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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:

  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. 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.


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