sed question...

[Nikos Vassiliadis]

On Tuesday 25 September 2007 06:07, Howard Goldstein wrote:
> Gary Kline wrote:
> > 	My earlier post about deleting the first N lines was answered by
> > 	this one-liner site {below}.   I wasn't including any
> > 	redirection; doing so finally resolved the problem.  Now I need
> > 	to delete every line from the 19th or so to the last line.

sed -e 18q
that is, quit after processing line 18.

> > 	Question one, can anybody explain the following syntax?  What do
> > 	"P", "D" "ba" represent, in other words?

The manual page explains sed in a very good way. For sure, better
than I could describe it here. You'd better read it.

> >
> >
> >  # delete the last 10 lines of a file
> >  sed -e :a -e '$d;N;2,10ba' -e 'P;D'   # method 1
> >  sed -n -e :a -e '1,10!{P;N;D;};N;ba'  # method 2
> >
> >
> > 	Question two, can sed do its thing inline?

Yes.
 -i extension
     Edit files in-place, saving backups with the specified extension.
     If a zero-length extension is given, no backup will be saved.  It
     is not recommended to give a zero-length extension when in-place
     editing files, as you risk corruption or partial content in situ-
     ations where disk space is exhausted, etc.

>
> Wouldn't it be easier to use  head -n 18 ?

No, it's the same. Some sed operation are trivial to read/write,
others aren't.

HTH

Nikos