CODEWITHSUNDEEP
pythonpopen
Pete Roberts
Pete Roberts

Reputation: 143

subprocess.Popen() IO redirect

Trying to redirect a subprocess' output to a file.

server.py:

while 1:
    print "Count " + str(count)
    sys.stdout.flush()
    count = count + 1
    time.sleep(1)

Laucher:

cmd = './server.py >temp.txt'
args = shlex.split(cmd)
server = subprocess.Popen( args )

The output appear on screen, temp.txt remains empty. What am I doing wrong?

As background I am trying to capture the output of a program that has already been written.

I cannot use:

server = subprocess.Popen(
                [exe_name],
                stdin=subprocess.PIPE, stdout=subprocess.PIPE)

as the program may not flush. Instead I was going to redirect output through a fifo. This works fine if I manually launch server.py but obviously not if I Popen() cause redirect doesnt work. ps -aux shows that server.py was launched correctly.

Upvotes: 14

Views: 21507

Answers (2)

jcollado
jcollado

Reputation: 40424

Altenatively, you can use the stdout parameter with a file object:

with open('temp.txt', 'w') as output:
    server = subprocess.Popen('./server.py', stdout=output)
    server.communicate()

As explained in the documentation:

stdin, stdout and stderr specify the executed program’s standard input, standard output and standard error file handles, respectively. Valid values are PIPE, an existing file descriptor (a positive integer), an existing file object, and None.

Upvotes: 16

AdamKG
AdamKG

Reputation: 14091

Output redirection with ">" is a feature of shells - by default, subprocess.Popen doesn't instantiate one. This should work:

server = subprocess.Popen(args, shell=True)

Upvotes: 9

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