On 27/01/2019 21:17, Stephane Chazelas wrote:
2019-01-27 02:29:04 +, Harald van Dijk:
[...]
This proposed resolution does not leave empty aliases (aliases not resulting
in any token) unspecified. I mentioned them before, because they are
mishandled by at least one shell:
$ dash -c 'alias empty=
empty'
dash: 2: Syntax error: end of file unexpected
I'd be perfectly okay with considering that a bug in dash (I personally
consider it exactly that, and it's easy to fix), but I do not know whether
there are different situations in other shells that also fail for other
reasons and require major changes to their implementations of aliases to
solve.
[...]
But then again
alias empty=
{ empty; }
or
(empty)
Also fails in many shells (and don't fail in some shells where
{ ;} or () otherwise fails).
The problem is not that much about empty aliases here but about
when alias expansion results in no command where that's not
expected.
I cannot find any shell in which { empty; } and { ; } behave
differently. ksh and zsh accept this, with or without the empty alias.
Everything else I tried (bash, bosh, dash, mksh, pdksh, yash) rejects
it, again with or without the empty alias.
I cannot find any shell in which (empty) and ( ) behave differently.
mksh, pdksh and zsh accept this, everything else I tried rejects this,
with or without the alias.
I can find one where () and ( ) behave differently, zsh, which also
makes it one where () and (empty) behave differently. That is because ()
is read as a single token, but either the space or the empty alias
forces it to be read as two separate single-character tokens.
Empty aliases are useful and often used in things like:
if [ "$DEBUG" ]; then
alias ifdebug=
else
alias ifdebug='#'
fi
ifdebug log "$(cmd)"
If it were implemented with a function instead, cmd would still
be run. aliases avoids that but you need to remember to keep
your debug commands on a single line.
You could avoid empty aliases here to let it work in dash and support
multi-line commands at the same time:
if [ "$DEBUG" ]; then
alias ifdebug=': && '
else
alias ifdebug=': || '
fi
And dash has no issue with that code.
It does have an issue with exactly what you wrote: try putting ifdebug
before the very last line in a script, in the non-DEBUG case where the
command is commented out.
Cheers,
Harald van Dijk
