[ILUG] Passwordless SSH access

[John Tobin]

On Wed, Mar 20, 2002 at 09:33:06AM +0000, Niall O Broin wrote:
> On Wed, Mar 20, 2002 at 12:11:30AM +0000, Ronan Waide wrote:
> [root at dunlop modules]# ps ax | grep foo
> 14801 pts/0    S      0:00 grep foo
> [root at dunlop modules]# ps ax | grep [f]oo
> [root at dunlop modules]#
> 
> What is it about grep and RE that causes this behaviour ? (note that the
> above example is not always correct. For example, 

The command line as shown by ps will be "grep [f]oo", which doesn't
match "foo".

> niall at bagend:~ >ps -ax|grep [f]oo
>  1944 pts/8    S      0:00 grep foo
> 
> but if you ensure that grep sees the RE like this
>  
> niall at bagend:~ >ps -ax|grep "[f]oo"
> niall at bagend:~ >
> 
> it's fine)

Do you by any chance have a file called foo in that directory?  bash
expands '[abcd]xxx' to 'axxx bxxx cxxx dxxx' ( quotes are for clarity
only ) if at least one of those files exists.  But when you quote the
argument filename expansion isn't performed on it.

> Does grep manipulate its arguments as saved or what ? Because I called my
> script ssh-agent-setup I matched on  "$USER.*ssh-agent$" and that works also
> (i.e. ps|grep doesn't find the grep) but leaving out the $ doesn't work

Because the $ is present in the command line as shown by ps.
Open two terminals and try this:
Terminal 1: ( ps auxwww; sleep 10 ) | grep "${USER}.*ssh-agent$"
Terminal 2 ( while the previous command is still running ): ps auxwww > file
Then take a look through file; you'll see something like ( wrapped ):
tobinjt   4568  0.1  0.1 1136  880 pts/37   S 11:26:23  0:00 grep
tobinjt.*ssh-agent$
Note the $ after ssh-agent so it doesn't match the RE.

> So, ps|grep won't match the grep when grep uses certain kinds of RE - oh
> lordy ! I'm leaving my grep -v grep in, thank you, in case the kind of RE is
> dependent on the phase of the moon :-)
> 
> Niall

-- 
John
"That would preempt a bunch of problems involved in trying to reconstruct
exactly how the Perl 5 parser thinks, which nobody entirely understands."
			Larry Wall, 2001/04/20, perl6-language at perl.org
"Finger to spiritual emptiness underlying everything." -- How a Japanese C
manual referred to a "pointer to void".