TCL exec syntax with unknown possible wildcard?

Question

I have the following line in a TCL script...

exec {*}$cmd

I understand that exec will run the command specified in the string $cmd, but cant find any reference to what the {*} does.... Can anyone tell me what the {*} does please?

I've thought that the {...} means that the first item is a group with no substitution allowed, so would it be like prefixing the string in $cmd with an asterisk... but this makes no sense to me... any ideas guys?

If I write

set cmd "ls"
puts {*}$cmd

I get

ls

So the asterisk is not being printed. Put anything else inside the braces and I get an error... possibly some really simple TCL syntax I'm not aware of but would appreciate a pointer

Answer 1

In Tcl, {*} will enumerate the elements of a list.

For instance, consider:

set cmd {$out Stuff}
puts $cmd
# => $out Stuff
puts {*}$cmd
# => can not find channel named "$out"

It is documented here.

Basically, the above code is puts-ing {$out Stuff} in the first case, but it is evaluating puts $out Stuff in the second. Of course, if you now have a channel named $out, you will have Stuff in that channel.