In [1]:
#Subprocess Module
#The subprocess module is used to run new applications or programs through Python code by creating new processes. It also helps to obtain the input/output/error pipes as well as the exit codes of various commands.
In [2]:
import subprocess
In [5]:
#it will run ls command and will print the output
subprocess.run('ls')
Out[5]:
CompletedProcess(args='ls', returncode=0)
In [7]:
#To use the suboptions of command you need to pass shell=True parameter also in subprocess.run
subprocess.run('ls -la',shell=True)
Out[7]:
CompletedProcess(args='ls -la', returncode=0)
In [8]:
#If you don't want to pass shell=True, you can pass it a list of arguments
subprocess.run(['ls','-la'])
Out[8]:
CompletedProcess(args=['ls', '-la'], returncode=0)
In [9]:
#this will print the command output in screen and will store CompletedProcess(args=['ls', '-la'], returncode=0)
p = subprocess.run(['ls','-la'])
In [10]:
#we can check arguments and return code, 0 meaning run with 0 errors
print(p.args)
print(p.returncode)

['ls', '-la']
0
In [27]:
#To store the standoutput and error in variable use capture_output=True
p = subprocess.run(['ls', '/home/kushagra/Desktop/udemy'],capture_output=True)
In [28]:
print(p)

CompletedProcess(args=['ls', '/home/kushagra/Desktop/udemy'], returncode=0, stdout=b'app.py\ntemplates\n', stderr=b'')
In [29]:
#if only want stdout without args and returncode
print(p.stdout)

b'app.py\ntemplates\n'
In [30]:
#we can decode this by using decode 
# it will print the output same as linux command would have done
print(p.stdout.decode())

app.py
templates
In [25]:
#another way is that instead of using decode we can use
p = subprocess.run(['ls','/home/kushagra/Desktop/udemy'],capture_output=True,text=True)
In [26]:
print(p.stdout)

app.py
templates
In [31]:
#To store Standard Output and not error
p = subprocess.run(['ls','/home/kushagra/Desktop/udemy'],stdout=subprocess.PIPE,text=True)
In [32]:
#will result as above
print(p.stdout)

app.py
templates
In [33]:
#we can also pipe the out to file. [can be used for logging purposes]
with open('output.txt','w') as f:
    p = subprocess.run(['ls','/home/kushagra/Desktop/udemy'],stdout=f,text=True)
In [34]:
#we can see that the output of the command is now saved in the file.
!cat output.txt

app.py
templates
In [41]:
#listing the contents of file that doesnot exist.
p = subprocess.run(['ls','-la','den'],capture_output=True,text=True)
In [42]:
#will give the returncode if not 0 means error
print(p.returncode)
#for checking the error
print(p.stderr)

2
ls: cannot access 'den': No such file or directory
In [46]:
#python won't throw exception if external command fails.. if you want python to do that you can pass check=True parameter in subprocess.run
p = subprocess.run(['ls','den'],capture_output=True,check=True,text=True) 
#we can see now python throws the exception which might be useful in some case.

---------------------------------------------------------------------------
CalledProcessError                        Traceback (most recent call last)
<ipython-input-46-2e01f752a78c> in <module>
      1 #python won't throw exception if external command fails.. if you want python to do that you can pass check=True parameter in subprocess.run
----> 2 p = subprocess.run(['ls','den'],capture_output=True,check=True,text=True)

~/anaconda3/lib/python3.7/subprocess.py in run(input, capture_output, timeout, check, *popenargs, **kwargs)
    485         if check and retcode:
    486             raise CalledProcessError(retcode, process.args,
--> 487                                      output=stdout, stderr=stderr)
    488     return CompletedProcess(process.args, retcode, stdout, stderr)
    489 

CalledProcessError: Command '['ls', 'den']' returned non-zero exit status 2.
In [47]:
#we can redirect out standard error to /dev/null if you don't wish to see any error message
p = subprocess.run(['ls','den'],stderr=subprocess.DEVNULL)
In [48]:
#we can see no error here.
print(p.stderr)

None
In [53]:
#case when we want to transfer output of one command to input of another. [output redirection]
p1 = subprocess.run(['cat','yo.txt'],capture_output=True,text=True)
In [56]:
#contents of file
print(p1.stdout)

This
is
a 
test 
file 
with 
7 lines.
In [58]:
#using the output of p1 in p2
p2= subprocess.run(['grep','-n','test'],capture_output=True,text=True,input=p1.stdout)
In [59]:
#printing result of grep command
print(p2.stdout)

4:test
In [60]:
#or we can directly use this we are not going to use parsing between commands anywhere else.
p1 = subprocess.run('cat yo.txt | grep -n test',capture_output=True,text=True,shell=True)
In [61]:
print(p1.stdout)

4:test