CODEWITHSUNDEEP
pythonpopen
Ryan
Ryan

Reputation: 842

How to use fifo named pipe as stdin in Popen python

How do I make Popen use a fifo named pipe as stdin?

import subprocess
import os
import time

FNAME = 'myfifo'
os.mkfifo(FNAME, mode=0o777)
f = os.open(FNAME, os.O_RDONLY)

process = subprocess.Popen(
    'wait2.sh',
    shell=True,
    stdout=subprocess.PIPE,
    stdin=f,
    stderr=subprocess.PIPE,
    universal_newlines=True,
)

while process.poll() is None:
    time.sleep(1)
    print("process.stdin", process.stdin)

If I run this script and in a terminal window

echo "Something" > myfifo

The process exits with process.stdin None. It seems its not getting the stdin from the fifo.

Upvotes: 5

Views: 4184

Answers (1)

Daniel Junglas
Daniel Junglas

Reputation: 5930

According to the documentation, the Popen.stdin is only not None if the argument for that field was PIPE, which is not the case in your code.

This code works fine for me, it prints "Line 1" and "Line 2" (from the child process) as expected

import subprocess
import os
import time

FNAME = 'myfifo'
os.mkfifo(FNAME, mode=0o777)

# Open read end of pipe. Open this in non-blocking mode since otherwise it
# may block until another process/threads opens the pipe for writing.
stdin = os.open(FNAME, os.O_RDONLY | os.O_NONBLOCK)

# Open the write end of pipe.
tochild = os.open(FNAME, os.O_WRONLY)
print('Pipe open (%d, %d)' % (stdin, tochild))

process = subprocess.Popen(
    ['/usr/bin/cat'],
    shell=True,
    stdout=None,
    stdin=stdin,
    stderr=None,
    universal_newlines=True,
)
print('child started: %s (%s)' % (str(process), str(process.stdin)))

# Close read end of pipe since it is not used in the parent process.
os.close(stdin)

# Write to child then close the write end to indicate to the child that
# the input is complete.
print('writing to child ...')
os.write(tochild, bytes('Line 1\n', 'utf-8'))
os.write(tochild, bytes('Line 2\n', 'utf-8'))
print('data written')
os.close(tochild)

# Wait for child to complete.
process.wait()
os.unlink(FNAME)

Upvotes: 7

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