Hello everyone,
I am new to Unix and I am stuck with a problem. I need only a single command to display the output of who and then add the total number of users and display at the bottom of that output.
Example-: (Expected output)
sreyan@debian:~$ <command>
sreyan tty7 2012-12-04 05:50 (:0)
sreyan pts/0 2012-12-04 05:51 (:0.0)
1
By the way I am using Debian 6.0.5 (64-bit). Thank You.

Something like this might do it

  w | tail -$(( $(w| wc -l) - 2 ))  | tee >(cut -d\  -f1|uniq | wc -l >/dev/stderr )

There is no such single command, because that would go against one of the most basic UNIX tenets: do one and do it as good as possible. That means: there is a command to display the users logged in ( who , as you have already figured out) and it will do this displaying as good as possible - but it won't count, sort or do any other "after-processing" of its display. This is simply not its job. There is a sort utility for sorting, but it will only do the sorting - it will not generate output of its own. The same goes for line counting (see the wc utility), filtering (see grep , sed , awk and a lot of similar tools) and any other purpose.

Think of the UNIX tools like instrumentalists in an orchestra. You don't expect the trumpet player to play the flute as well. No, you have a world-class flutist to play the flute and a world-class trumpet player to play the trumpet. It is your job to conduct them accordingly to bring out the brilliant and stunning sound they are capable to produce.

I hope this helps.

bakunin

Can you please break up this command and explain it to me. I am completely new to unix and don't understand it. But it has worked flawlessly.

w | tail -$(( $(w| wc -l) - 2 ))  | tee >(cut -d\  -f1|uniq | wc -l >/dev/stderr )

w | tail -$(( $(w| wc -l) - 2 ))

$(( $(w| wc -l) - 2 ))

$(w| wc -l)

w | tail -$(( $(w| wc -l) - 2 ))

<w minus first 2 lines> | tee >(cut -d\  -f1|uniq | wc -l >/dev/stderr )

<w minus first 2 lines> >(cut -d\  -f1|uniq | wc -l >/dev/stderr )

w | tail -$(( $(w| wc -l) - 2 ))  | cut -d\  -f1|uniq | wc -l

Thank You so much for your detailed explanation. But I have some doubts-:

1) In the following code-:

$(( $(w| wc -l) - 2 ))

$(w| wc -l)

(cut -d\  -f1|uniq | wc -l >/dev/stderr )

kill $!

Actually, these are very good questions and i am glad that you asked. It is always much more satisfying to teach someone willing to learn.

These are different language constructs. First the "$(( ... ))": this means: replace the numeric expression inside by its outcome. For instance you could write

foo -x $(( 5 - 3 + 1 ))

foo -x 3

z=0
x="abc5"
x="${x#???}"    # this chops off the first 3 characters, leaving "5"
(( z = x * 5 ))
echo $z

>(cut -d\  -f1|uniq | wc -l >/dev/stderr )

Originally Posted by sreyan32 2) When we use tee the stream is duplicated and by default the first stream is always printed out. Am I right in assuming this ? Or is there a way to control the output of both the divided streams?

Originally Posted by sreyan32 3)The duplicate stream that tee creates is also an output stream. And we redirect that second duplicate stream to the following operations-: Code: (cut -d\ -f1|uniq | wc -l >/dev/stderr ) But then if there is already an output stream then why are we redirecting the output to the error stream ?

ls -l /etc/passwd /some/file/blabla

ls -l /etc/passwd /some/file/blabla > /dev/null
ls -l /etc/passwd /some/file/blabla 2> /dev/null

Originally Posted by sreyan32 By the way do you know what the following command does-: Code: kill $!