(make) Shell Function

 
 The `shell' Function
 ====================
 
    The `shell' function is unlike any other function except the
 `wildcard' function ( The Function `wildcard' Wildcard Function.)
 in that it communicates with the world outside of `make'.
 
    The `shell' function performs the same function that backquotes
 (``') perform in most shells: it does "command expansion".  This means
 that it takes an argument that is a shell command and returns the
 output of the command.  The only processing `make' does on the result,
 before substituting it into the surrounding text, is to convert each
 newline or carriage-return / newline pair to a single space.  It also
 removes the trailing (carriage-return and) newline, if it's the last
 thing in the result.
 
    The commands run by calls to the `shell' function are run when the
 function calls are expanded.  In most cases, this is when the makefile
 is read in.  The exception is that function calls in the commands of
 the rules are expanded when the commands are run, and this applies to
 `shell' function calls like all others.
 
    Here are some examples of the use of the `shell' function:
 
      contents := $(shell cat foo)
 
 sets `contents' to the contents of the file `foo', with a space (rather
 than a newline) separating each line.
 
      files := $(shell echo *.c)
 
 sets `files' to the expansion of `*.c'.  Unless `make' is using a very
 strange shell, this has the same result as `$(wildcard *.c)'.
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