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I'm using Bash on both client and server. When running a command over SSH:

  • ssh <host> 'declare' gives a list of shell variables.

  • ssh <host> 'mount' gives a list of mountpoints.

However, declare is a Bash builtin, while mount is an external command. How does SSH know which to run if there is a shell builtin and an external command with the same name on the server?

  • 2
    ssh always run your shell, which is stored in /etc/passwd. If it's /usr/sbin/nologin you can't log in. – Ipor Sircer Dec 23 '16 at 21:26
22

The ssh runs the commands you provide in the remote user's shell (obtained from the /etc/passwd), as visible from the source code:

argv[0] = (char *) shell0;
argv[1] = "-c";
argv[2] = (char *) command;
argv[3] = NULL;
execve(shell, argv, env);

Therefore the respective commands that is executed for your example on the remote server are:

  • bash -c declare
  • bash -c mount

Both of them are passed to the bash and evaluated. Built-ins are evaluated inside, and the external commands are called as if you would do that from your local command line prompt.

  • If a shell is always started by ssh daemon, then the interesting thing is, when user calls ssh with a command, then that shell is not a login shell, even if user performed some kind of login. Any idea? – Cyker Dec 24 '16 at 9:15
  • Yes. The shell running the command is 1) No a login shell 2) Not interactive one. You can force the interactivity by the -t switch, but it will still not load the rc files. – Jakuje Dec 24 '16 at 9:33
  • 1
    Note that sshd (the server) is doing this, not ssh (the client). – ysdx Dec 24 '16 at 13:02
  • @ysdx They're working together. The client tells the server to execute the command through a shell, and the server does it. – Barmar Dec 28 '16 at 18:44

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