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 12 Features of GNU `make'
 Here is a summary of the features of GNU `make', for comparison with
 and credit to other versions of `make'.  We consider the features of
 `make' in 4.2 BSD systems as a baseline.  If you are concerned with
 writing portable makefiles, you should not use the features of `make'
 listed here, nor the ones in  Missing.
    Many features come from the version of `make' in System V.
    * The `VPATH' variable and its special meaning.   Searching
      Directories for Prerequisites Directory Search.  This feature
      exists in System V `make', but is undocumented.  It is documented
      in 4.3 BSD `make' (which says it mimics System V's `VPATH'
    * Included makefiles.   Including Other Makefiles Include.
      Allowing multiple files to be included with a single directive is
      a GNU extension.
    * Variables are read from and communicated via the environment.
       Variables from the Environment Environment.
    * Options passed through the variable `MAKEFLAGS' to recursive
      invocations of `make'.   Communicating Options to a
      Sub-`make' Options/Recursion.
    * The automatic variable `$%' is set to the member name in an
      archive reference.   Automatic Variables.
    * The automatic variables `$@', `$*', `$<', `$%', and `$?' have
      corresponding forms like `$(@F)' and `$(@D)'.  We have generalized
      this to `$^' as an obvious extension.   Automatic Variables.
    * Substitution variable references.   Basics of Variable
      References Reference.
    * The command-line options `-b' and `-m', accepted and ignored.  In
      System V `make', these options actually do something.
    * Execution of recursive commands to run `make' via the variable
      `MAKE' even if `-n', `-q' or `-t' is specified.   Recursive
      Use of `make' Recursion.
    * Support for suffix `.a' in suffix rules.   Archive Suffix
      Rules.  This feature is obsolete in GNU `make', because the
      general feature of rule chaining ( Chains of Implicit Rules
      Chained Rules.) allows one pattern rule for installing members in
      an archive ( Archive Update) to be sufficient.
    * The arrangement of lines and backslash-newline combinations in
      commands is retained when the commands are printed, so they appear
      as they do in the makefile, except for the stripping of initial
    The following features were inspired by various other versions of
 `make'.  In some cases it is unclear exactly which versions inspired
 which others.
    * Pattern rules using `%'.  This has been implemented in several
      versions of `make'.  We're not sure who invented it first, but
      it's been spread around a bit.   Defining and Redefining
      Pattern Rules Pattern Rules.
    * Rule chaining and implicit intermediate files.  This was
      implemented by Stu Feldman in his version of `make' for AT&T
      Eighth Edition Research Unix, and later by Andrew Hume of AT&T
      Bell Labs in his `mk' program (where he terms it "transitive
      closure").  We do not really know if we got this from either of
      them or thought it up ourselves at the same time.   Chains of
      Implicit Rules Chained Rules.
    * The automatic variable `$^' containing a list of all prerequisites
      of the current target.  We did not invent this, but we have no
      idea who did.   Automatic Variables.  The automatic variable
      `$+' is a simple extension of `$^'.
    * The "what if" flag (`-W' in GNU `make') was (as far as we know)
      invented by Andrew Hume in `mk'.   Instead of Executing the
      Commands Instead of Execution.
    * The concept of doing several things at once (parallelism) exists in
      many incarnations of `make' and similar programs, though not in the
      System V or BSD implementations.   Command Execution
    * Modified variable references using pattern substitution come from
      SunOS 4.   Basics of Variable References Reference.  This
      functionality was provided in GNU `make' by the `patsubst'
      function before the alternate syntax was implemented for
      compatibility with SunOS 4.  It is not altogether clear who
      inspired whom, since GNU `make' had `patsubst' before SunOS 4 was
    * The special significance of `+' characters preceding command lines
      ( Instead of Executing the Commands Instead of Execution.) is
      mandated by `IEEE Standard 1003.2-1992' (POSIX.2).
    * The `+=' syntax to append to the value of a variable comes from
      SunOS 4 `make'.   Appending More Text to Variables Appending.
    * The syntax `ARCHIVE(MEM1 MEM2...)' to list multiple members in a
      single archive file comes from SunOS 4 `make'.   Archive
    * The `-include' directive to include makefiles with no error for a
      nonexistent file comes from SunOS 4 `make'.  (But note that SunOS 4
      `make' does not allow multiple makefiles to be specified in one
      `-include' directive.)  The same feature appears with the name
      `sinclude' in SGI `make' and perhaps others.
    The remaining features are inventions new in GNU `make':
    * Use the `-v' or `--version' option to print version and copyright
    * Use the `-h' or `--help' option to summarize the options to `make'.
    * Simply-expanded variables.   The Two Flavors of Variables
    * Pass command-line variable assignments automatically through the
      variable `MAKE' to recursive `make' invocations.   Recursive
      Use of `make' Recursion.
    * Use the `-C' or `--directory' command option to change directory.
       Summary of Options Options Summary.
    * Make verbatim variable definitions with `define'.   Defining
      Variables Verbatim Defining.
    * Declare phony targets with the special target `.PHONY'.
      Andrew Hume of AT&T Bell Labs implemented a similar feature with a
      different syntax in his `mk' program.  This seems to be a case of
      parallel discovery.   Phony Targets Phony Targets.
    * Manipulate text by calling functions.   Functions for
      Transforming Text Functions.
    * Use the `-o' or `--old-file' option to pretend a file's
      modification-time is old.   Avoiding Recompilation of Some
      Files Avoiding Compilation.
    * Conditional execution.
      This feature has been implemented numerous times in various
      versions of `make'; it seems a natural extension derived from the
      features of the C preprocessor and similar macro languages and is
      not a revolutionary concept.   Conditional Parts of
      Makefiles Conditionals.
    * Specify a search path for included makefiles.   Including
      Other Makefiles Include.
    * Specify extra makefiles to read with an environment variable.
       The Variable `MAKEFILES' MAKEFILES Variable.
    * Strip leading sequences of `./' from file names, so that `./FILE'
      and `FILE' are considered to be the same file.
    * Use a special search method for library prerequisites written in
      the form `-lNAME'.   Directory Search for Link Libraries
    * Allow suffixes for suffix rules ( Old-Fashioned Suffix Rules
      Suffix Rules.) to contain any characters.  In other versions of
      `make', they must begin with `.' and not contain any `/'
    * Keep track of the current level of `make' recursion using the
      variable `MAKELEVEL'.   Recursive Use of `make' Recursion.
    * Provide any goals given on the command line in the variable
      `MAKECMDGOALS'.   Arguments to Specify the Goals Goals.
    * Specify static pattern rules.   Static Pattern Rules Static
    * Provide selective `vpath' search.   Searching Directories for
      Prerequisites Directory Search.
    * Provide computed variable references.   Basics of Variable
      References Reference.
    * Update makefiles.   How Makefiles Are Remade Remaking
      Makefiles.  System V `make' has a very, very limited form of this
      functionality in that it will check out SCCS files for makefiles.
    * Various new built-in implicit rules.   Catalogue of Implicit
      Rules Catalogue of Rules.
    * The built-in variable `MAKE_VERSION' gives the version number of
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