i want to write a script to delete all lines of files except a line with specific word. like delete every line but not one with "media" word.

Hi and welcome to LQ.

Have a look at a sed tutorial. Some hints:
-n
-i
/<media/>
p

Yes, I have been using sed but wasn't getting to the point that how to delete everything except that line which contains media.

It would be more helpful if you provided an actual example of the text, to help avoid any trouble spots in our answers. It's hard to give specific advice when we have to guess the requirements.

In any case, I'd now recommend using ed instead of sed for simple modifications like this, as it's an actual text editor, and should be fully portable across platforms.

First, I used printf pipe the commands into ed, separated by newlines.

'v' runs the command on all lines that don't contain '/pattern/', and 'd' deletes them.

The ',p' command prints it to the screen (for confirmation), and the 'w' command writes the changes back to the file (only use it when you're ready to commit).

How to use ed:
http://wiki.bash-hackers.org/howto/edit-ed
http://snap.nlc.dcccd.edu/learn/nlc/ed.html
(also read the info page)

It's also possible to use vi/vim in a similar way.

2 members found this post helpful.

will give you the result

1 members found this post helpful.

When it comes to sed, it would be easier in your case to think: print only lines containing the pattern instead of delete everything apart.......
See also the -i flag for permanent chages.

Or, if you are stuck with a sed that does not support the -n option,

Will work also.