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Re: ulimit and ssh?


From: Bob Proulx
Subject: Re: ulimit and ssh?
Date: Mon, 5 Oct 2009 15:36:17 -0600
User-agent: Mutt/1.5.18 (2008-05-17)

peter360 wrote:
> Thanks Adreas.  That was what I suspected in my reply to Bob.  But Bob
> disagreed.   Looks like there were some confusion about this feature even
> among experts.  Seems another reason to deprecate the feature.

I don't think anything I said disagreed with what Andreas said.  It is
just confusing you since we are talking about different points in the
process.  What does the local shell do, what does the ssh do, what
does the remote shell do, what is the end-to-end effect and so forth.
All three processes are handling the arguments and each has an affect
at different points in the timeline.  I was talking end-to-end
(shell-ssh-shell) and Andreas was talking about ssh.

And by the way but this feature can't really be changed or it would
break twenty years or so of scripts which rely upon the existing
behavior.

Greg Wooledge wrote:
> imadev:~$ $HOME/bin/args 'ulimit -a'
> 1 args: 'ulimit -a'
> imadev:~$ remsh localhost $HOME/bin/args 'ulimit -a'
> 2 args: 'ulimit' '-a'
> imadev:~$ ssh localhost $HOME/bin/args 'ulimit -a'
> address@hidden's password:
> 2 args: 'ulimit' '-a'

Nice!  It would also be illustrative to show what happens without any
quoting and with quoting for two shell layers.

  $ $HOME/bin/show-args 'ulimit -a'
  arg1: ulimit -a

  $ ssh -n localhost $HOME/bin/show-args ulimit -a
  arg1: ulimit
  arg2: -a

  $ ssh -n localhost $HOME/bin/show-args 'ulimit -a'
  arg1: ulimit
  arg2: -a

  $ ssh -n localhost $HOME/bin/show-args '"ulimit -a"'
  arg1: ulimit -a

Since there are two shells splitting words in the end-to-end result
then the effect is that you need to quote your arguments twice in
order to have an argument containing whitespace in one argument by the
time it has been processed through two shell's argument processing.

Without sufficient quoting you have this following case, which isn't
what you want.  The bash shell on the remote end would not see args
"-c" "ulimit -a" but would see "-a" as a separate

  $ ssh -n localhost $HOME/bin/show-args bash -c "ulimit -a"
  arg1: bash
  arg2: -c
  arg3: ulimit
  arg4: -a

That isn't what you want.  You need to quote the args twice.

  $ ssh -n localhost $HOME/bin/show-args 'bash -c "ulimit -a"'
  arg1: bash
  arg2: -c
  arg3: ulimit -a

In the end you want something like this:

  $ ssh -n localhost 'bash -c "ulimit -a"'
  core file size          (blocks, -c) 0
  data seg size           (kbytes, -d) unlimited
  scheduling priority             (-e) 0
  file size               (blocks, -f) unlimited
  pending signals                 (-I) 8185
  max locked memory       (kbytes, -l) 32
  max memory size         (kbytes, -m) unlimited
  open files                      (-n) 1024
  pipe size            (512 bytes, -p) 8
  POSIX message queues     (bytes, -q) 819200
  real-time priority              (-r) 0
  stack size              (kbytes, -s) 8192
  cpu time               (seconds, -t) unlimited
  max user processes              (-u) 8185
  virtual memory          (kbytes, -v) unlimited
  file locks                      (-x) unlimited

However I think it is simpler to avoid the argument processing and
instead using stdin to the shell.

  $ echo ulimit -a | ssh localhost bash

Isn't that simpler?

Bob




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