We've already covered many of the basic elements of make commands, but just to make sure we're all on the same page, let's review a little.
Commands are essentially one-line shell scripts. In effect, make grabs each line and passes it to a subshell for execution. In fact, make can optimize this (relatively) expensive fork/ exec algorithm if it can guarantee that omitting the shell will not change the behavior of the program. It checks this by scanning each command line for shell special characters, such as wildcard characters and i/o redirection. If none are found, make directly executes the command without passing it to a subshell.
By default, /bin/sh is used for the shell. This shell is controlled by the make variable SHELL but it is not inherited from the environment. When make starts, it imports all the variables from the user 's environment as make variables, except SHELL . This is because the user's choice of shell should not cause a makefile (possibly included in some downloaded software package) to fail. If a user really wants to change the default shell used by make , he can set the SHELL variable explicitly in the makefile . We will discuss this issue in the Section 5.2 later in this chapter.
