On 27/03/2018 10:55, Herbert Xu wrote:
Here is a better example:
a="/*/\nullx" b="/*/\null"; printf "%s\n" $a $b
dash currently prints
/*/\nullx
/*/\null
bash prints
/*/\nullx
/dev/null
You may argue the bash behaviour is inconsistent but it actually
makes sense. What happens is that quote removal only applies to
the original token as seen by the shell. It is never applied to
the result of parameter expansion.
Huh? Here's what POSIX says:
> The order of word expansion shall be as follows:
> 1. ... parameter expansion (see Parameter Expansion) ... shall be
performed, ...
> ...
> 3. Pathname expansion (see Pathname Expansion) shall be performed,
unless set -f is in effect.
> 4. Quote removal (see Quote Removal) shall always be performed last.
If you say that quote removal takes place on the original token (meaning
before parameter expansion), and if parameter expansion takes place
before pathname expansion, then there's nothing left to allow \* to
behave differently from *.
Now you may ask why on earth does the second line say "/dev/null"
instead of "/dev/\null". Well that's because it is not the quote
removal step that removed the backslash, but the pathname expansion.
The fact that the /de\v does not become /dev even though it exists
is just the result of the optimisation to avoid unnecessarily
calling stat(2). I have checked POSIX and I don't see anything
that forbids this behaviour.
POSIX never actually says this optimisation is allowed. The only thing
that allows it is the fact that it produces the same results anyway. If
that stops, then checking the file system for matches becomes required.
Cheers,
Harald van Dijk
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