Brace expansion in command position

Brace expansion in command position

[Daniel Shahaf]

[[[
% type pwd
pwd is a shell builtin
% {pwd,-P}
zsh: command not found: pwd,-P
% builtin {pwd,-P}
/home/daniel
]]]

I expected the second command to do what the third command does.

Cheers,

Daniel

Re: Brace expansion in command position

[Peter Stephenson]

On Sat, 2020-11-21 at 15:31 +0000, Daniel Shahaf wrote:
> [[[
> % type pwd
> pwd is a shell builtin
> % {pwd,-P}
> zsh: command not found: pwd,-P
> % builtin {pwd,-P}
> /home/daniel
> ]]]
> 
> I expected the second command to do what the third command does.

"{" in command position introduces a list to be run in the current
shell, not a brace expansion.  So this is treated as { ... } where the
... here is the single-word command "pwd,-P" as a comma has no special
meaning at this point.

Actually, other shells require a space after the "{", as well as a
command terminator ";" or newline before the "}", but zsh doesn't.
Other shells don't need a space or terminator in the subshell form,
with parentheses (...), so this is another of those minefields it's
quite hard to pronounce definitely on.  (This may not stop other people
trying, excuse me if I don't take the bait :-).)

pws

Re: Brace expansion in command position

[Daniel Shahaf]

Peter Stephenson wrote on Sat, Nov 21, 2020 at 17:56:35 +0000:
> On Sat, 2020-11-21 at 15:31 +0000, Daniel Shahaf wrote:
> > [[[
> > % type pwd
> > pwd is a shell builtin
> > % {pwd,-P}
> > zsh: command not found: pwd,-P
> > % builtin {pwd,-P}
> > /home/daniel
> > ]]]
> > 
> > I expected the second command to do what the third command does.
> 
> "{" in command position introduces a list to be run in the current
> shell, not a brace expansion.  So this is treated as { ... } where the
> ... here is the single-word command "pwd,-P" as a comma has no special
> meaning at this point.
> 
> Actually, other shells require a space after the "{", as well as a
> command terminator ";" or newline before the "}", but zsh doesn't.
> Other shells don't need a space or terminator in the subshell form,
> with parentheses (...), so this is another of those minefields it's
> quite hard to pronounce definitely on.  (This may not stop other people
> trying, excuse me if I don't take the bait :-).)

It's okay, there are plenty of fish in the sea ;-)

Seriously, thanks for the answer.  This seems to be partially covered
by A02alias.ztst:"Aliasing reserved tokens", but I don't see any test
specifically about this, so:

diff --git a/Test/A01grammar.ztst b/Test/A01grammar.ztst
index 35a04e7d5..c9a381812 100644
--- a/Test/A01grammar.ztst
+++ b/Test/A01grammar.ztst
@@ -939,3 +939,8 @@ F:Note that the behaviour of 'exit' inside try-list inside a function is unspeci
  $ZTST_testdir/../Src/zsh -fc '{ ( ) } always { echo foo }'
 0:exec last command optimization inhibited for try/always
 >foo
+
+ function 'ls,/' () {echo success}
+ {ls,/}
+0:current-shell blocks masquerading as brace expansion (test to ensure behaviour changes aren't unintentional)
+>success