My question can be seen as an extension of How do diff over ssh? with a little twist.


macOS Sierra (10.12.3)
OpenSSH 7.3

Example situation

I am trying to print a diff between two files, one of them is located on a remote host and requires sudo for reading.

If this remote file was readable by my remote user (or if I could execute the command with the sudo flag NOPASSWD) I would simply execute:

diff LOCALFILE <(ssh host 'cat REMOTEFILE')

However I do need to authenticate in order to execute the remote command, which means I have to allocate a pseudo-terminal with ssh -t, and the following execution never completes:

diff LOCALFILE <(ssh -t host 'sudo cat REMOTEFILE')


ps shows me that the ssh process is stopped:

S+    0:00.00   diff LOCALFILE /dev/fd/12
T     0:00.03   ssh -t host sudo cat REMOTEFILE

The ssh process does not respond to SIGTERM at that point, and the file descriptor above does not exist:

❯ ls -l /dev/fd
total 0
crw--w----  1 me     tty     16,   2 Feb 23 17:36 0
crw--w----  1 me     tty     16,   2 Feb 23 17:36 1
crw--w----  1 me     tty     16,   2 Feb 23 17:36 2
dr--r--r--  1 root   wheel         0 Feb 23 09:33 4

Extra notes

I can observe the exact same behavior any time I use ssh -t within a process substitution, regardless of whether a password is needed or not for sudo (that said I understand the use of -t is questionable if no input is expected on the remote host).

Bottom line

I'm using diff as an example here, but my question is actually more general: is there any way to use a Bash process substitution together with a SSH pseudo-terminal allocation?

  • I don't have requiretty set, besides my problem is that I actually want to use a tty in that case. Feb 23, 2017 at 17:27
  • Why do you think you want a tty? What would it possibly gain you? Feb 23, 2017 at 17:29
  • 1
    That was the whole point of the question - It is often best to make your question title the most specific version of your question. Something like "Provide sudo password over ssh while using process substitution" instead of the more generic option you used. Having a specific title and first sentence really is really the best way to get good answers and comments.
    – Zoredache
    Feb 23, 2017 at 17:44
  • 1
    This honestly sounds like a job for expect, if it is/can be installed on the server. You might be able to get it to work with expect installed locally. If you just run ssh -t host sudo cat REMOTEFILE alone, do you see the sudo password prompt?
    – DerfK
    Feb 23, 2017 at 18:25

1 Answer 1


You can force sudo to take the password from STDIN using the -S option, per this superuser post. Thus the syntax diff LOCALFILE <(ssh host 'echo <password> | sudo -S cat REMOTEFILE') should do what you need, without the need for a pseudo-terminal.

  • 1
    That works indeed, thanks! I decided to export my password in the environment and use diff LOCALFILE <(echo $SUDOPASS | ssh host 'sudo -S cat REMOTEFILE'). Feb 24, 2017 at 11:40

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