File: autoconf.info,  Node: The Make Macro SHELL,  Next: Parallel Make,  Prev: The Make Macro MAKEFLAGS,  Up: Portable Make

12.9 The Make Macro ‘SHELL’
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Many ‘make’ implementations use the ‘$(SHELL)’ macro to spawn shell
processes and execute Make rules.  This is a builtin macro with a
default value upplied by ‘make’; the default can be overridden by a
makefile or by a command-line argument, though not by the environment.

   Other ‘make’ implementations use other ways to spawn shell processes,
and the POSIX standard for ‘make’ says that portable makefiles should
neither define nor use the ‘$(SHELL)’ macro.

   Despite this prohibition, in practice it does not hurt to define and
then possibly use ‘SHELL’ in your makefiles and in some cases it can
help your builds use a better shell to spawn shell processes.  So it’s a
good idea to define ‘SHELL’ in your makefiles.  If you use Autoconf, you
can use its standard output variable ‘SHELL’ as follows:

     SHELL = @SHELL@

If you use Automake, this is done for you.

   Do not force ‘SHELL = /bin/sh’ because that is not correct
everywhere.  Remember, ‘/bin/sh’ is not POSIX compliant on some systems,
such as Solaris 10.  Additionally, 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.  Using ‘@SHELL@’ means that
your makefile will benefit from the same improved shell, such as ‘bash’
or ‘ksh’, that was discovered during ‘configure’, so that you aren’t
fighting two different sets of shell bugs between the two contexts.

   Do not rely on whether ‘make’’s ‘SHELL’ settings are exported to
subprocesses, as implementations differ:

     $ cat Makefile
     all:
             @printf '%s\n' '$(SHELL)'
             @printenv SHELL
     $ env SHELL=/bin/sh make -e SHELL=/bin/ksh  # BSD make, AIX make
     /bin/ksh
     /bin/ksh
     $ env SHELL=/bin/sh make -e SHELL=/bin/ksh  # GNU make
     /bin/ksh
     sh