How to get python script to differentiate if output is going to pipe vs. process substitution

Question

Trying to differentiate if stdout is going to pipe or process substitution.

my_python_script.py

#!/usr/bin/python3.6
if sys.__stdin__.isatty():
    print("__stdin__ is TTY")
else:
    print("__stdin__ is not TTY")
if sys.__stdout__.isatty():
    print("__stdout__ is TTY")
else:
    print("__stdout__ is not TTY")
if sys.__stderr__.isatty():
    print("__stderr__ is TTY")
else:
    print("__stderr__ is not TTY")

if sys.stdin.isatty():
    print("stdin is TTY")
else:
    print("stdin is not TTY")
if sys.stdout.isatty():
    print("stdout is TTY")
else:
    print("stdout is not TTY")
if sys.stderr.isatty():
    print("stderr is TTY")
else:
    print("stderr is not TTY")

The output below is identical if pipe or process substitution.

> my_python_script.py | cat

__stdin__ is TTY
__stdout__ is not TTY
__stderr__ is TTY
stdin is TTY
stdout is not TTY
stderr is TTY

> cat <(my_python_script.py)

__stdin__ is TTY
__stdout__ is not TTY
__stderr__ is TTY
stdin is TTY
stdout is not TTY
stderr is TTY

Is there a way to differentiate between output to a pipe vs. process substitution inside a python script?

Thank you

Philippe · Accepted Answer · 2021-03-13 22:15:01Z

1

Following script should differentiate two cases :

import os
import re
import sys

def get_fds(id):
    fds = [os.path.realpath(f"/proc/{id}/fd/{i}") 
           for i in list(os.walk(f'/proc/{id}/fd'))[0][2]]
    return [re.sub('.*/', '', i) 
            for i in fds if "pipe" in i]

if  any(i == j for i in get_fds(os.getpid())
               for j in get_fds(os.getppid())):
    print("Process substitution", file=sys.stderr)
else:
    print("Normal pipe", file=sys.stderr)

answered Mar 13, 2021 at 22:15

Philippe

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1 Comment

Sultan Over a year ago

Thank you very much. Just tried it and seems to work. Will test it a bit more and also break it up to understand what exactly your code is doing.

user2849202 · Accepted Answer · 2021-03-13 20:54:04Z

0

The process subsitiution creates a named pipe.

$ echo <(uptime)
/dev/fd/63

$ file <(uptime)
/dev/fd/63: broken symbolic link to pipe:[11474138]

So the answer is NO.

answered Mar 13, 2021 at 20:54

user2849202

1 Comment

Shawn Over a year ago

Bash only uses named pipes/fifos for process substitution when the OS doesn't support the /dev/fd pseudo-filesystem. That's just the /dev/fd entry for the read end of an anonymous pipe.

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