Re: difference in how ~ and HOME are treated when trying to remove a matching prefix.

[Cristian Zoicas]

Tim Visher wrote:
> On Tue, Mar 2, 2021 at 9:43 AM Cristian Zoicas <zoicas@medialab.sissa.it 
> <mailto:zoicas@medialab.sissa.it>> wrote:
>
>     Hello all
>
>     There is a difference in how ~ and HOME are treated when trying to
>     remove a matching prefix. Here is an example:
>
>     # ~ and $HOME are the same:
>     #
>     wj@rogers:~$ echo ~
>     /home/wj
>     wj@rogers:~$ echo $HOME
>     /home/wj
>
>     # Nothing is removed (although the entire contents should be removed).
>     #
>     wj@rogers:~$ (A="/home/wj////"; echo "A (with prefix)   : ${A}"; echo "A 
> (without prefix): ${A##~*}")
>     A (with prefix)   : /home/wj////
>     A (without prefix): /home/wj////
>
>     # The prefix is removed with all following characters (due to *)
>     #
>     wj@rogers:~$ (A="/home/wj////"; echo "A (with prefix)   : ${A}"; echo "A 
> (without prefix): ${A##${HOME}*}")
>     A (with prefix)   : /home/wj////
>     A (without prefix):
>
>     Is this a bug or am I missing something?
>
>
> I think you may be misunderstanding Tilde expansion. Tilde expansion only 
> occurs when the tilde is at the beginning of a word or during variable 
> assignment when it follows any `:` character or the first `=` character. The 
> reasons for the `:` character are because of its prevalence in variables like 
> `PATH`. Thus the `~` character is _not_ a variable like `HOME` and can't be 
> used in all the contexts that a variable can be used, one of which is Parameter 
> Expansion.
>
> https://www.gnu.org/software/bash/manual/bash.html#Tilde-Expansion
> https://www.gnu.org/software/bash/manual/bash.html#Shell-Parameter-Expansion

The explanation is interesting, but I would add the following:

In the first case, the prefix is nto removed because the ~ is found in an 
innapropriate place. For example,
if i add a / after the ~, the prefix is removed:

(A="/home/wj////"; echo "A (with prefix)   : ${A}"; echo "A (without prefix): 
${A##~/*}")
A (with prefix)   : /home/wj////
A (without prefix):

According to these examples (in the firs email and in this email) what is going 
on is
that ~ followed by * does not work but if there is a / between ~ and * then it 
works.

Cristian