Makefile自定义函数cap输出异常问题咨询：执行make f为何打印@echo指令而非预期内容

Why This Happens #

The issue comes down to how Make validates recipe lines and the structure of your cap function.

In your original Makefile, the cap function generates @echo $1 without a leading tab. When foreach expands, only the first line of the resulting recipe has a leading tab (inherited from the original f: rule's recipe line). The subsequent lines (@echo x2, @echo x3, etc.) lack this required tab.

Make requires every recipe line to start with a tab (or the custom character set in .RECIPEPREFIX). Lines without this tab aren't treated as valid command lines. Instead of executing @echo x2 as a command, Make ends up printing the literal @echo x2 text.

Fixes #

Here are a few straightforward ways to resolve this:

Fix 1: Include a Tab in the cap Function

Modify the cap function to include a leading tab before @echo. This ensures every line generated by foreach is a valid recipe line:

define cap
	@echo $1
endef
f:
$(foreach x,$(shell command),$(call cap,$x))

(Important: The line inside define cap must start with a tab, not spaces.)

Fix 2: Use a Single Shell Command with Semicolons

If you don't mind running all echo commands in a single shell instance, simplify the recipe to a single line with @ to suppress command echoing:

f:
	@$(foreach x,$(shell command),echo $(x);)

This will print each xN on a new line without showing any underlying echo commands.

Fix 3: Add Tab and @ Directly in the foreach Call

Keep the cap function simple, and handle the tab and @ prefix within the foreach loop:

define cap
echo $1
endef
f:
$(foreach x,$(shell command),	@$(call cap,$x))

Again, ensure the line inside the foreach starts with a tab.

Expected Output #

Any of these fixes will produce your desired result:

x1
x2
x3
x4
x5

内容的提问来源于stack exchange，提问作者NinjaGaiden