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’
===========================

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@

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 many systems,
such as FreeBSD 4, NetBSD 3, AIX 3, Solaris 10, or Tru64.  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.

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

   Conversely, ‘make’ is not supposed to export any changes to the macro
‘SHELL’ to child processes.  Again, many implementations break this
rule:

     $ cat Makefile
     all:
             @echo $(SHELL)
             @printenv SHELL
     $ env SHELL=sh make -e SHELL=/bin/ksh   # BSD Make, GNU make 3.80
     /bin/ksh
     /bin/ksh
     $ env SHELL=sh gmake -e SHELL=/bin/ksh  # GNU make 3.81
     /bin/ksh
     sh