(autoconf.info)The Make Macro SHELL

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11.9 The Make Macro `SHELL'

Posix-compliant `make' internally uses the `$(SHELL)' macro to spawn
shell processes and execute Make rules.  This is a builtin macro
supplied by `make', but it can be modified by a makefile or by a
command-line argument.

   Not all `make' implementations define this `SHELL' macro.  Tru64
`make' is an example; this implementation always uses `/bin/sh'.  So
it's a good idea to always define `SHELL' in your makefiles.  If you
use Autoconf, do

     SHELL = @SHELL@

   Do not force `SHELL = /bin/sh' because that is not correct
everywhere.  For instance DJGPP lacks `/bin/sh', and when its GNU
`make' port sees such a setting it enters a special emulation mode
where features like pipes and redirections are emulated on top of DOS's
`command.com'.  Unfortunately this emulation is incomplete; for
instance it does not handle command substitutions.  On DJGPP `SHELL'
should point to Bash.

   Posix-compliant `make' should never acquire the value of $(SHELL)
from the environment, even when `make -e' is used (otherwise, think
about what would happen to your rules if `SHELL=/bin/tcsh').

   However not all `make' implementations have this exception.  For
instance it's not surprising that Tru64 `make' doesn't protect `SHELL',
since it doesn't use it.

     $ cat Makefile
     SHELL = /bin/sh
     FOO = foo
             @echo $(SHELL)
             @echo $(FOO)
     $ env SHELL=/bin/tcsh FOO=bar make -e   # Tru64 Make
     $ env SHELL=/bin/tcsh FOO=bar gmake -e  # GNU make

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