make syntax

There is some basic stuff you need to know if you want to extend the provided luggage.makefile. I was surprised by the nice structured syntax and by how well it actually fits what you try to do when buiding a package.

Rules

Makefiles are governed by rules, basic syntax is:

target: dependency1, dependency2, etc.
	command to create target
	optional other command

If the target exists, make skips the rule. The dependencies are also rules, make checks if the dependencies are up-to-date and builds them if necessary. The commands are regular shell commands indented by a tab. make echoes every command before it is executed unless it is preceded with a @. Here’s an example from luggage.make:

payload_d:
	@sudo mkdir -p ${PAYLOAD_D}

The target is payload_d, it has no other dependencies and it is built using mkdir -p ${PAYLOAD_D} (${PAYLOAD_D} is a variable declared earlier)

Extended rule syntax

It took some time to understand this one:

unbz2-applications-%: %.tar.bz2 l_Applications
	sudo ${TAR} xjf $< -C ${WORK_D}/Applications
	@sudo chown -R root:admin ${WORK_D}/Applications/$(shell echo $< | sed s/\.tar\.bz2//g)

It matches all rules that start with unbz2-applications-, using ‘%’ as a wildcard (like * in bash). Then it substitutes the contents of ‘%’ in the first dependency (%.tar.bz2) and then uses the first dependency in the first command using $<.

So if we have a rule that states unbz2-applications-mail.app, make is going to look for a file or rule that is named mail.app.tar.bz2, then it is going to extract that using ${TAR} xjf mail.app.tar.bz2

Syntax

@ prevents the line (not the output) being echoed
- continue after an error occurs
% wildcard, matches any chars
$< matches the first dependency