Re: errors in sed manual 4.2.2

On Sat, 9 Aug 2014 15:15:51 +0100, bamber ward <[email protected]> wrote:

> I have tried to construct examples to test out \`(backslash backtick) but
> have not succeeded  in getting the responses  I want. I assume that
> backtick is the same as backquote. Backticks as in `command`.
> for instance,
> echo cat | sed "s/\`cat/bingo/"
> gives cat

You're hitting shell escaping issues. If you're in doubt, a way to check is
putting the sed program in a file, then running with sed -f prog.sed.

In this first example, sed sees:

s/`cat/bingo/

as your shell is most likely removing the backslash before the backtick.
So the result is correct: no replacement is made, as the input does not
contain 

`cat

Instead, you probably want one of these:

$ echo cat | sed 's/\`cat/bingo/'
bingo
$ echo cat | sed "s/\\\`cat/bingo/"


> echo cat | sed "s/\`cat\'/bingo" # double quotes because of single quote
> gives cat

As written, that gives an error:

sed: -e expression #1, char 14: unterminated `s' command

If, as I think, you meant instead

echo cat | sed "s/\`cat\'/bingo/"

then again the shell removes the first backslash and sed sees

s/`cat\'/bingo/

and the result is again correct, no replacement as the input does not
contain 

`cat

Working alternatives:

$ echo cat | sed "s/\\\`cat\'/bingo/"
bingo
$ echo cat | sed 's/\`cat'\\\''/bingo/'
bingo

> but
> echo cat | sed "s/cat\'/=/" gives
> =

Here sed sees 

s/cat\'/=/

so there is a match and the replacement is performed.

Note that \` and \' are GNU sed specific extensions and will not work on
other sed implementations. 


On a related note, it's quite difficult to find the download link for the
latest source, as this page is outdated:

http://directory.fsf.org/wiki/Sed

and here

http://www.gnu.org/software/sed/sed.html

there's no download link, nor do the docs
(http://www.gnu.org/software/sed/manual/) say where it can be downloaded. 
And finally, http://www.gnu.org/software/sed/manual/sed.html seems still
to be for 4.2.1.

-- 
D.