Backticks/paths was: sudo blocks aliases

[Peter Garrett]

On Fri, 30 Dec 2005 06:01:57 +0100
"J.Markoll" <j.markoll at free.fr> wrote:

> by the way, the newbie who talks to you (me) would be most happy to 
> learn about this one command line: ls -l `which sh`
> it avoids you to 'cd' to '/bin' before invoking 'ls -l', or does it do 
> more ?

The "which" command returns the full path for an executable thet is in
your $PATH . For instance in my case

peter at prospero:~ $ echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/bin/X11:/usr/games

peter at prospero:~ $ which firefox
/usr/bin/firefox

The "back quotes" or "back ticks"  (  `` ) evaluate the expression within
them, so when I type 

ls -l `which sh`

It is equivalent to typing

ls -l  /bin/sh

In this case  ls -l /bin/sh would have been easier - it's just a bad habit
of mine ;-)

If you aren't sure of the complete path though, it can be a handy
shortcut. It can also be used in scripts, of course, although the
equivalent $(which sh) is easier to read. 

If , for example, you are installing linux-headers and want an exact match
to your current kernel (as you normally do), you can type

sudo apt-get install linux-headers-$(uname -r)

which expands to the correct kernel version.

Hope that isn't confusing :)

Peter
-- 
Unix is hard to learn. The process of learning it is one of multiple small
epiphanies. -- Neal Stephenson