[GTALUG] Hardcore sed foo

[Giles Orr]

On Mon, 4 Nov 2019 at 18:36, William Witteman <wwitteman at gmail.com> wrote:

> No sed knowledge here, but what if you turn it around, and grab two
> characters and conditionally truncate, rather than the other way around?
>
> My only useful programming advice is when you get stuck, turn your problem
> around.
>
> On Mon., Nov. 4, 2019, 18:31 Giles Orr via talk, <talk at gtalug.org> wrote:
>
>> I've tinkered with Bash prompts for a lot of years.  That's where this
>> problem originates, but it can be considered just as a thought experiment
>> if you prefer.
>>
>> We have the directory we're in, for example:
>>
>>     /Users/gorr/.bashprompt/really/deep/directory/structure/even/deeper
>>
>> (I keep my prompts in ~/.bashprompt/ , and a very long directory name is
>> often a cause of breakage, so I keep one around to break them ...)
>>
>> I want to shorten the directory name to just the first letters:
>>
>>     export newPWD=$(echo ${PWD} | sed -e "s@${HOME}@~@" -e 's@
>> \(/.\)[^/]*@\1 at g')
>>     echo $newPWD
>>     ~/./r/d/d/s/e/d
>>
>> This does '~' replacement for $HOME and then substitutes the first letter
>> of each directory for the full directory name.
>>
>> Here's the question: if the first letter of the directory name is a dot
>> '.', can sed then capture one character more so that the output would
>> become:
>>
>>     ~/.b/r/d/d/s/e/d
>>
>> I think this would be pretty easy with Bash and a loop, but that's a lot
>> of processing so I'd rather not go down that road.  I suspect sed is
>> capable of this, but I haven't delved deeply enough into the tool to even
>> know where to start.  This may in fact be a regex problem more than a sed
>> problem - either way I'm kind of stumped.  I'm open to simpler
>> implementations using other (standard system) tools as well.
>>
>
All that was needed was a walk home to think and solve it:

     export newPWD=$(echo ${PWD} | sed -e "s@${HOME}@~@" -e 's@
\(/[.].\|/.\)[^/]*@\1 at g') ; echo $newPWD
    ~/.b/r/d/d/s/e/d

It's a regex problem: capture a group that's a slash, a dot, and a
character, OR a slash and a character.

-- 
Giles
https://www.gilesorr.com/
gilesorr at gmail.com
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