语句¶

条件语句¶

条件语句的语法:

1. # if ...:
<conditional-directive>
<text-if-true>
endif

2. # if ... else ...:
<conditional-directive>
<text-if-true>
else
<text-if-false>
endif

ifeq：条件判断，判断是否相等:

示例: 判断当前编译器是否 gcc ，然后指定不同的库文件:
ifeq ($(CC),gcc)
  libs=$(libs_for_gcc)
else
  libs=$(normal_libs)
endif

例:
判断变量 ROOT_PACKAGE 是否为空:
ifeq ($(ROOT_PACKAGE),)

ifneq：条件判断，判断是否不相等:

格式:
ifneq (<arg1>, <arg2>)
ifneq '<arg1>' '<arg2>'
ifneq "<arg1>" "<arg2>"

ifdef：条件判断，判断变量是否已定义:
```
格式:
ifdef <variable-name>
```
ifndef：条件判断，判断变量是否未定义:
```
格式:
ifndef <variable-name>
```

循环语句¶

LIST = one two three
all:
    for i in $(LIST); do \
        echo $$i; \
    done

# 等同于

all:
    for i in one two three; do \
        echo $i; \
    done