Write to file descriptor 3 of a Python subprocess.Popen object

Question

How do I write to file descriptor 3 of a subprocess.Popen object?

I'm trying to accomplish the redirection in the following shell command with Python (without using named pipes):

$ gpg --passphrase-fd 3 -c 3<passphrase.txt < filename.txt > filename.gpg

Answer 1

The subprocess proc inherits file descriptors opened in the parent process. So you can use os.open to open passphrase.txt and obtain its associated file descriptor. You can then construct a command which uses that file descriptor:

import subprocess
import shlex
import os

fd=os.open('passphrase.txt',os.O_RDONLY)
cmd='gpg --passphrase-fd {fd} -c'.format(fd=fd)
with open('filename.txt','r') as stdin_fh:
    with open('filename.gpg','w') as stdout_fh:        
        proc=subprocess.Popen(shlex.split(cmd),
                              stdin=stdin_fh,
                              stdout=stdout_fh)        
        proc.communicate()
os.close(fd)

---

To read from a pipe instead of a file, you could use os.pipe:

import subprocess
import shlex
import os

PASSPHRASE='...'

in_fd,out_fd=os.pipe()
os.write(out_fd,PASSPHRASE)
os.close(out_fd)
cmd='gpg --passphrase-fd {fd} -c'.format(fd=in_fd)
with open('filename.txt','r') as stdin_fh:
    with open('filename.gpg','w') as stdout_fh:        
        proc=subprocess.Popen(shlex.split(cmd),
                              stdin=stdin_fh,
                              stdout=stdout_fh )        
        proc.communicate()
os.close(in_fd)