bug#70511: Option to grep into compressed files

[Mary]

> Thanks for the suggestion. You're right, this would be better than zgrep
> etc.
> 
> I have some qualms though, as the new option would increase the attack
> surface for 'grep', in that you could then execute arbitrary code by
> passing certain options to 'grep'. Is there some safer way to get what
> you want?


There is still the possibility of including the respective compression 
libraries directly in grep and using the `-Z` and `-J` as proposed, but this 
wouldn't allow to use less popular compression algorithms.

One possibility, but I'm not sure what it's worth, would be to give grep a 
special arg0 to enable shell commands, like `jgrep zcat pattern123 file.gz`. 
But I'm not sure if it's worth the trouble.


> One supposes that if the file extension is not trustworthy, one can taste 
> file like the file command, and use libraries like the gzip libraries to 
> handle gzipped files as a stream.  There are so many others: zip files could 
> be treated like directories and all the files in them that match the glob 
> could be searched, and then there is bzip2, 7zip, ....  It becomes a 
> popularity contest!  One can do all this with shell scripting, and leave poor 
> old grep out of it!


The reason why I wanted to do this in grep directly is because it's difficult 
to implement this with shell scripting. I noticed that neither zgrep, bzgrep 
nor xzgrep support the `-r` option, among others, presumably because it's too 
difficult to implement in a portable way.

I made my patch use a shell command specifically to provide maximum flexibility 
with minimum maintenance cost. But it does open the door to security risks, so 
I understand if it's not worth adding to grep.