Why are the arguments supplied for the command run through ssh interpreted by shell before they are passed to the command on the server side?

Colin Watson cjwatson at debian.org
Sun Jan 12 00:38:53 AEDT 2020

I do think it's a problem that, as far as I can see, the exact behaviour
of ssh command interpretation isn't explained anywhere in its man pages,
even though it seems like the only possibility if you understand the
protocol.  ssh(1) just says "If command is specified, it is executed on
the remote host instead of a login shell", but that doesn't provide any
hint about what quoting of metacharacters might be required, which is
important since ssh is often used as plumbing.

Now, I understand that ssh doesn't itself control how the command is
parsed: it just executes the user's shell with -c and the command as
arguments (passing the whole command as a single argument).  However,
that's not the only possible interpretation of the bit from ssh(1) that
I quote above, and it would be helpful to clarify the documentation to
say so explicitly.  People who need to work out subtle details of
quoting rules could then at least know to consult the documentation of
the appropriate shell.

Another thing that's poorly-explained in ssh(1) is the handling of the
case where the command is passed to ssh as multiple arguments (e.g. 'ssh
host echo foo' rather than 'ssh host "echo foo"').  The behaviour is
that all the arguments are concatenated into a single string with a
space character between them, but as far as I can see ssh(1) makes no
mention of this whatsoever and so I don't think this is well-understood.
It's important to explain this because the following example is not
handled in the way that one might naturally expect:

  ssh host cat 'path with spaces'

The command sent to the server is the string "cat path with spaces",
which will then typically be split into four words by the shell at the
other end.  If you want to preserve the quoting then you need to write
it as something like this:

  ssh host "cat 'path with spaces'"

Colin Watson                                       [cjwatson at debian.org]

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