[patch] Re: -n vs batch_mode vs batch_flag

[Tom Holroyd]

On Sun, 8 Apr 2001, Markus Friedl wrote:

> i did not invent '-n' but the manpage reads:
>
>      -n      Redirects stdin from /dev/null (actually, prevents reading from
>              stdin).  This must be used when ssh is run in the background.  A
>              common trick is to use this to run X11 programs on a remote ma-
>              chine.  For example, ssh -n shadows.cs.hut.fi emacs & will start
>              an emacs on shadows.cs.hut.fi, and the X11 connection will be au-
>              tomatically forwarded over an encrypted channel.  The ssh program
>              will be put in the background.  (This does not work if ssh needs
> 					     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>              to ask for a password or passphrase; see also the -f option.)
> 	     ^^^^^^^^^^^^^^^^^^^^^			   ^^^^^^^^^^^^^

Yes, I read that.  Notice how ssh doesn't do what that says.

> > 	% ssh -n localhost &
> > 	[1] 5220
> > 	% tomh at localhost's password:
> > 	[1]  + Suspended (tty input)         ssh -n localhost

According to the man page, -n should _prevent_ reading from stdin, but it
doesn't.  There is a mechanism for this (batch mode) but it isn't used.  I
was suggesting that -n set options.batch_mode (the stuff I said about
batch_flag was all wrong; if -n sets options.batch_mode it works).

Here's a patch.  With this, you can put
	ssh -n host command &
in a background script (or a menu item) and it will die gracefully if a
passwordless method (such as pubkey) isn't available.  If you say -f then
it's an error to use batch mode.

--- ssh.c.old   Mon Apr  9 10:44:52 2001
+++ ssh.c       Mon Apr  9 10:53:35 2001
@@ -335,6 +335,7 @@
                        break;
                case 'n':
                        stdin_null_flag = 1;
+                       options.batch_mode = 1;
                        break;
                case 'f':
                        fork_after_authentication_flag = 1;
@@ -533,6 +534,8 @@
        /* Cannot fork to background if no command. */
        if (fork_after_authentication_flag && buffer_len(&command) == 0 && !no_shell_flag)
                fatal("Cannot fork into background without a command to execute.");
+       if (fork_after_authentication_flag)
+               options.batch_mode = 0;

        /* Allocate a tty by default if no command specified. */
        if (buffer_len(&command) == 0)


Dr. Tom