paramiko combine stdout and stderr

Question

I am trying to combine the output of stdout and stderr. My belief is that this can be done with the set_combine_stderr() of a Channel object.

This is what I am doing:

SSH = paramiko.SSHClient()
#I connect and everything OK, then:
chan = ssh.invoke_shell()
chan.set_combine_stderr(True)
chan.exec_command('python2.6 subir.py')
resultado = chan.makefile('rb', -1.)

However, I get the following error when I try to store the result (last line above, chan.makefile() ):

Error: Channel closed.

Any help would be greatly appreciated

score 22 · Answer 1 · answered Nov 08 '11 at 03:53

While it is true that set_combine_stderr diverts stderr to the stdout stream, it does so in chaotic order, so you do not get the result you probably want, namely, the lines combined in the order written, as if you were running the command in a local terminal window. Instead, use get_pty. That will cause the server to run the lines through a pseudo-terminal, keeping them in chronological sequence.

Here's a test program, outerr.py, that writes alternating lines on stdout and stdin. Assume it's sitting in the home directory of llmps@meerkat2.

#!/usr/bin/env python

import sys

for x in xrange(1, 101):
    (sys.stdout, sys.stderr)[x%2].write('This is line #%s, on std%s.\n' %
                (x, ('out', 'err')[x%2]))

Now try the following code to run it remotely:

#!/usr/bin/env python

import paramiko

def connect():
    ssh = paramiko.SSHClient()
    ssh.set_missing_host_key_policy(paramiko.AutoAddPolicy())
    ssh.connect('meerkat2', username='llmps', password='..')
    return ssh

def runTest(ssh):
    tran = ssh.get_transport()
    chan = tran.open_session()
    # chan.set_combine_stderr(True)
    chan.get_pty()
    f = chan.makefile()
    chan.exec_command('./outerr.py')
    print f.read(),

if __name__ == '__main__':
    ssh = connect()
    runTest(ssh)
    ssh.close()

If you run the above, you should see 100 lines in order as written. If, instead, you comment out the chan.get_pty() call and uncomment the chan.set_combine_stderr(True) call, you will get clumps of stdout and stderr lines interspersed randomly from run to run.

score 6 · Answer 2 · answered Sep 23 '16 at 15:01

6

Ok, I know this is quite an old topic, but I run into the same problem and I got a (maybe not-so-)pretty solution. Just call the command on the remote server redirecting the stderr to stdout and then always read from the stdout. For example:

client = paramiko.SSHClient()
client.load_system_host_keys()
client.set_missing_host_key_policy(paramiko.AutoAddPolicy())
client.connect('hostname', username='user', password='pass')

stdin,stdout,stderr = client.exec_command('python your_script.py 2> \&1')
print stdout.read()

answered Sep 23 '16 at 15:01

robotic_chaos

570
5
10

thanks for this answer! the backslash on `\&1` is what i was missing. why is that needed? – starwarswii Mar 25 '22 at 18:23
It has been a while since I wrote this answer. If I remember correctly, you need to escape the '&' char, so bash doesn't interpret it as a signal to run the command in the background. – robotic_chaos Mar 27 '22 at 13:59

score 3 · Answer 3 · edited Apr 14 '17 at 09:53

@AaronMcSmooth: I am referring to the stdout and stderr of the computer I am connecting to (via SSH).

I ended up doing this:

stdin, stdout, stderr = ssh.exec_command(...)

output = stdin.read().strip() + stdout.read().strip()

For the purpose of my application, it doesn't matter to distinguish between stdout and stderr, but I don't think that's the best way to combine the two.

The code of SSHClient.exec_command() is (looking at paramiko's source code):

def exec_command(self, command, bufsize=-1):
    chan = self._transport.open_session() 
    chan.exec_command(command) 
    stdin = chan.makefile('wb', bufsize) 
    stdout = chan.makefile('rb', bufsize) 
    stderr = chan.makefile_stderr('rb', bufsize) 
    return stdin, stdout, stderr

I am performing the same actions on the channel but receive the Channel is closed error.

Looks like you're trying to read from `stdin`? I think you may have meant to read from `stderr`. — greezybacon, Mar 10 '21 at 15:36

paramiko combine stdout and stderr

3 Answers3