Assign shell script output to python variable ignoring error messages

Question

I have a python script that I am using to call a bash script that renames a file. I then need the new name of the file so python can do some further processing on it. I'm using subprocess.Popen to call the shell script. The shell script echos the new file name so I can use stdout=subprocess.PIPE to get the new file name.

The problem is that sometimes the bash script tries to rename the file with it's old name depending on the circumstances and so gives the message that the two files are the same from the mv command. I have cutout all the other stuff and included a basic example below.

$ ls -1
test.sh
test.txt

This shell script is just an example to force the error message.

$ cat test.sh
#!/bin/bash
mv "test.txt" "test.txt"
echo "test"

In python:

$ python
>>> import subprocess
>>> p = subprocess.Popen(['/bin/bash', '-c', './test.sh'], stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
>>> p.stdout.read()
"mv: `test.txt' and `test.txt' are the same file\ntest\n"

How can I ignore the message from the mv command and only get the output of the echo command? If all goes well the only output of the shell script would be the result of the echo so really I just need to ignore the mv error message.

Thanks,

Geraint

Don't send `stderr` to `stdout` if you don't want to see error messages on `stdout`. — Etan Reisner, Jan 27 '15 at 17:41
so you don't want to know if the command has not been successful? — Padraic Cunningham, Jan 27 '15 at 17:56

user590028 · Accepted Answer · 2015-01-28T00:26:03.943

0

Direct stderr to null, Thusly

$ python
>>> import os
>>> from subprocess import *
>>> p = Popen(['/bin/bash', '-c', './test.sh'], stdout=PIPE, stderr=open(os.devnull, 'w'))
>>> p.stdout.read()

edited Jan 28 '15 at 00:26

answered Jan 27 '15 at 17:53

user590028

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do not set `stderr=PIPE` unless you read from the pipe. [`os.devnull` is portable](http://stackoverflow.com/a/28181836/4279) – jfs Jan 27 '15 at 23:17
I see no problem with allowing the GC to reap the unused PIPE. But the `os.devnull` is a nice trick I didn't know. – user590028 Jan 28 '15 at 00:19
unread PIPE is a deadlock waiting to happen. Count number of warnings about it in the subprocess docs. – jfs Jan 28 '15 at 00:22
I thought of that, but my take was that OP was talking about a single line of error from his script. Any deadlock would not take place until a buffer was full (typically 4k). But you're right -- it's bad form and should be avoided. I've revised my answer – user590028 Jan 28 '15 at 00:27

score 0 · Answer 2 · edited May 23 '17 at 12:22

0

To get subprocess' output and ignore its error messages:

#!/usr/bin/env python
from subprocess import check_output
import os

with open(os.devnull, 'wb', 0) as DEVNULL:
    output = check_output("./test.sh", stderr=DEVNULL)

check_output() raises an exception if the script returns with non-zero status.

See How to hide output of subprocess in Python 2.7.

edited May 23 '17 at 12:22

Community

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answered Jan 27 '15 at 23:16

jfs

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I assume if the OP wanted to ignore `stderr`, he doesn't want an exception either, no? – user590028 Jan 28 '15 at 00:27
I don't think so. If OP wants to ignore errors completely; the explicit `try/except` could be added – jfs Jan 28 '15 at 00:44
Thanks for this. Although in this case I don't want an exception this is somethiing that I am sure will be useful where It matters if there has been an error. – Geraint Anderson Jan 28 '15 at 11:03

Assign shell script output to python variable ignoring error messages

2 Answers2