Getting the path right for Mercurial over SSH

[Ben Schmidt]

[This is a repeat of the earlier post; I absentmindedly wrote git in the subject 
as it was mentioned in the original post. It is better now, and I hope a more 
inspiring subject line to readers of this list!]

Hi,

I wrote the post below on the Apple Mac OS X Server mailing list, though
not subscribed to that list, because I found a solution that I thought
others might appreciate having documented, as I found it quite a bit of
trouble getting it right myself. Though written for Mac OS X, the
essentials apply to any Unix-like system. As Mercurial was the reason I
was investigating this, and particularly since a post or two I found on
the web deemed the current documentation in Bryan's book regarding SSH to
be lacking in these kinds of details, I thought perhaps some other users
may appreciate the ideas as well. So I am forwarding this mail here. If
you think it would be helpful, do post it to an FAQ somewhere, or
incorporate it into the book, or whatever else you like; Google will
hopefully find it and help other users with it. The original message and
my followup, which isn't threaded, are here (and pasted below):

http://lists.apple.com/archives/macos-x-server/2008/May/msg00678.html
http://lists.apple.com/archives/macos-x-server/2008/Jun/msg00251.html

I'm planning to stick around for a bit and perhaps contribute a little
to Mercurial development, though I know no Python yet, so I definitely
won't be diving deeply into code just yet. But if there's somewhere
you'd like to point me to take this issue further, go right ahead.

Ben.



> Hi all,
>
> I've recently begun administering a Mac OS X Server 10.5.3 Leopard
> server used, in part, for my company's developers. We use a couple of
> tools (i.e., git, a version control system) that typically invoke non-
> interactive, non-login shells to do stuff. However, I discovered that
> when a user logs in via one of these tools, which in turn invokes SSH,
> the bash shell that user uses has a very minimal $PATH set. Thus, my
> question:
>
> Is there some place on Mac OS X Server to alter a non-interactive,
> non- login bash shell's $PATH in a system-wide way? The only solutions
> I've come across are all per-user solutions (e.g., ~/.bashrc or
> ~/.profile, ~/.ssh/environment, and so on).

These kinds of issues have been annoying me, too, and Googling to find a
solution led me to your post. After more trouble than I would have
liked, I discovered and came up with the following. (Don't get
disheartened part way through; there is a pretty good climax at the
end!) (Also, though I am on Mac OS X Client/User 10.4.9, I expect all
this applies to Mac OS X Server, and probably to 10.5, too.)

- When ssh is started without a commandline, an interactive login shell
   is started. This is nice! Everything works!

- When ssh is started with a commandline, a non-interactive non-login
   shell is started. However...bash does not use $BASH_ENV in this case,
   so setting it in ~/.ssh/environment (e.g. to /etc/profile) doesn't
   help. What bash does is source /etc/bashrc and ~/.bashrc--it evidently
   treats ssh like rsh and does a little more than is documented even for
   rsh (search for rsh in the bash man page for details). This is pretty
   much never what I want, as I would like all ssh logins to be 'login
   shells' and use /etc/profile and ~/.profile, but only those that are
   interactive to use ~/.bashrc. I came up with this fairly robust
   per-user solution, which I flooded with comments to help me remember
   what's used when:

   In /etc/profile

     # System-wide .profile for sh(1)
     PATH=...
     export PATH
     # Source /etc/bashrc for interactive shells.
     [ -n "$PS1" -a -r /etc/bashrc ] && source /etc/bashrc
     ... stuff for login shells (more environment mostly)

   In /etc/bashrc

     # System-wide .bashrc file for interactive bash(1) shells.
     ... stuff for interactive shells (aliases and terminal stuff mostly)

   In ~/.profile

     # For login shells (interactive or not).
     # For interactive non-login shells see ~/.bashrc.
     # For non-interactive non-login ssh shells see ~/.bashrc.
     # For other non-interactive non-login shells see $BASH_ENV.
     # At present this is sourced by ~/.bashrc for the ssh case, so
     # all ssh sessions act like login sessions.
     PATH=...
     export PATH
     # Source ~/.bashrc for interactive shells.
     if [ -n "$PS1" -a -r ~/.bashrc ]; then source ~/.bashrc ; fi
     ... stuff for login shells (more environment mostly)

   In ~/.bashrc

     # For interactive non-login shells
     # and non-interactive non-login ssh shells.
     # For login shells (interactive or not) see first existent of
     # ~/.bash_profile, ~/.bash_login and ~/.profile.
     # For other non-interactive non-login shells see $BASH_ENV.
     # At present, this is sourced by ~/.profile so runs for all
     # interactive shells (login or not).
     # If the shell isn't actually interactive it is an ssh session, and
     # we want to source /etc/profile and ~/.profile instead. We can't
     # use a simple test of $PS1 and must test $- because /etc/bashrc
     # (also sourced when bash detects ssh) sets $PS1. (We empty it
     # here.)
     if echo $- | grep -q i ; then : ; else
       [ -r /etc/profile ] && source /etc/profile
       PS1=
       source ~/.profile
       return
     fi
     ... stuff for interactive shells (aliases and terminal stuff mostly)

   This is OK if you're a single user wanting your shell to behave like
   you want it to, particularly if you're setting up your own account on
   a remote machine. Note that I have made one functional modification to
   /etc/profile: only sourcing /etc/bashrc if interactive; on my version
   of OS X it is sourced for all login shells, interactive or not. I
   have written ~/.bashrc so it will work OK without that change, though,

   But if you're a sysadmin, you don't want to deal with all this crap
   for every user, particularly if/when they break it.

- Setting $PATH and/or other environment variables directly via
   ~/.ssh/environment, rather than trying to get bash to run a script
   does work, but is another per-user solution.

- Eventually I came up with this system-wide solution. Add

     ForceCommand /usr/local/bin/ssh_session

   to /etc/sshd_config which makes ssh run that command regardless of
   what the user requested. It sets $SSH_ORIGINAL_COMMAND, though, so we
   can just use that to do what the user originally asked for, and seems
   to reliably work when used with eval. We can fall back to login if a
   command is not requested. This is what my /usr/local/bin/ssh_session
   script does:

     #!/bin/bash
     export SSH_SESSION=1
     if [ -z "$SSH_ORIGINAL_COMMAND" ] ; then
       export SSH_LOGIN=1
       exec login -fp "$USER"
     else
       export SSH_LOGIN=
       [ -r /etc/profile ] && source /etc/profile
       [ -r ~/.profile ] && source ~/.profile
       eval exec "$SSH_ORIGINAL_COMMAND"
     fi

   It is of course important to

     chmod +x /usr/local/bin/ssh_session

   I obviously haven't allowed for ~/.bash_profile or ~/.bash_login, but
   others are likely to want a different solution for these specifics
   anyway. The important thing is that you can easily see where to add
   environment variables or other commands for all cases, the login case,
   and the non-login (command execution) case. I believe the command
   execution case also applies to subsystems (e.g. sftp).

   There are potentially some difficulties if using X forwarding, due to
   use of the login command, as documented in the sshd_config man page at
   the UseLogin directive; but I think these would be minor--they would
   only occur if you start an interactive ssh session with X forwarding,
   but more likely than not you would make a different ssh process do
   this with -Nf or run a specific command rather than an interactive
   session, I think. And they may not occur at all when -fp is used.

   All that said, this seems to be working well so far for me!

Hope this can help some others.

Ben Schmidt.
Melbourne, Australia.