How can I make my own working shell using ssh?

I am creating a custom shell in Python for a very limited user on the server who logs in via ssh with public key authentication. They must be able to run ls

, find -type d

and cat

in certain directories with certain restrictions. This works great if you run something like ssh user@server -i keyfile

, because you see an interactive prompt and can run these commands. However, something like ssh user@server -i keyfile "ls /var/log"

no. ssh

just freezes, no response. Using the switch -v

I found that the connection succeeds, so the problem is in my shell. I'm also pretty sure the script isn't even running since it print sys.argv

doesn't do anything at the beginning of the program. Here's the code:

#!/usr/bin/env python

import subprocess
import re
import os

with open(os.devnull, 'w') as devnull:
    proc = lambda x: subprocess.Popen(x, stdout=subprocess.PIPE, stderr=devnull)

    while True:
        try:
            s = raw_input('> ')
        except:
            break

        try:
            cmd = re.split(r'\s+', s)
            if len(cmd) != 2:
                print 'Not permitted.'
                continue
            if cmd[0].lower() == 'l':
                # Snip: verify directory
                cmd = proc(['ls', cmd[1]])
                print cmd.stdout.read()
            elif cmd[0].lower() == 'r':
                # Snip: verify directory
                cmd = proc(['cat', cmd[1]])
                print cmd.stdout.read()
            elif cmd[0].lower() == 'll':
                # Snip: verify directory
                cmd = proc(['find', cmd[1], '-type', 'd'])
                print cmd.stdout.read()
            else:
                print 'Not permitted.'
        except OSError:
            print 'Unknown error.'

      

        
        
        
      

    

And here's the relevant line from ~ / .ssh / authorized_keys:

command="/path/to/shell $SSH_ORIGINAL_COMMAND" ssh-rsa [base-64-encoded-key] user@host

      

        
        
        
      

    

How can I make a shell script when a command is passed on the command line, so it can be used in scripts without starting an interactive shell?