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make: File Function

 
 8.6 The 'file' Function
 =======================
 
 The 'file' function allows the makefile to write to or read from a file.
 Two modes of writing are supported: overwrite, where the text is written
 to the beginning of the file and any existing content is lost, and
 append, where the text is written to the end of the file, preserving the
 existing content.  In both cases the file is created if it does not
 exist.  It is a fatal error if the file cannot be opened for writing, or
 if the write operation fails.  The 'file' function expands to the empty
 string when writing to a file.
 
    When reading from a file, the 'file' function expands to the verbatim
 contents of the file, except that the final newline (if there is one)
 will be stripped.  Attempting to read from a non-existent file expands
 to the empty string.
 
    The syntax of the 'file' function is:
 
      $(file OP FILENAME[,TEXT])
 
    When the 'file' function is evaluated all its arguments are expanded
 first, then the file indicated by FILENAME will be opened in the mode
 described by OP.
 
    The operator OP can be '>' to indicate the file will be overwritten
 with new content, '>>' to indicate the current contents of the file will
 be appended to, or '<' to indicate the contents of the file will be read
 in.  The FILENAME specifies the file to be written to or read from.
 There may optionally be whitespace between the operator and the file
 name.
 
    When reading files, it is an error to provide a TEXT value.
 
    When writing files, TEXT will be written to the file.  If TEXT does
 not already end in a newline a final newline will be written (even if
 TEXT is the empty string).  If the TEXT argument is not given at all,
 nothing will be written.
 
    For example, the 'file' function can be useful if your build system
 has a limited command line size and your recipe runs a command that can
 accept arguments from a file as well.  Many commands use the convention
 that an argument prefixed with an '@' specifies a file containing more
 arguments.  Then you might write your recipe in this way:
 
      program: $(OBJECTS)
              $(file >$@.in,$^)
              $(CMD) $(CMDFLAGS) @$@.in
              @rm $@.in
 
    If the command required each argument to be on a separate line of the
 input file, you might write your recipe like this:
 
      program: $(OBJECTS)
              $(file >$@.in) $(foreach O,$^,$(file >>$@.in,$O))
              $(CMD) $(CMDFLAGS) @$@.in
              @rm $@.in
 

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