5.2 Command Execution
When it is time to execute commands to update a target, they are executed
by making a new subshell for each line. (In practice, make may
take shortcuts that do not affect the results.)
Please note: this implies that shell commands such as cd
that set variables local to each process will not affect the following
command lines. (2) If you want to use cd
to affect the next command, put the two on a single line with a
semicolon between them. Then make will consider them a single
command and pass them, together, to a shell which will execute them in
sequence. For example:
| | foo : bar/lose
cd bar; gobble lose > ../foo
|
If you would like to split a single shell command into multiple lines of
text, you must use a backslash at the end of all but the last subline.
Such a sequence of lines is combined into a single line, by deleting the
backslash-newline sequences, before passing it to the shell. Thus, the
following is equivalent to the preceding example:
| | foo : bar/lose
cd bar; \
gobble lose > ../foo
|
The program used as the shell is taken from the variable SHELL.
By default, the program `/bin/sh' is used.
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:
-
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.
-
In the current directory.
-
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
says `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.
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. See section Variables from the Environment. 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; this will
override the value of SHELL.
