UNIX for Dummies Questions & Answers

I need to extract a particular block from a file whose locations are not known but the only identity is a word. For example

in a file I have

ABC

asdklf
asdfk

FGH

dfdfg

asdlfk
asdfl

...

JHK

ppoi
lkjkj


FGH
sdjsd
sdf
sd



-eof-

I have to extract the block whichever begins with FGH like

FGH

dfdfg

asdlfk
asdfl

FGH
sdjsd
sdf
sd

Here the significance is in between two blocks (ex FGH and JHK) i have double spacing.

How can I do this ?

This is always a fun programming problem. There are three approaches I can think of. I am sure there are many more. I suggest you do what you feel comfortable with or maybe get some ideas from what I suggest:

1. Using awk

I can't even come up with an example for you. You'll have to use a BEGIN an END section, probably with FS="<text you are looking for>". For RS, I am not sure what to use if they are separated by double blank lines. I am sure there is a way to specify this but I don't know what that would be. You may even have to do a sed first to replace some stuff to get awk to work. Eitherway, it probably isn't going to be pretty.

2. Using Perl

Text parsing is definitely where perl shines. There is a range regular expession you can use that will probably do what you want in less then 5 lines of code. You may even be able to do it in 1. I would check with perlmonks.com for advice on the complete script but the regular experssion would be similar to (from memory, syntax is probably wrong):

/FGH/ .. /\n\n

This will find the block between and including FGH and 2 blank lines.

3. c

You could alway write a c program to pump the data into an array and parse it that way.

#2 would be my choice, but I thought you would like to know more options incase you don't know perl (if you don't, you should check it out, it is pretty sweet)

TioTony

Code:

#! /usr/bin/ksh
read next
printing=0
while this="$next"; read this ; do
     [[ $this = "FGH" ]] && printing=1
     ((printing)) && print - "$this"
     [[ $this = "" && $next = "" ]] && printing=0
done
((printing)) && print - "$next"
exit 0

thanks Tony for your kind and nice information.
Perdarbo,
thanks for your code, but I am sorry I could not know where to specify the input filename in that script.
could you please tell me how to use this.

thanks

1) Use an editor to create a file and type in the code. I will call it "printblock".

2) Make the script executable with "chmod +x printblock".

3) Run the script and redirect input from your file. "./printblock < datafile". You may want to save the output to a second file. You can do that with "./printblock < datafile > secondfile".

Now I don't want to sound unkind, but if you're struggling with concepts like running a script and redirecting input, then you shouldn't be trying to write complex scripts yet. You need to walk before you run. I strongly recommend that you spend some time on unix basics via a book or class.

Thanks Perderabo for your kind advice and help.
That code is working fine. I had totally forgotten the redirection concept in the whole tension.
I am sorry for having run as I have hardly started walking. I am still going through the unix basics and I know it is never gonna be complete to master UNIX, at least for me. Thanks for your help.