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makefile styleguide as task runner

Using make as task runner, this is style guide of makefile for me

Style Rules

Assignment to variables

Use := as possible. The variables defined by = is referred recursively and it is little tricky.

See 6.5 Setting Variables

Order or targets

Here is the preferable target order.

1. all (any name you like)
3. (other tasks)
4. clean
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all is optional and should contains default tasks, like download, build, start. If you do not allow running without specifying goal, just remove this target.

.PHONY target should contain all phony targets, which name is not build output. There are two reasons to use phony target. to make sure to run target if a file of the same name exists, and to skip checking a file timestamp of the same name.

see Phony Targets (GNU make).

clean is used to be placed in the last, as I know.

Formatting Rules

Command substitution

Use $$() (double dollar) instead of \`` (backquotes).

Some articles say that backquotes has a little bit more compatibility to ancient make. However, escaping backquotes in nested command substitution is bother jobs.

TIPS: $$(command) vs $(shell command)

$(shell command) will be evaluated at the first time. $$(command) will be evaluated at the execution time.

Semicolon vs new line and TAB

Use new line and TAB for command. To keep the format constant regardless of the number of lines in the command.

Shell variables

Use $${VAR} instead of $$VAR. It help to distinguish shell variables from make variables.

Meta Rules

File name

Use Makefile, start with capital.

The manual says We recommend Makefile because it appears prominently near the beginning of a directory listing, right near other important files such as README.

References and Inspirations

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