Info Catalog make: Values make: Using Variables make: Appending

make: Setting

 
 6.5 Setting Variables
 =====================
 
 To set a variable from the makefile, write a line starting with the
 variable name followed by '=', ':=', or '::='.  Whatever follows the
 '=', ':=', or '::=' on the line becomes the value.  For example,
 
      objects = main.o foo.o bar.o utils.o
 
 defines a variable named 'objects'.  Whitespace around the variable name
 and immediately after the '=' is ignored.
 
    Variables defined with '=' are "recursively expanded" variables.
 Variables defined with ':=' or '::=' are "simply expanded" variables;
 these definitions can contain variable references which will be expanded
 before the definition is made.  The Two Flavors of Variables
 Flavors.
 
    The variable name may contain function and variable references, which
 are expanded when the line is read to find the actual variable name to
 use.
 
    There is no limit on the length of the value of a variable except the
 amount of memory on the computer.  You can split the value of a variable
 into multiple physical lines for readability (Splitting Long
 Lines Splitting Lines.).
 
    Most variable names are considered to have the empty string as a
 value if you have never set them.  Several variables have built-in
 initial values that are not empty, but you can set them in the usual
 ways (Variables Used by Implicit Rules Implicit Variables.).
 Several special variables are set automatically to a new value for each
 rule; these are called the "automatic" variables (Automatic
 Variables).
 
    If you'd like a variable to be set to a value only if it's not
 already set, then you can use the shorthand operator '?=' instead of
 '='.  These two settings of the variable 'FOO' are identical (The
 'origin' Function Origin Function.):
 
      FOO ?= bar
 
 and
 
      ifeq ($(origin FOO), undefined)
      FOO = bar
      endif
 
    The shell assignment operator '!=' can be used to execute a shell
 script and set a variable to its output.  This operator first evaluates
 the right-hand side, then passes that result to the shell for execution.
 If the result of the execution ends in a newline, that one newline is
 removed; all other newlines are replaced by spaces.  The resulting
 string is then placed into the named recursively-expanded variable.  For
 example:
 
      hash != printf '\043'
      file_list != find . -name '*.c'
 
    If the result of the execution could produce a '$', and you don't
 intend what follows that to be interpreted as a make variable or
 function reference, then you must replace every '$' with '$$' as part of
 the execution.  Alternatively, you can set a simply expanded variable to
 the result of running a program using the 'shell' function call.  
 The 'shell' Function Shell Function.  For example:
 
      hash := $(shell printf '\043')
      var := $(shell find . -name "*.c")
 
    As with the 'shell' function, the exit status of the just-invoked
 shell script is stored in the '.SHELLSTATUS' variable.
 

Info Catalog make: Values make: Using Variables make: Appending