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.<

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

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 3

Check for commands in your shell startup files (I would assume ~/.cshrc from your prompt; in a non-interactive session, ~/.login shouldn't matter) that require a terminal for some reason.

Answer 4

I eventually found the "$-" var which works for me:

if [[ $- =~ i ]] ; then
    [ -x /bin/tcsh ] && exec /bin/tcsh
    # Bash startup stuff goes here...
fi

Got this from: https://www.gnu.org/software/bash/manual/html_node/Is-this-Shell-Interactive_003f.html

Answer 5

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.

Answer 6

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