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. (1) 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:
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 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. Variables from the Environment
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'.
---------- Footnotes ----------
(1) On MS-DOS, the value of current working directory is *global*,
so changing it _will_ affect the following command lines on those
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