SSH Command Execution Hangs, although interactive shell functions fine

Question

When I attempt to execute a command on a remote server with ssh, the ssh command hangs after the exec request accepted debug message, and eventually times out.

The failing command: ssh -v -v <username>@<server> uptime (also tried echo hello etc.)

debug1: Authentication succeeded (publickey).
Authenticated to <server> (<ip>:22).
debug1: channel 0: new [client-session]
debug2: channel 0: send open
debug1: Requesting [email protected]
debug1: Entering interactive session.
debug2: callback start
debug2: client_session2_setup: id 0
debug2: fd 4 setting TCP_NODELAY
debug1: Sending environment.
debug1: Sending env LANG = en_US.UTF-8
debug2: channel 0: request env confirm 0
debug1: Sending command: uptime
debug2: channel 0: request exec confirm 1
debug2: callback done
debug2: channel 0: open confirm rwindow 0 rmax 32768
debug2: channel 0: rcvd adjust 2097152
debug2: channel_input_status_confirm: type 99 id 0
debug2: exec request accepted on channel 0

And there it hangs, indefinitely.

When I ssh without a command into my remote server, however, I get an interactive shell and all is good.

Successful Command: ssh -v -v <username>@<server>

Output:

debug1: Authentication succeeded (publickey).
Authenticated to <server> (<ip>:22).
debug1: channel 0: new [client-session]
debug2: channel 0: send open
debug1: Requesting [email protected]
debug1: Entering interactive session.
debug2: callback start
debug2: client_session2_setup: id 0
debug2: fd 4 setting TCP_NODELAY
debug2: channel 0: request pty-req confirm 1
debug1: Sending environment.
debug1: Sending env LANG = en_US.UTF-8
debug2: channel 0: request env confirm 0
debug2: channel 0: request shell confirm 1
debug2: callback done
debug2: channel 0: open confirm rwindow 0 rmax 32768
debug2: channel_input_status_confirm: type 99 id 0
debug2: PTY allocation request accepted on channel 0
debug2: channel 0: rcvd adjust 2097152
debug2: channel_input_status_confirm: type 99 id 0
debug2: shell request accepted on channel 0
Welcome!
<prompt>%
...

Has anyone an idea why an interactive session would be successful but a command execution not?

Has been haunting me for months now because I cannot use unison to sync my files any more (it used to work). Any help much appreciated.

Answer 1

The problem was indeed my login script, although not to do with requiring a terminal (I'd suspected that and tested with the -t and -T options). The problem was that my .bashrc was running an exec (in this case to zsh - because our system doesn't allow chsh to zsh).

The offending line:

test -f /usr/bin/zsh && exec /usr/bin/zsh

Solved by first checking for interactive shell and exiting if so:

[ -z "$PS1" ] && return
test -f /usr/bin/zsh && exec /usr/bin/zsh

So, essentially, because the shell was execing into zsh, ssh was waiting for this to finish - which never happened.

I am a little confused why my .bashrc was being called at all - I thought this was only for interactive shells, but the exact purpose and order of the various init scripts is something I don't think I'll ever learn.

I hope this can be useful to others who have some kind of exec in their startup scripts.

BTW - the other two answers were on the right track so I was completely unsure if I should 'answer' or just comment their answers. If answering my own question is morally wrong on stackoverflow, let me know and I'll do penitence. Thank you to the other answerers.

Answer 2

We fixed this by adding adding -n (to redirect std in from /dev/null) and -t (force pseudo-tty allocation)

Example:

ssh -t -n user@host command

Answer 3

Your problem most likely lies in your shell startup or shell logout scripts. Without knowing what's in there, it's hard to guess the actual problem.

Answer 4

I recently encountered a problem with the same symptoms, but determined that the issue was not a problem in my login scripts. Rather, my local .ssh/config file was configured with RequestTTY force for the host that I was trying to copy to.