Re: shortcut of ":" but not /bin/true

[David Boyce]

To clarify for anyone following along at home: this is different from the 
normal GNU make "fast path" feature where it will run <command> directly 
where possible rather than using the "traditional path" of running '/bin/sh 
-c <command>'. The ":" is given a third way which we might call the 
"blazing fast path"; it's not executed at all. The discussion is of whether 
and how to promote true and false from the fast path to the blazing fast path.

Which raises the question: why wasn't the fast path in use in the truss 
output I showed? The answer is apparently that the generated Perl makefiles 
have an explicit SHELL=/bin/sh which disables the fast path. I wonder if it 
would be possible to detect when assignment to SHELL is a no-op and leave 
fast path on? Might be harder than it seems when recursively expanded 
variables are involved. Unfortunately a lot of generated makefiles have 
this tic. In fact ISTR reading somebody's "best practices with make" paper 
which included an injunction to always assign SHELL=/bin/sh explicitly. Not 
saying I agree with that but it's out there and clearly a lot of people do.

-David Boyce