UNIX for Dummies Questions & Answers

I need someone to tell me how exactly works the following code αfter /etc/passwd :

Code:

cat /etc/passwd|grep "^[^:][^:][^:][^:]:"|sed '1,$s/^\([^:]*\):[^:]*:\([^:]*\):.*$/ \1 \2 /'|sort -nrk3 -t:

I want a good explanation to understand the code please

Well... It's a cluster. The person who wrote it seems to me to be insane.

Code:

cat /etc/passwd | grep "^[^:][^:][^:][^:]:"

This greps for lines beginning with "XXXX:", where X is any character except a colon. So basically, this will only show you 4-letter users. ... Kinda weird, but.. okay. Also, the person that wrote it is perpetuating the misunderstanding that cat serves a function for single files. (Grep can take files as arguments.)

Code:

| sed '1,$s/^\([^:]*\):[^:]*:\([^:]*\):.*$/ \1 \2 /'

That is the most ridiculous thing I've ever seen. Unless I'm missing something big, the purpose of it is apparently to pull out only the username and the uid. It could have been accomplished so much more simply, clearly, and efficiently by replacing it with a cut command (this is what cut was made for!) like this:

Code:

| cut -d: -f1,3 --output-delimiter=' '

Lastly...

Code:

| sort -nrk3 -t:

What I think this is trying to do: sort the 4-letter users in descending order of their uid (so bob with uid 1100 would be before aaron with uid 505). However, the code is broken, probably because the person's over-complicated sed string is broken. To fix it, just change that trailing colon to a quoted space character:

Code:

' '

Look up those options in the sort man page if you're curious.

Long story short, here's what the command should be:

Code:

grep "^[^:][^:][^:][^:]:" /etc/passwd | cut -d: -f1,3 --output-delimiter=' ' | sort -nr -k2 -t' '

Thank you very much for your help!I will try to learn it now!And you are right!The person who wrote this is completely insane!In three hours he taught us the grep,sort,tr and sed command(not cut)!I dont know how can i learn this code with 3 hours of lesson,because i see these things for first time!Anyway if i have any more questions i will post here to help me again

---------- Post updated at 03:28 AM ---------- Previous update was at 03:00 AM ----------

As i said before he didn't taught us the cut command and now i have to learn the code without cut
In this command how do i understand pulling out only the username and the uid?

Code:

| sed '1,$s/^\([^:]*\):[^:]*:\([^:]*\):.*$/ \1 \2 /'

As i know according to this

Code:

http://www.cyberciti.biz/faq/understanding-etcpasswd-file-format/

the username is on the first field and the uid on the third,but i can't understand nothing on this code!Could you explain it without the cut command?

I'm sorry man. I love sed, and I use it for all kinds of things, but I would never use it for this. ... I don't understand how it's being used here, and I don't quite care to try and muddle through it and figure it out, though I'd be curious to have someone explain it. If your uh... unorthodox.. instructor won't explain it to you (and no one else answers), you could try to puzzle through it with the help of the sed info page (type info sed).

As a consolation prize, here's a command that does the same thing your instructor's original command was supposed to do, but more simply and more efficient. Maybe he'll be impressed.

As far as i know he will teach us the awk command on the next lesson!I may not know how to use the awk but as i see now this command seems a lot helpful!