为何Makefile中被两个目标依赖的write目标仅执行一次？

I have this Makefile:

write:
	@echo `date`
file_1: write file_2
	@echo "file_1 begin"
	$(shell sleep 3)
	@echo "file_1 end"
file_3: write file_4
	@echo "file_3 begin"
	$(shell sleep 3)
	@echo "file_3 end"
all: file_1 file_3
.PHONY: write

When I run make, the output is:

Sat Oct 9 15:22:45 CST 2021
file_1 begin
file_3 begin
file_1 end
file_3 end

I noticed that the write target only runs once, even though it's a dependency for both file_1 and file_3. My goal is to use the write target to log start times so I can calculate the execution time of file_1 and file_3, but this behavior breaks that plan.

Why does this happen? And is there a better way to calculate the execution time of Makefile targets?

Great question! Let's break this down step by step.

Why write only runs once #

This is core Make behavior: every target (including .PHONY ones) is executed exactly once per make session, no matter how many other targets list it as a dependency.

The .PHONY marker just tells Make that write isn't associated with a physical file, so it should always run the target's commands instead of checking if a file is up-to-date. But it doesn't override Make's fundamental rule of avoiding redundant work. When Make processes file_1, it runs write; when it later gets to file_3's dependencies, it sees write has already been executed in this session, so it skips running it again.

Better ways to calculate target execution time #

If you want to track the start/end time (and elapsed duration) for each individual target, here are a few clean approaches:

1. Inline time tracking per target #

Add start/end time capture directly in each target's command sequence. This is straightforward and easy to read:

file_1: file_2
	@start_time=$$(date +%s); \
	echo "file_1 begin at $$(date)"; \
	sleep 3; \
	end_time=$$(date +%s); \
	echo "file_1 end at $$(date), elapsed time: $$((end_time - start_time)) seconds"

file_3: file_4
	@start_time=$$(date +%s); \
	echo "file_3 begin at $$(date)"; \
	sleep 3; \
	end_time=$$(date +%s); \
	echo "file_3 end at $$(date), elapsed time: $$((end_time - start_time)) seconds"

all: file_1 file_3

2. Use the time command #

If you just need a quick duration without custom logging, use the shell's built-in time command. Wrap your target's commands in parentheses to time the entire sequence:

file_1: file_2
	@echo "file_1 begin"
	@time (sleep 3)
	@echo "file_1 end"

file_3: file_4
	@echo "file_3 begin"
	@time (sleep 3)
	@echo "file_3 end"

all: file_1 file_3

This will output standard time stats (real, user, sys) after the target finishes.

3. Reusable Make function for consistency #

If you have many targets to track, define a reusable function to avoid repetition:

# Define a function that takes target name and command as arguments
define timed_target
	@start_time=$$(date +%s); \
	echo "$(1) started at $$(date)"; \
	$(2); \
	end_time=$$(date +%s); \
	echo "$(1) finished at $$(date) | Elapsed: $$((end_time - start_time)) seconds"
endef

file_1: file_2
	$(call timed_target,file_1,sleep 3)

file_3: file_4
	$(call timed_target,file_3,sleep 3)

all: file_1 file_3

4. Per-target start logging (if you still want per-target timestamps) #

If you specifically want a "start time" log entry for each target (like your original write idea), don't use a shared dependency. Instead, call a logging snippet directly in each target:

# Helper to log start time for a target
define log_start
	@echo "$$(date) - $(1) starting"
endef

file_1: file_2
	$(call log_start,file_1)
	@sleep 3
	@echo "file_1 end"

file_3: file_4
	$(call log_start,file_3)
	@sleep 3
	@echo "file_3 end"

all: file_1 file_3

This way, each target gets its own start timestamp, which you can pair with an end timestamp to calculate duration.

内容的提问来源于stack exchange，提问作者zcfh