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The syntax of a simple conditional with no else
is as follows:
conditional-directive text-if-true endif |
The text-if-true may be any lines of text, to be considered as part of the makefile if the condition is true. If the condition is false, no text is used instead.
The syntax of a complex conditional is as follows:
conditional-directive text-if-true else text-if-false endif |
If the condition is true, text-if-true is used; otherwise, text-if-false is used instead. The text-if-false can be any number of lines of text.
The syntax of the conditional-directive is the same whether the conditional is simple or complex. There are four different directives that test different conditions. Here is a table of them:
ifeq (arg1, arg2)
ifeq 'arg1' 'arg2'
ifeq "arg1" "arg2"
ifeq "arg1" 'arg2'
ifeq 'arg1' "arg2"
Often you want to test if a variable has a non-empty value. When the
value results from complex expansions of variables and functions,
expansions you would consider empty may actually contain whitespace
characters and thus are not seen as empty. However, you can use the
strip
function (see section 8.2 Functions for String Substitution and Analysis) to avoid interpreting
whitespace as a non-empty value. For example:
ifeq ($(strip $(foo)),) text-if-empty endif |
will evaluate text-if-empty even if the expansion of
$(foo)
contains whitespace characters.
ifneq (arg1, arg2)
ifneq 'arg1' 'arg2'
ifneq "arg1" "arg2"
ifneq "arg1" 'arg2'
ifneq 'arg1' "arg2"
ifdef variable-name
Note that ifdef
only tests whether a variable has a value. It
does not expand the variable to see if that value is nonempty.
Consequently, tests using ifdef
return true for all definitions
except those like foo =
. To test for an empty value, use
ifeq ($(foo),)
. For example,
bar = foo = $(bar) ifdef foo frobozz = yes else frobozz = no endif |
sets `frobozz' to `yes', while:
foo = ifdef foo frobozz = yes else frobozz = no endif |
sets `frobozz' to `no'.
ifndef variable-name
Extra spaces are allowed and ignored at the beginning of the conditional directive line, but a tab is not allowed. (If the line begins with a tab, it will be considered a command for a rule.) Aside from this, extra spaces or tabs may be inserted with no effect anywhere except within the directive name or within an argument. A comment starting with `#' may appear at the end of the line.
The other two directives that play a part in a conditional are else
and endif
. Each of these directives is written as one word, with no
arguments. Extra spaces are allowed and ignored at the beginning of the
line, and spaces or tabs at the end. A comment starting with `#' may
appear at the end of the line.
Conditionals affect which lines of the makefile make
uses. If
the condition is true, make
reads the lines of the
text-if-true as part of the makefile; if the condition is false,
make
ignores those lines completely. It follows that syntactic
units of the makefile, such as rules, may safely be split across the
beginning or the end of the conditional.
make
evaluates conditionals when it reads a makefile.
Consequently, you cannot use automatic variables in the tests of
conditionals because they are not defined until commands are run
(see section Automatic Variables).
To prevent intolerable confusion, it is not permitted to start a
conditional in one makefile and end it in another. However, you may
write an include
directive within a conditional, provided you do
not attempt to terminate the conditional inside the included file.
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