Do Curly Brace Wildcards work in GNU Make 4 (or even POSIX Make)? - unix

Question

I found a difference of behaviour between GNU Make 4.1 and 3.81 and wonder whether my code is not POSIX compliant which 4 is enforcing more strictly, or whether something else is going on.
I distilled the failure case to this Makefile
.POSIX:
all: test-b
test-a:
cat a.txt b.txt c.txt >results.txt
test-b:
cat {a,b,c}.txt >results.txt
Assuming those files have been created with cat {a,b,c}.txt, target test-a always works, yet test-b works on Make 3.81 but fails on 4.1.
The output for 3.81:
$ make --version
GNU Make 3.81
Copyright (C) 2006 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.
There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE.
This program built for i386-apple-darwin11.3.0
$ make
cat {a,b,c}.txt >results.txt
$ echo $?
0
The output for 4.1:
$ make --version
GNU Make 4.1
Built for x86_64-pc-linux-gnu
Copyright (C) 1988-2014 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
$ make
cat {a,b,c}.txt >results.txt
cat: {a,b,c}.txt: No such file or directory
Makefile:9: recipe for target 'test-b' failed
make: *** [test-b] Error 1
$ echo $?
2
It's possible the cat command is actually failing in 3.81 and it just isn't pointing it out, as later versions of GNU Make mention passing the -e flag to the shell when invoking target commands to make it more POSIX compliant, but I can't see how that command could be failing.
I assume the wildcards are handled solely by the shell, so I can't see how invoking the shell via a make target command should be any different.
Which is these behaviours are correct? If wildcards like that don't work in Makefiles, which other wildcards can I assume to work?
test-b still fails in 4.1 even if .POSIX: is removed from the file.

Answer 1

Recipes are sent to the shell. They are not interpreted by make. So your question is really, are curly-brace expansions supported by the shell?
That depends on which shell make uses. They are not supported by POSIX standard sh. They are supported by bash (and many other shells).
Make always invokes /bin/sh, regardless of what shell you personally use, unless you specifically set the make SHELL variable to something else. On some systems, /bin/sh is a symlink to /bin/bash so they are the same thing (bash runs in a "POSIX emulation" mode when invoked as /bin/sh but most bash features are still available). Other systems use different shells, such as dash, as /bin/sh which do not have extra bash features.
So, you can either (a) not have a portable makefile and assume /bin/sh is the same as /bin/bash, (b) set SHELL := /bin/bash in your makefile to force it to use bash always (but fail on systems that don't have bash installed), or (c) write your makefile recipes to use only POSIX sh features so it works regardless of which shell is used for /bin/sh.