(make) Choosing the Shell

 
 5.3.1 Choosing the Shell
 ------------------------
 
 The program used as the shell is taken from the variable `SHELL'.  If
 this variable is not set in your makefile, the program `/bin/sh' is
 used as the shell.
 
    Unlike most variables, the variable `SHELL' is never set from the
 environment.  This is because the `SHELL' environment variable is used
 to specify your personal choice of shell program for interactive use.
 It would be very bad for personal choices like this to affect the
 functioning of makefiles.   Variables from the Environment
 Environment.
 
    Furthermore, when you do set `SHELL' in your makefile that value is
 _not_ exported in the environment to commands that `make' invokes.
 Instead, the value inherited from the user's environment, if any, is
 exported.  You can override this behavior by explicitly exporting
 `SHELL' ( Communicating Variables to a Sub-`make'
 Variables/Recursion.), forcing it to be passed in the environment to
 commands.
 
    However, on MS-DOS and MS-Windows the value of `SHELL' in the
 environment *is* used, since on those systems most users do not set
 this variable, and therefore it is most likely set specifically to be
 used by `make'.  On MS-DOS, if the setting of `SHELL' is not suitable
 for `make', you can set the variable `MAKESHELL' to the shell that
 `make' should use; if set it will be used as the shell instead of the
 value of `SHELL'.
 
 Choosing a Shell in DOS and Windows
 ...................................
 
 Choosing a shell in MS-DOS and MS-Windows is much more complex than on
 other systems.
 
    On MS-DOS, if `SHELL' is not set, the value of the variable
 `COMSPEC' (which is always set) is used instead.
 
    The processing of lines that set the variable `SHELL' in Makefiles
 is different on MS-DOS.  The stock shell, `command.com', is
 ridiculously limited in its functionality and many users of `make' tend
 to install a replacement shell.  Therefore, on MS-DOS, `make' examines
 the value of `SHELL', and changes its behavior based on whether it
 points to a Unix-style or DOS-style shell.  This allows reasonable
 functionality even if `SHELL' points to `command.com'.
 
    If `SHELL' points to a Unix-style shell, `make' on MS-DOS
 additionally checks whether that shell can indeed be found; if not, it
 ignores the line that sets `SHELL'.  In MS-DOS, GNU `make' searches for
 the shell in the following places:
 
   1. In the precise place pointed to by the value of `SHELL'.  For
      example, if the makefile specifies `SHELL = /bin/sh', `make' will
      look in the directory `/bin' on the current drive.
 
   2. In the current directory.
 
   3. In each of the directories in the `PATH' variable, in order.
 
 
    In every directory it examines, `make' will first look for the
 specific file (`sh' in the example above).  If this is not found, it
 will also look in that directory for that file with one of the known
 extensions which identify executable files.  For example `.exe',
 `.com', `.bat', `.btm', `.sh', and some others.
 
    If any of these attempts is successful, the value of `SHELL' will be
 set to the full pathname of the shell as found.  However, if none of
 these is found, the value of `SHELL' will not be changed, and thus the
 line that sets it will be effectively ignored.  This is so `make' will
 only support features specific to a Unix-style shell if such a shell is
 actually installed on the system where `make' runs.
 
    Note that this extended search for the shell is limited to the cases
 where `SHELL' is set from the Makefile; if it is set in the environment
 or command line, you are expected to set it to the full pathname of the
 shell, exactly as things are on Unix.
 
    The effect of the above DOS-specific processing is that a Makefile
 that contains `SHELL = /bin/sh' (as many Unix makefiles do), will work
 on MS-DOS unaltered if you have e.g. `sh.exe' installed in some
 directory along your `PATH'.
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