How to read file of subprocess output

Question

In this code, subprocess.Popen creates /tmp/test.txt with date output. Why doesn't the 2nd "print lines" work ? I would appreciate for your help.

test]# touch /tmp/test.txt
test]# ./x1.py
/tmp/test.txt
()
/tmp/test.txt
()
test]# ./x1.py
/tmp/test.txt
('Sun Jun 19 15:10:21 PDT 2016\n',)
/tmp/test.txt
()
test]# cat x1.py
#!/usr/bin/python

from subprocess import Popen, PIPE

filename = "/tmp/test.txt"

lines = tuple(open(filename, 'r'))
print filename
print  lines # this is not empty

file_ = open(filename, "w")
Popen("date", shell=True, stdout=file_)
file_.close()

lines = tuple(open(filename, 'r'))
print filename
# why is this empty even if file_ definitely created the /tmp/test.txt ?
print  lines    
test]#

Have you considered .wait()ing for the Popen to finish before closing its stdout? — Tadhg McDonald-Jensen
– Tadhg McDonald-Jensen, Commented Jun 19, 2016 at 22:52

Tadhg McDonald-Jensen · Accepted Answer · 2016-06-19 23:05:22Z

2

You need to .wait() for the Popen to finish before closing the stdout, otherwise you get undefined behaviour.

Popen("date", shell=True, stdout=file_).wait()
#                                       ^ add this part!

I can't say for sure how the file was written to after you tried to read from it but I imagine that is an implementation detail for your OS.

answered Jun 19, 2016 at 23:05

Tadhg McDonald-Jensen

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