using the subprocesses, which work simultaneously.

Let us discuss the concept of subprocess with a real-time example. Let us consider a software company in which the tasks that are given by the higher companies should be completed on time. So, the head of the company is not going to do all the work that is supposed to be done. He will hire a few people to work under him in order to complete the given work on time by distributing the work so that those hired people can work simultaneously.

The major advantage of this is that the work is being done in a very less amount of time and the people need not work very hard to complete the work. The company here is Computer. The head of the company is Process. The people that the process hired is Subprocess. So, in order to complete the task within less amount of time, the processes are divided into subprocesses which begin their work simultaneously.

The process is referred to be a Parent, whereas the subprocess is referred to be a child of that particular parent. As we know, one parent can have many children. In the same way, one process can have a huge number of subprocesses. The processes and subprocesses always intend to perform independently so that the destruction of any one of the processes or subprocesses will not lead to the termination of the entire procedure. Sometimes, the subprocesses inherit few characteristics from the processes for completing the desired task or operation. The information is mostly maintained in the form of a table within the process so that the fragmentation of the process into subprocesses becomes easier.  

Why is the Subprocess module used in Python?

• It is used to run any type of operating system commands and manipulate the output later.
• It helps us call external commands, including methods, functions, etc.
• With the help of subprocess modules, you can be able to capture the output of the commands or type convert the output from one command to another.
• It acts like a communicator between the shell and the output script.

Common steps that must be done to implement the subprocess module in Python:

1. Firstly the subprocess module must be imported by using “ import “.

# import subprocess module
import subprocess

2. After importing the subprocess module, you can give the desired command using the subprocess module by calling the required method with the word or keyword " subprocess ". The methods that we use and the commands we produce completely depend on the Operating system we use. Mostly, the " Popen() " method or function is used. Along with the method " Popen() ", we can also use the " run() " method.

The methods with subprocess and that can be used in implementing subprocess module are:

• Popen()
• check_call()
• check_output()
• run()  [ This is a general function ]

# using the run() function
subprocess.run(command)


#         or


# using the Popen() function
subprocess.Popen(command)


#         or


# using check_call() function
subprocess.check_call(command)


#         or


# using check_output() function
subprocess.check_output(command)

Simple program syntax which can implement subprocess module:

# to implement the subprocess module in python


# import subprocess module
import subprocess


# using the run() function
subprocess.run(command)

Let us discuss an example problem using all functions:

1. run() :

# to implement the subprocess module in python using run()
# import subprocess module
import subprocess
# using the run() function
subprocess.run('dir')

2. Popen() :

# to implement the subprocess module in python using Popen()
# import subprocess module
import subprocess
# using the Popen() function
subprocess.Popen('dir')

3. check_call() :

# to implement the subprocess module in python using check_call()
# import subprocess module
import subprocess
# using the check_call() function
subprocess.check_call(['False'])

4. check_output() :

# to implement the subprocess module in python using check_output()
# import subprocess module
import subprocess
# using the check_output() function
subprocess.check_output('dir')

All the programs that are listed above will produce errors when they are executed. The reason for producing errors is that the given commands are not accepted in the form of strings. They are considered to be normal statements but not string commands.

To overcome these errors, we generally introduce a command, normally called an argument “ shell “, and we set it to True to express the given command in the form of a string. You can also produce the output without using the " shell " argument. For doing so, you need to put the commands inside the list, i.e., represent the commands in the form of a list so that the compiler identifies it as a string.  

Let us examine the same program again by using “ self “ and setting it to “ True “:

Program using run() function:

# to implement the subprocess module in python using run()
# import subprocess module
import subprocess
# using the run() function
subprocess.run('dir',shell=True)

Output:

Volume in drive D is Data
Volume Serial Number is 40D9-C745 
Directory of D:\downloads hdd\2-2


09-07-2022  21:38    <DIR>          .
08-07-2022  21:48    <DIR>          ..
30-06-2022  20:03             1,515 #class HumanSpecies is being declared fo.py
21-06-2022  14:01           528,681 B.Tech_2-2 Sem R20_Regular_July 2022.pdf
23-04-2022  20:51         3,612,851 daa unit 1.pdf
23-04-2022  20:51         1,774,354 daa unit 2.pdf
23-04-2022  20:52         4,855,826 daa unit 3.pdf
23-04-2022  20:56         1,971,782 dbms.pdf
30-06-2022  21:23               575 import csv.py
23-04-2022  20:45           128,387 p&s ASSIGNMENT-I.pdf
29-06-2022  22:09             1,513 test.py
27-06-2022  22:49                38 Untitled-1.py
09-07-2022  21:38               157 Untitled-11.py
27-06-2022  23:00                32 Untitled-2.py
27-06-2022  23:10                32 Untitled-3.py
30-06-2022  20:03             2,018 Untitled-4.py
30-06-2022  20:58               843 Untitled-6.py
30-06-2022  22:02               259 Untitled-7.py
              16 File(s)     12,878,863 bytes
               2 Dir(s)  989,647,040,512 bytes free

These are the files present in my directory. All the details, including time, date, and space occupancy, will also be printed along with the name of the files.

Program using Popen() function:

# to implement the subprocess module in python using Popen()
# import subprocess module
import subprocess
# using the Popen() function
subprocess.Popen('dir',shell=True)

Output

Volume in drive D is Data
Volume Serial Number is 40D9-C745


Directory of D:\downloads hdd\2-2


09-07-2022  21:38    <DIR>          .
08-07-2022  21:48    <DIR>          ..
30-06-2022  20:03             1,515 #class HumanSpecies is being declared fo.py
21-06-2022  14:01           528,681 B.Tech_2-2 Sem R20_Regular_July 2022.pdf
23-04-2022  20:51         3,612,851 daa unit 1.pdf
23-04-2022  20:51         1,774,354 daa unit 2.pdf
23-04-2022  20:52         4,855,826 daa unit 3.pdf
23-04-2022  20:56         1,971,782 dbms.pdf
30-06-2022  21:23               575 import csv.py
23-04-2022  20:45           128,387 p&s ASSIGNMENT-I.pdf
29-06-2022  22:09             1,513 test.py
27-06-2022  22:49                38 Untitled-1.py
09-07-2022  21:38               157 Untitled-11.py
27-06-2022  23:00                32 Untitled-2.py
27-06-2022  23:10                32 Untitled-3.py
30-06-2022  20:03             2,018 Untitled-4.py
30-06-2022  20:58               843 Untitled-6.py
30-06-2022  22:02               259 Untitled-7.py
       16 File(s)     12,878,863 bytes
       2 Dir(s)  989,647,040,512 bytes free

The above program prints all the folders that are present in the current directory using the command “ dir “. We can observe that the output of the first program is equal to the output of the second program. So, any one of these methods can be used in order to implement a subprocess in Python.