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when i do ssh host it will log me in to host and then run bash. but when i do ssh host bash it will just sit there doing seemingly nothing. to get a usable bash prompt i have to provide -t like this: ssh -t host bash.

actually it does not just sit there doing nothing. when i do ssh host bash (without -t) it seems that i get a working bash but no prompt. i can do commands like echo foo or ps -ef and will get usable output. just no prompt.

why do i need -t for ssh host bash to get a usable bash prompt but ssh host runs bash just fine?

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When you run:

ssh host some    command

ssh doesn't request a pseudo-tty device to be used on the remote end. The stdout and stderr or the remote commands are pipes instead. sshd runs:

exec("remote-user-login-shell", ["remote-user-login-shell", "-c", "some command"])

That's the rsh mode. When you don't specify a command, it enters the rlogin mode, where it does start a pseudo-terminal, and runs a login interactive shell session, where it runs:

exec("remote-user-login-shell", ["-remote-user-login-shell"])

(with a leading - in the argv[0] to tell the shell it is to behave as a login shell).

You need -t when you need to run a specific command interactively (like bash or vi) in which case sshd does create a pseudo-terminal on the remote host to interact with the remote command. If not, you do not want to use -t.

In:

ssh host bash

bash doesn't do nothing, it's just not interactive. It will process the input the same way as it would process a script or like you would get by running cat | bash. You can still enter commands and see their output.

Historically, rsh (which ssh replaced) was actually running rlogin when not given any command to run, and rlogin was invoking a different service (on a different TCP port) from the rsh service.

  • Can the answer explain what a "remote tty" is? – fraxture Jul 29 at 14:42
  • @fraxture, see edit where I've tried to make the wording more consistent. – Stéphane Chazelas Jul 29 at 14:46

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