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46 GNU make

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 6.10 [Variables from the Environment], page 70.

Furthermore, when you do set SHELL in your makeﬁle that value is not exported in the

environment to recipe lines 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 (see Section 5.7.2 [Communicating Variables to a Sub-make], page 52), forcing it to

be passed in the environment to recipe lines.

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.

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