Commands and execution
Command Echoing/Silencing
在每一个命令之前加上@可以禁止打印输出，你也可以在运行make时加上-s实现在每一个命令之前加上@的效果。
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all:
@echo "This make line will not be printed"
echo "But this will"
Command Execution
每个命令运行在一个新的终端中（or at least the effect is as such）。
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all:
cd ..
# The cd above does not affect this line, because each command is effectively run in a new shell
echo `pwd`
# This cd command affects the next because they are on the same line
cd ..;echo `pwd`
# Same as above
cd ..; \
echo `pwd`
Default Shell
默认的终端是/bin/sh，你可以通过设置SHELL来改变终端
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SHELL=/bin/bash
cool:
echo "Hello from bash"
Double dollar sign
在字符串中，如果想要像bash/sh中那样使用$，在make中可以使用$$
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make_var = I am a make variable
all:
# Same as running "sh_var='I am a shell variable'; echo $sh_var" in the shell
sh_var='I am a shell variable'; echo $$sh_var
# Same as running "echo I am a amke variable" in the shell
echo $(make_var)
Error handling with -k, -i, and -
在运行make时：加上-k，可以使得：make运行即使出现错误，也能够继续运行。这一般用在：需要一次性查看所有的错误
在运行命令时，加上-，可以忽略此命令的错误
在运行make时：加上-i，可以忽略所有错误
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one:
# This error will be printed but ignored, and make will continue to run
-false
touch one
Recursive use of make
如果要递归的调用makefile，应当使用$(MAKE)而不是make（because it will pass the make flags for you and won’t itself be affected by them）。
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new_contents = "hello:\n\ttouch inside_file"
all:
mkdir -p subdir
printf $(new_contents) | sed -e 's/^ //' > subdir/makefile
cd subdir && $(MAKE)
clean:
rm -rf subdir
Export, environments, and recursive make
当make启动时，其会根据当时终端中的环境变量自动创建make变量。
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# Run this with "export shell_env_var='I am an environment variable'; make"
all:
# Print out the Shell variable
echo $$shell_env_var
# Print out the Make variable
echo $(shell_env_var)
export可以将变量设置为终端环境可见
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shell_env_var=Shell env var, created inside of Make
export shell_env_var
all:
echo $(shell_env_var)
echo $$shell_env_var
当在make中运行make命令时，可以通过export设置变量，从而让sub-make命令也获得此变量。在下面这个例子中，cooly正是如此。
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new_contents = "hello:\n\techo \$$(cooly)"
all:
mkdir -p subdir
printf $(new_contents) | sed -e 's/^ //' > subdir/makefile
@echo "---MAKEFILE CONTENTS---"
@cd subdir && cat makefile
@echo "---END MAKEFILE CONTENTS---"
cd subdir && $(MAKE)
# Note that variables and exports. They are set/affected globally.
cooly = "The subdirectory can see me!"
export cooly
# This would nullify the line above: unexport cooly
clean:
rm -rf subdir
变量必须通过export，才能在终端运行
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one=this will only work locally
export two=we can run subcommands with this
all:
@echo $(one)
@echo $$one
@echo $(two)
@echo $$two
可以使用.EXPORT_ALL_VARIABLES来export所有变量
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.EXPORT_ALL_VARIABLES:
new_contents = "hello:\n\techo \$$(cooly)"
cooly = "The subdirectory can see me!"
# This would nullify the line above: unexport cooly
all:
mkdir -p subdir
printf $(new_contents) | sed -e 's/^ //' > subdir/makefile
@echo "---MAKEFILE CONTENTS---"
@cd subdir && cat makefile
@echo "---END MAKEFILE CONTENTS---"
cd subdir && $(MAKE)
clean:
rm -rf subdir
Arguments to make
make的命令参数可以参考list of options
你可以运行多个target，例如make clean run test，其会先运行目标clean，再运行run，再运行test
Variables Pt. 2
Flavors and modification
变量定义的两种形式
- recursive（使用
=）：当命令使用到这个变量时，才会去寻找变量的定义（比如例子中的later_variable） - simply expanded（使用
:=）：变量定义时就直接进行展开（later_variable未被定义，直接输出为空）
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# Recursive variable. This will print "later" below
one = one ${later_variable}
# Simply expanded variable. This will not print "later" below
two := two ${later_variable}
later_variable = later
all:
echo $(one)
echo $(two)
Simply expanded (using :=)可以避免循环引用的错误
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one = hello
# one gets defined as a simply expanded variable (:=) and thus can handle appending
one := ${one} there
all:
echo $(one)
?= 只会设置未被定义的变量
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one = hello
one ?= will not be set
two ?= will be set
all:
echo $(one)
echo $(two)
在末尾的空格不会被忽略，但是在开头的空格会被忽略。如果想要将变量设置为空格，可以使用$(nullstring)
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with_spaces = hello # with_spaces has many spaces after "hello"
after = $(with_spaces)there
nullstring =
space = $(nullstring) # Make a variable with a single space.
all:
echo "$(after)"
echo start"$(space)"end
没有被定义的变量为空
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all:
# Undefined variables are just empty strings!
echo $(nowhere)
使用+=可以用来增加变量定义的内容
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foo := start
foo += more
all:
echo $(foo)
String Substitution 也是一个很常见的修改变量的方式。可以参考 Text Functions， Filename Functions
Command line arguments and override
可以通过override将make命令中的变量定义进行覆盖。比如我们在这里运行：make option_one=hi
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# Overrides command line arguments
override option_one = did_override
# Does not override command line arguments
option_two = not_override
all:
echo $(option_one)
echo $(option_two)
List of commands and define
define/endef可以将多个命令赋值给一个变量，下面这个例子比较了两种定义方式的不同，变量one同样也是被赋值了多个命令，但是变量two的多个命令时运行在不同的shell中的
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one = export blah="I was set!"; echo $$blah
define two
export blah="I was set!"
echo $$blah
endef
all:
@echo "This prints 'I was set'"
@$(one)
@echo "This does not print 'I was set' because each command runs in a separate shell"
@$(two)
Target-specific variables
变量可以仅仅被设置给某个target，如下面的例子所示
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all: one = cool
all:
echo one is defined: $(one)
other:
echo one is nothing: $(one)
Pattern-specific variables
类似的，也可以将变量设置给某种target patterns
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%.c: one = cool
blah.c:
echo one is defined: $(one)
other:
echo one is nothing: $(one)