A while back there was a thread on one of our company mailing lists about SSH quoting, and I posted a long answer to it. Since then a few people have asked me questions that caused me to reach for it, so I thought it might be helpful if I were to anonymize the original question and post my answer here.
The question was why a sequence of commands involving
ssh and fiddly
quoting produced the output they did. The first example was this:
$ ssh firstname.lastname@example.org bash -lc "cd /tmp;pwd" /home/user
Oh hi, my dubious life choices have been such that this is my specialist subject!
This is because SSH command-line parsing is not quite what you expect.
First, recall that your local shell will apply its usual parsing, and the
actual OS-level execution of
ssh will be like this:
: ssh : email@example.com : bash : -lc : cd /tmp;pwd
Now, the SSH wire protocol only takes a single string as the command, with the expectation that it should be passed to a shell by the remote end. The OpenSSH client deals with this by taking all its arguments after things like options and the target, which in this case are:
: bash : -lc : cd /tmp;pwd
It then joins them with a single space:
bash -lc cd /tmp;pwd
This is passed as a string to the server, which then passes that entire string to a shell for evaluation, so as if you’d typed this directly on the server:
sh -c 'bash -lc cd /tmp;pwd'
The shell then parses this as two commands:
bash -lc cd /tmp pwd
The directory change thus happens in a subshell (actually it doesn’t quite
even do that, because
bash -lc cd /tmp in fact ends up just calling
because of the way
bash -c parses multiple arguments), and then that
subshell exits, then
pwd is called in the outer shell which still has the
original working directory.
The second example was this:
$ ssh firstname.lastname@example.org bash -lc "pwd;cd /tmp;pwd" /home/user /tmp
Following the logic above, this ends up as if you’d run this on the server:
sh -c 'bash -lc pwd; cd /tmp; pwd'
The third example was this:
$ ssh email@example.com bash -lc "cd /tmp;cd /tmp;pwd" /tmp
And this is as if you’d run:
sh -c 'bash -lc cd /tmp; cd /tmp; pwd'
Now, I wouldn’t have implemented the SSH client this way, because I agree
that it’s confusing. But
/usr/bin/ssh is used as a transport for other
things so much that changing its behaviour now would be enormously
disruptive, so it’s probably impossible to fix. (I have occasionally
agitated on openssh-unix-dev@ for at least documenting this better, but
haven’t made much headway yet; I need to get round to preparing a
documentation patch.) Once you know about it you can use the proper
quoting, though. In this case that would simply be:
ssh firstname.lastname@example.org 'cd /tmp;pwd'
Or if you do need to specifically invoke
bash -l there for some reason
(I’m assuming that the original example was reduced from something more
complicated), then you can minimise your confusion by passing the whole
thing as a single string in the form you want the remote
sh -c to see, in
a way that ensures that the quotes are preserved and sent to the server
rather than being removed by your local shell:
ssh email@example.com 'bash -lc "cd /tmp;pwd"'
Shell parsing is hard.