Looping Through API Calls in a Makefile

When working with Makefiles, you might encounter scenarios where you need to perform repetitive tasks, such as calling an API to list information from a server. Instead of duplicating code, it’s more efficient to extract a function and loop through an array of parameters. Let’s walk through an example and address a common issue that arises.

Initial Task

Consider the following task in a Makefile, which fetches data files for app1 and app2:

list.data.files:
	@echo "Data files for app1"
	@curl https://example.com/file/list?app=app1
	@echo "Data files for app2"
	@curl https://example.com/file/list?app=app2

Refactoring with a Function

To avoid code duplication, we can define a function and use an array to loop through the apps:

APPS = app1 app2

define list_data_files
	@echo "Data files for $(1)"
	@curl https://example.com/file/list?app=$(1)
endef

list.data.files:
	@$(foreach app,$(APPS),$(call list_data_files,$(app));)

Encountering Issues

However, running the above code results in an error:

/bin/sh: @echo: command not found
make: *** [list.data.files] Error 127

The first app executes correctly, but the second one fails. Further simplification to isolate the issue shows the same problem:

APPS = app1 app2

define list_data_files
	@echo "Data files for $(1)"
endef

list.data.files:
	@$(foreach app,$(APPS),$(call list_data_files,$(app));)

Output:

Data files for app1
/bin/sh: @echo: command not found
make: *** [list.data.files] Error 127

Removing the @ symbol before the command in the function leads to the same issue with curl:

APPS = app1 app2

define list_data_files
	@curl https://example.com/file/list?app=$(1)
endef

list.data.files:
	@$(foreach app,$(APPS),$(call list_data_files,$(app));)

Output:

...
/bin/sh: @curl: command not found
make: *** [list.data.files] Error 127

Solution

The solution is to remove the @ symbol before the commands in the function. Here’s the final, working Makefile:

APPS = app1 app2

define list_data_files
	echo "Data files for $(1)"
	curl https://example.com/file/list?app=$(1)
endef

list.data.files:
	@$(foreach app,$(APPS),$(call list_data_files,$(app));)

With this approach, the commands won't be printed when executed in the function, achieving the desired behavior without errors.