Managing file interactions with make

Environment variables

Environment variables are read and added to the macro definitions each time make executes. Precedence is a prime consideration in doing this properly. The following describes make's interaction with the environment. A macro, MAKEFLAGS, is maintained by make. The macro is defined as the collection of all input flag arguments into a string (without minus signs). The macro is exported and thus accessible to recursive invocations of make. Command line flags and assignments in the makefile update MAKEFLAGS. Thus, to describe how the environment interacts with make, the MAKEFLAGS macro (environment variable) must be considered.

When executed, make assigns macro definitions in the following order:

  1. Read the MAKEFLAGS environment variable. If it is not present or null, the internal make variable MAKEFLAGS is set to the null string. Otherwise, each letter in MAKEFLAGS is assumed to be an input flag argument and is processed as such. (The only exceptions are the -f, -p, and -r flags.)

  2. Read the internal list of macro definitions.

  3. Read the environment. The environment variables are treated as macro definitions and marked as exported (in the shell sense).

  4. Read the makefile(s). The assignments in the makefile(s) override the environment. This order is chosen so that when a makefile is read and executed, you know what to expect. That is, you get what is seen unless the -e flag is used. The -e is the input flag argument, which tells make to have the environment override the makefile assignments. Thus, if make -e is entered, the variables in the environment override the definitions in the makefile. Also MAKEFLAGS overrides the environment if assigned. This is useful for further invocations of make from the current makefile.
It may be clearer to list the precedence of assignments. Thus, in order from least binding to most binding, the precedence of assignments is as follows:

  1. internal definitions

  2. environment

  3. makefile(s)

  4. command line
The -e flag has the effect of rearranging the order to:

  1. internal definitions

  2. makefile(s)

  3. environment

  4. command line
This order is general enough to allow a programmer to define a makefile or set of makefiles whose parameters are dynamically definable.
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