On Sat, 11 Jan 2020 at 20:59, Yuri <yuri@xxxxxxxxx> wrote:
[...]
> It's not obvious why does it have to be this way. ssh sends the command
> as an array of strings.
>
> The first string is the command, and the
> subsequent strings are arguments. It can easily call the same command
> with the same arguments on the remote host.
Why do you say that? From RFC 4254 section 6.5:
"""
byte SSH_MSG_CHANNEL_REQUEST
uint32 recipient channel
string "exec"
boolean want reply
string command
This message will request that the server start the execution of the
given command. The 'command' string may contain a path. Normal
precautions MUST be taken to prevent the execution of unauthorized
commands.
"""
Note the single string specifying the command.
> Also this sentence from the man page seems to be false:
>
> > If a command is specified, it is executed on the remote host instead
> of a login shell.
>
> Login shell still interprets the command.
No, it's interpreted by a non-login shell. A login shell is a subset
of shell invocations with some specific behaviour. The bash man page
describes some of this:
"""
INVOCATION
A login shell is one whose first character of argument zero is a -, or
one started with the --login option.
[...]
Login shells:
On login (subject to the -noprofile option):
if /etc/profile exists, source it.
if ~/.bash_profile exists, source it,
else if ~/.bash_login exists, source it,
else if ~/.profile exists, source it.
On exit:
if ~/.bash_logout exists, source it.
"""
> Interestingly, the shell
> process isn't running on the remote host, the command is a direct child
> of sshd.
That's probably because the shell is skipping the fork in the simple
command case as an optimization. If you repeat the experiment with a
non-trivial command you'll see something like this:
$ ssh localhost "sleep 1000 | sleep 1000"
[...]
$ ps -eaf | grep "sleep 1000"
dtucker 22321 1 0 22:56 ? 00:00:00 bash -c sleep 1000 | sleep 1000
[...]
$ pstree -s -p 22355
systemd(1)---bash(22321)---sleep(22355)
--
Darren Tucker (dtucker at dtucker.net)
GPG key 11EAA6FA / A86E 3E07 5B19 5880 E860 37F4 9357 ECEF 11EA A6FA (new)
Good judgement comes with experience. Unfortunately, the experience
usually comes from bad judgement.
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