How can I get stderr from os.popen()?

Question

If I call

os.popen('foo').read()

I want to capture

sh: foo: not found

as well.
I don't have the subprocess module since this is a minimal install on an embedded system.

popen3,4 doesn't work either:

 File "/usr/lib/python2.7/os.py", line 667, in popen3
import subprocess
ImportError: No module named subprocess

I suppose I could do

os.popen(command + " 2>&1").read()

to pipe it to stdout, but ideally I'd want to get it separately.

Use `os.popen4()`; it returns a tuple with `child_stdin, child_stderr`. You could also use `popen3()` which returns `child_stdin, child_stdout, child_stderr`. **Disclaimer**: I think. — zondo, Feb 24 '16 at 20:10
I don't know about embedded systems but if you've got Python, you've got filesystem rw, right? — cat, Feb 24 '16 at 20:15
You could even pipe it to another character device in `/dev/tty` and `open` that — cat, Feb 24 '16 at 20:16
@zondo `os.popen()` is a system call, not a wrapper -- subprocess would be preferred here — cat, Feb 24 '16 at 20:17

score 3 · Answer 1 · answered Oct 11 '17 at 11:22

3

Since subprocess should be use in place of os.popen, you can do something like that

test.py:

from subprocess import PIPE, Popen

p = Popen("foo", shell=True, stdout=PIPE, stderr=PIPE)
stdout, stderr = p.communicate()
print "stdout: '%s'" % stdout
print "stderr: '%s'" % stderr

Now execute:

python test.py 
stdout: ''
stderr: '/bin/sh: 1: foo: not found
'

Notice the CR in stderr.

answered Oct 11 '17 at 11:22

wilfriedroset

217
1
8

1

Would you care to elaborate as to why it is better? – user2589273 Jan 15 '20 at 15:56
1

@user2589273, (1) allows operating without a shell, which cures a series of security issues; (2) _whether or not_ operating without a shell, allows arguments to be passed out-of-band from the code being run, which lets someone write secure code even while using a shell (albeit requiring some effort). (3) offers vastly more flexibility -- lets one run a process in a different directory, perform redirections, control status of file descriptors, etc. (4) preferred as the replacement by upstream Python; as the current upstream docs say: ... – Charles Duffy Sep 21 '20 at 18:34
@user2589273, "*This is implemented using subprocess.Popen; see that class’s documentation for more powerful ways to manage and communicate with subprocesses*" -- so right now `os.popen` is just a less-powerful wrapper around `subprocess.Popen`; use the `subprocess` module and you get the full power. – Charles Duffy Sep 21 '20 at 18:34

How can I get stderr from os.popen()?

1 Answers1