Merging a Python script's subprocess' stdout and stderr while keeping them distinguishable

python subprocess stdout stderr
beemtee picture beemtee · Jul 24, 2011 · Viewed 17.7k times · Source

I would like to direct a python script's subprocess' stdout and stdin into the same file. What I don't know is how to make the lines from the two sources distinguishable? (For example prefix the lines from stderr with an exclamation mark.)

In my particular case there is no need for live monitoring of the subprocess, the executing Python script can wait for the end of its execution.

Answer

mossman picture mossman · Oct 3, 2011 
tsk = subprocess.Popen(args,stdout=subprocess.PIPE,stderr=subprocess.STDOUT)

subprocess.STDOUT is a special flag that tells subprocess to route all stderr output to stdout, thus combining your two streams.

btw, select doesn't have a poll() in windows. subprocess only uses the file handle number, and doesn't call your file output object's write method.

to capture the output, do something like:

logfile = open(logfilename, 'w')

while tsk.poll() is None:
    line = tsk.stdout.readline()
    logfile.write(line)

Related questions

"subprocess.Popen" - checking for success and errors

I want to check if a subprocess has finished execution successfully or failed. Currently I have come up with a solution but I am not sure if it is correct and reliable. Is it guaranteed that every process outputs its …

Read More
Python read from subprocess stdout and stderr separately while preserving order

I have a python subprocess that I'm trying to read output and error streams from. Currently I have it working, but I'm only able to read from stderr after I've finished reading from stdout. Here's what it looks like: process = …

Read More
Python: get output from a command line which exits with nonzero exit code

I am Using Python 2.7.1 on a Windows Server 2008 R2 x64 box. I'm trying to get the output of a command line process which gives a nonzero exit status after outputting the information I need. I was initially using subprocess.check_…

Read More