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

[Chet Ramey]

On 3/2/21 9:43 AM, Cristian Zoicas wrote:
> Hello all
>
> There is a difference in how ~ and HOME are treated when trying to
> remove a matching prefix.

Yes, there is. They are not the same thing.

> # ~ 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////

This is the result of a misunderstanding about what constitutes a "tilde
prefix." POSIX says the following:

"A "tilde-prefix" consists of an unquoted <tilde> character at the
beginning of a word, followed by all of the characters preceding the first
unquoted <slash> in the word, or all the characters in the word if there is
no <slash>."

In this case, the word is "~*". There is no slash. The tilde-prefix is 
"~*", which means that the shell tries to look up a user `*', since

"If none of the characters in the tilde-prefix are quoted, the characters
in the tilde-prefix following the <tilde> are treated as a possible login
name from the user database."

That fails, of course, so the tilde prefix is left unchanged. The pattern
is `~*', including the literal tilde, which doesn't match anything in $A,
so the expansion doesn't remove anything.

--
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU    chet@case.edu    http://tiswww.cwru.edu/~chet/