File: make.info,  Node: Setting,  Next: Appending,  Prev: Values,  Up: Using Variables

6.5 Setting Variables
=====================

To set a variable from the makefile, write a line starting with the
variable name followed by one of the assignment operators '=', ':=',
'::=', or ':::='.  Whatever follows the operator and any initial
whitespace on the line becomes the value.  For example,

     objects = main.o foo.o bar.o utils.o

defines a variable named 'objects' to contain the value 'main.o foo.o
bar.o utils.o'.  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.  Variables defined with ':::=' are
"immediately expanded" variables.  The different assignment operators
are described in *Note 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 (*note 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 (*note 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 (*note 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 (*note 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.  *Note
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.