how.wtf

Write a multi-line command in a Makefile

· Thomas Taylor

Occasionally, there is a need for adding multiple continuous lines for one target in a Makefile.

Writing multiple lines for a target

Executing multiple commands in a target is trivial:

1test:
2	@echo 1
3	@echo 2

Output:

11
22

However, each line is executed in its own shell. If a continuous line is needed, the following rules must be applied:

  1. Use $$ instead of $
  2. Terminate each line using a semicolon ;
  3. Add a trailing backslash at the ending of the line \
1test:
2	var1=test; \
3	echo $$var1

Output:

1var1=test; \
2echo $var1
3test

A real use case using multi-line targets

For how.wtf, the CI/CD process invalidates a CloudFront distribution via id.

For context, the aws dependency verifies that aws is executable prior to the command execution. More information on that here.

1.PHONY: invalidate
2invalidate: aws
3	distribution_id=$$(aws cloudfront list-distributions --query "DistributionList.Items[?starts_with(Origins.Items[0].DomainName, '$(BUCKET)')].Id" --output text); \
4	aws cloudfront create-invalidation \
5		--distribution-id $$distribution_id \
6		--paths "/*" \
7		--query "Invalidation.Id" \
8		--output text

1distribution_id=$(aws cloudfront list-distributions --query "DistributionList.Items[?starts_with(Origins.Items[0].DomainName, 'how.wtf')].Id" --output text); \
2aws cloudfront create-invalidation \
3	--distribution-id $distribution_id \
4	--paths "/*" \
5	--query "Invalidation.Id" \
6	--output text
7ID6WOFMWKLCYDYRUA6AGPXXXXX

Writing multiple lines using ONESHELL

The ONESHELL directive allows multiple lines to executed in the same shell; however, special characters like @, -, and + are interpreted differently.

1.ONESHELL:
2test:
3	var1=test
4	echo $$var1

Output:

1var1=test
2echo $var1
3test

#Makefile  

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