Lazy Set
VARIABLE = value
Normal setting of a variable, but any other variables mentioned with the value
field are recursively expanded with their value at the point at which the variable is used, not the one it had when it was declared
Immediate Set
VARIABLE := value
Setting of a variable with simple expansion of the values inside - values within it are expanded at declaration time.
Lazy Set If Absent
VARIABLE ?= value
Setting of a variable only if it doesn't have a value. value
is always evaluated when VARIABLE
is accessed. It is equivalent to
ifeq ($(origin VARIABLE), undefined)
VARIABLE = value
endif
See the documentation for more details.
Append
VARIABLE += value
Appending the supplied value to the existing value (or setting to that value if the variable didn't exist)
It is actually executing the command, changing the directory to some_directory
, however, this is performed in a sub-process shell, and affects neither make nor the shell you're working from.
If you're looking to perform more tasks within some_directory
, you need to add a semi-colon and append the other commands as well. Note that you cannot use newlines as they are interpreted by make as the end of the rule, so any newlines you use for clarity needs to be escaped by a backslash.
For example:
all:
cd some_dir; echo "I'm in some_dir"; \
gcc -Wall -o myTest myTest.c
Note also that the semicolon is necessary between every command even though you add a backslash and a newline. This is due to the fact that the entire string is parsed as a single line by the shell. As noted in the comments, you should use '&&' to join commands, which mean they only get executed if the preceding command was successful.
all:
cd some_dir && echo "I'm in some_dir" && \
gcc -Wall -o myTest myTest.c
This is especially crucial when doing destructive work, such as clean-up, as you'll otherwise destroy the wrong stuff, should the cd
fail for whatever reason.
A common usage though is to call make in the sub directory, which you might want to look into. There's a command line option for this so you don't have to call cd
yourself, so your rule would look like this
all:
$(MAKE) -C some_dir all
which will change into some_dir
and execute the Makefile
in there with the target "all". As a best practice, use $(MAKE)
instead of calling make
directly, as it'll take care to call the right make instance (if you, for example, use a special make version for your build environment), as well as provide slightly different behavior when running using certain switches, such as -t
.
For the record, make always echos the command it executes (unless explicitly suppressed), even if it has no output, which is what you're seeing.
Best Answer
You can print out variables as the makefile is read (assuming GNU make as you have tagged this question appropriately) using this method (with a variable named "var"):
You can add this construct to any recipe to see what make will pass to the shell:
Now, what happens here is that make stores the entire recipe (
$(info $$var is [${var}])echo Hello world
) as a single recursively expanded variable. When make decides to run the recipe (for instance when you tell it to buildall
), it expands the variable, and then passes each resulting line separately to the shell.So, in painful detail:
$(info $$var is [${var}])echo Hello world
$(info $$var is [${var}])
$$
becomes literal$
${var}
becomes:-)
(say)$var is [:-)]
appears on standard out$(info...)
though is emptyecho Hello world
echo Hello world
on stdout first to let you know what it's going to ask the shell to doHello world
on stdout.