[TriLUG] Changing slashes to backslashes in prompt

Fri Apr 19 18:05:09 EDT 2019

GACK: should *NOT* try to pretend...

On 4/19/19 6:02 PM, Stephen P. Schaefer via TriLUG wrote:
> Yes, you should try to pretend that a linux shell is anything like
> Windows cmd prompt, but the Linux shell is Turing equivalent (ignoring
> various value limits induced by variable bits and RAM size that render
> it, like any actual program, a finite state machine), so if the problem
> is Turing evaluable, as this one is, the shell can do it; the question
> is "merely" how elegantly it can do so.
> 
> Here's how I approached this.
> 
> First, I checked what my default PS1 was doing, since, by default it is
> set to show the basename of the directory I'm in.  So
> 
> [sps at corydon ~]$ set | grep PS1
> GUESTFISH_PS1='\[\e[1;32m\]><fs>\[\e[0;31m\] '
> PS1='[\u@\h \W]\$ '
> 
> (you can ignore the GUESTFISH_PS1)
> 
> From that I see that PS1 is assigned a string in single quotes, and that
> several magic backslashes are getting interpolated from within that
> string *every time the prompt gets displayed*.  So there is extra magic
> going on that doesn't happen in the normal course of command line variables.
> 
> So next, I took a look at "man bash" to see what guidance it could give:
> 
> PROMPTING
>        When executing interactively, bash displays the primary prompt
> PS1 when
>        it  is  ready  to  read a command, and the secondary prompt PS2
> when it
>        needs more input to complete  a  command.   Bash  allows  these
> prompt
>        strings  to  be  customized  by inserting a number of
> backslash-escaped
>        special characters that are decoded as follows:
> 
> ...hoo boy, so, inded there is an extra layer of interpolation going on,
> and specifically involves backslashes.  There follows a list of about
> twenty different characters that can follow the backslash, including:
> 
>               \\     a backslash
> 
> Now, after that layer of backslashing, there will be another layer
> performed on any shell invocation, e.g., from $().  So I tried
> 
> PS1='$(pwd | sed -e "s,/,\\\\,g)" '
> 
> ...which didn't work.  Why?  We've got one special reduction of
> backslashes because this is a prompt, so at variable interpolation
> (which includes the $()) what's getting evaluated is
> 
> pwd | sed -e "s,/,\\,g"
> 
> ...and then, when it gets to sed, sed will see an argument
> 
> s,/,\,g
> 
> Aha.  sed is *also* interpreting backslashes, so it complains that the
> substitution is unterminated, since the backslashing of the trailing
> comm made it part of the substitution string, and then there wasn't the
> terminating character.  So what *did* work was:
> 
> PS1='$(pwd | sed -e "s,/,\\\\\\,g") '
> 
> So: magic prompt interpolations turn it into
> 
> $(pwd | sed -e "s,/,\\\,g") '
> 
> Shell interpolations turn it into
> 
> pwd | sed -e s,/,\\,g
> 
> and, finally, sed's interpolation turns the substitution string between
> the , delimiters into
> 
> \
> 
> ...and that succeeded.
> 
> Why, yes, I frequently write code I warn my employer to erase :-)
> 
> Next exercise for the reader:
> 
> Don't use sed; instead use the variable expansion glob patterns.
> 
> 
>     - Stephen
> On 4/19/19 5:33 PM, bick via TriLUG wrote:
>> When I first started using Linux a few years ago, I went through
>> tutorials on how to change the command prompt.  I changed the colors,
>> etc.  I'm just going through this book, because I thought it would be a
>> fun refresher.  I don't get a lot of practice with the command line
>> tools in my daily use.
>>
>> I am really not concerned with changing my prompt to make it look like
>> Windows.  I am really just interested to know why what I'm doing isn't
>> working for the backslash, when it works to replace the slashes with any
>> other character or combination of characters I have tried.
>>
>> Thanks again for your time.
>>
>> On 4/19/19 3:39 PM, David Both wrote:
>>> Why not try something different to learn about changing your command
>>> prompt? Here are a couple articles available on Opensource.com. I
>>> write for them requently but neither of these articles is one of mine.
>>>
>>> https://opensource.com/article/17/7/bash-prompt-tips-and-tricks
>>>
>>> https://opensource.com/article/18/12/linux-toy-bash-prompt
>>>
>>> Here is an article of mine about Bash command line programming. You
>>> might find it useful.
>>>
>>> https://opensource.com/article/18/11/control-operators-bash-shell
>>>
>>> There are many more articles there to help you learn the Bash shell.
>>> Just search for Bash.
>>>
>>>
>>> On 4/19/19 4:27 PM, Bick via TriLUG wrote:
>>>> To be fair, this is not part of an overall strategy of the book to
>>>> make things more Windows like.  It is just a one-off example, not
>>>> meant to be actually used in practice, and I think the author is
>>>> having a bit of fun.
>>>>
>>>> As for me, I am annoyed that I have spent this much time on the
>>>> problem, because I would never want to make the prompt look more like
>>>> Windows.  However, it bothers me that it doesn’t work, and as far as
>>>> I can tell, none of the bash prompt how-tos I have found online
>>>> explain why.  If you have any ideas, or know what documentation I
>>>> should be reading to find the answer, I would greatly appreciate it.
>>>>
>>>>> On Apr 19, 2019, at 3:14 PM, David Both via TriLUG
>>>>> <trilug at trilug.org> wrote:
>>>>>
>>>>> First - kudos to you for trying to learn Linux and the command line.
>>>>>
>>>>> <rant>
>>>>>
>>>>> I understand what you are doing and why, but the author of that book
>>>>> should never be allowed to write another book with the word Linux in
>>>>> it. Ignore that exercise once you have finished it. Never think of
>>>>> it again. Then find a better book.
>>>>>
>>>>> There is a horrible problem with Windows people trying to make Linux
>>>>> look and work like Windows. In my articles and books I caution
>>>>> against this approach to learning Linux. You will never really learn
>>>>> Linux if you try to make it work like Windows. My primary example of
>>>>> this is using aliases so the user can type a Windows command that is
>>>>> then interpreted by the shell into its Linux equivalent. I will add
>>>>> this example to my list at the next opportunity.
>>>>>
>>>>> What this tells me is that the author is stuck in the Windows world
>>>>> and is, probably unconsciously but nonetheless, trying to infect
>>>>> readers in the same way. Linux is not Windows, and trying to make it
>>>>> so will only impede your efforts to learn Linux.
>>>>>
>>>>> I can think of a slew of different things to do with the command
>>>>> prompt than making it look like Windows.
>>>>>
>>>>> </rant>
>>>>>
>>>>> Good luck and keep learning!
>>>>>
>>>>>
>>>>>> On 4/19/19 3:55 PM, Bick via TriLUG wrote:
>>>>>> I am reading through a Linux book right now and one of the
>>>>>> exercises in the chapter about environment variables is to make the
>>>>>> prompt look more Windows like.  Specifically, I am supposed to make
>>>>>> the prompt display C:\directory.  I first attempted to use sed to
>>>>>> accomplish it like this: PS1='C:$(echo $(pwd) | sed 's:/:\\:g') '. 
>>>>>> The result is C:parentDirchildDir.  It does not print slashes.  If
>>>>>> I just put the command I am calling directly into the bash prompt,
>>>>>> I get the proper output: C:\parentDir\childDir.
>>>>>>
>>>>>> I thought maybe the problem was that the single slash was being
>>>>>> reinterpreted as an escape character when PS1 was being read, so I
>>>>>> tried using four slashes (so that two would be present in the PS1
>>>>>> string).  Again typing this at the command prompt worked (output
>>>>>> was C:\\parentDir\\childDir), but it didn't work as part of
>>>>>> assigning it to PS1.
>>>>>>
>>>>>> I have also tried assigning the commands I want to run to a
>>>>>> variable, $NEWPWD, I created in PROMPT_COMMAND.  I then used the
>>>>>> variable in the PS1 assignment.  This didn't work either.
>>>>>>
>>>>>> I have successfully replaced the slashes with letters, numbers, and
>>>>>> even an exclamation point (which needed to be escaped by a
>>>>>> backslash) using ALL of the above methods.  Any ideas why this
>>>>>> doesn't work with backslashes?
>>>>>>
>>>>>> Thanks,
>>>>>>
>>>>>> Chris
>>>>> -- 
>>>>>
>>>>> *********************************************************
>>>>> David P. Both
>>>>> *********************************************************
>>>>> The value of any software lies in its usefulness
>>>>> not in its price.
>>>>>
>>>>> — Linus Torvalds
>>>>> *********************************************************
>>>>>
>>>>> -- 
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