Re: sed one-liner?

Dear jkilbour@xxxxxxx,

   Thanks for writing! On 20 Dec 2005 at 21:33, you said:

> > I decided to give it a try also. The following sed script
> > will print the penultimate line (the next to last line) on a file of
> > any length except a file which has only one line, in which case it
> > prints nothing:
> >
> > sed -e '$!{h;d;};x'
> >
> > --
> > Eric Pement
> >
> 
> Could you annotate your solution a little, Eric, for those of us who
> are interested in your solution but not yet experienced enough to
> follow it?
> 
> Thanks.

   Your reply went to me personally, not to the sed-users mailing 
list, so I'm adding the mailing list to the CC: field. Here is my 
explanation:

   $! {      # On every line EXCEPT the last one, ..

      h;     # .. copy the line into the hold space, overwriting
             # whatever was in there previously, and

      d;     # delete that line from the pattern space.  As soon
             # as the 'd' command is issued, sed gets a fresh line,
             # puts it in the pattern space, and runs the script
             # again. The result is that nothing EVER gets printed
             # at all for this address range, but exactly one line
             # is ALWAYS saved ... for files larger than 1 line.

   };   # This brace closes the address range.

   x;   # To reach this command, we MUST be at the last line
        # because the $!{...} portion restricted us to every line
        # of the input except the last line.

        # 'x' means to exchange the pattern space and the hold
        # space, so whatever was on the last line of the file is
        # put in the hold space, and what was in the hold space
        # (which MUST have been the next-to-the-last line) is put
        # into the pattern space.

        # Since there are no further commands, sed prints out what
        # was in the pattern space, which must be the penultimate
        # line of the file. The last line of the file, which is
        # still in the hold space, is not printed. Then sed exits.

Hope this helps!

-- 
Eric Pement - eric.pement@xxxxxxxxx
Educational Technical Services, MBI



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