How to Use Python Subprocess to Write a String to a File

Question

I am trying to figure out what happens in subprocess. So, I write this code:

import subprocess

p1 = subprocess.Popen('grep a', stdin=subprocess.PIPE, stdout=subprocess.PIPE,shell=True, universal_newlines=True)
p2 = subprocess.Popen('grep a', stdin=p1.stdout, stdout=open('test', 'w'),shell=True, universal_newlines=True)

p1.stdin.write("asdads"*700)

After I write the string to p1.stdin, I expected the string to be written to the file test. But there is nothing in the file.

When I tried another way:

out, err = p1.communicate("adsad"*700)

The string is in out.

Why not use `open` to open a file and write it the normal way? — Devesh Kumar Singh, May 17 '19 at 04:46
Maybe start with a simpler problem, like calling a command like `ls` or `date` and seeing it's output? — Devesh Kumar Singh, May 17 '19 at 04:49

score 2 · Accepted Answer · answered May 17 '19 at 05:23

Your code wasn't working because your stdin stream is not closed. To prove what I'm saying:

p1 = subprocess.Popen('grep l', stdin=subprocess.PIPE, stdout=subprocess.PIPE, shell=True, universal_newlines=True)
out, err = p1.communicate('hello')

>>> out
'hello\n'

Now test with communicate, which automatically closes stream for you.

p1 = subprocess.Popen('grep l', stdin=subprocess.PIPE, stdout=subprocess.PIPE, shell=True, universal_newlines=True)
p2 = subprocess.Popen('grep h', stdin=p1.stdout, stdout=open('test', 'w'),shell=True, universal_newlines=True)

# test is already created and empty
# but call communicate again can write to file
>>> p1.communicate('hello')
('', None)

$cat test
hello

And another way:

# use stdin.write
>>> p1.stdin.write('hello') # empty file
>>> p1.stdin.close() # flushed

References:

How to Use Python Subprocess to Write a String to a File

1 Answers1