Dependency Files

Now we've made a README.md file, we can show how a makefile looks in the README:

README.md: Makefile
	echo "Basic makefile example." > $@
	echo "" >> $@
	echo '```' >> $@
	cat $< >> $@
	echo '```' >> $@

Note the order:

The first thing is the file you want, then a colon (:).
After the colon, any file it depends on.
Finally, the shell commands to execute.

Strange Sigils

Notice that the file above can print into the README by using echo "" >> $@. The $@ stands for 'the file which we want', and $< stands for 'the first dependency file'. The make program starts by replacing those variables, and the result it:

README.md: Makefile
	echo "Basic makefile example." > README.md
	echo "" >> README.md
	echo '```' >> README.md
	cat Makefile >> README.md
	echo '```' >> README.md

Sigil	Meaning
`$@`	The file we want
`$<`	First dependency file
`$^`	All dependency files
`$(@F)`	Filename of the file we want
`$(@D)`	Directory path of the file we want
`$(<F)`	Filename of the first dependency
`$(@D)`	Directory path of the first dependency

Basic Variables

You can assign a variable normally, but must refer to it in brackets.

storage_directory = backups

README.md: Makefile
	echo "Basic makefile example." > $@
	echo "" >> $@
	echo '```' >> $@
	cat $< >> $@
	echo '```' >> $@

$(storage_directory)/README.md: README.md
    mkdir $(@D)
    cp $< $@

Now you can tell make to create the backup:

make backups/README.md

Command Variables

The backup README.md could be named after the current minute of the day, using date +%M. This allows up-to-the-minute backups:

current_minute != date +%M

storage_directory = backups

README.md: Makefile
	echo "Basic makefile example." > $@
	echo "" >> $@
	echo '```' >> $@
	cat $< >> $@
	echo '```' >> $@

$(storage_directory)/backup_$(current_minute).md: README.md
	mkdir $(@D)
	cp $< $@

...but the repeated use of mkdir is causing an error, because that directory already exists.

We can solve this by using mkdir -p.

Phony Targets

But we don't want to look up the current minute of the day to make backups. Better to just say make backup. However, this will confuse make, because make thinks everything is a file, so it would try to make a file called backup.

The solution is to tell make that backup is a phony target.

  [ ... ]

.PHONY: backup
backup: $(storage_directory)/backup_$(current_minute).md
$(storage_directory)/backup_$(current_minute).md: README.md
	mkdir -p $(@D)
	cp $< $@

Now run make backup to create an up-to-date backup.

Order

Makefile thinks like this:

Fill in all the variables in the file, from top to bottom.
If variables are missing, go through the file again.
Figure out the order the files should be built in.

In this case, the makefile can see that backup depends on the current backup file (with the minute in the filename), which depends on the README.md file, which depends on the Makefile itself.


┌──────────────────────┐
│       Makefile       │
└──────────────────────┘
  │
  │
  ▼
┌──────────────────────┐
│      README.md       │
└──────────────────────┘
  │
  │
  ▼
┌──────────────────────┐
│ backups/backup_06.md │
└──────────────────────┘
  │
  │
  ▼
┌──────────────────────┐
│        backup        │
└──────────────────────┘

4.6 KiB Raw Blame History