Parallel building with gnumake and prerequisites

My first question (yay!) is about gnumake and parallel builds. Here's a quick example file:

.PHONY: tool_1 tool_2 tool_3 tool_4 all tools

all: | tools

tools: | tool_2 tool_3 tool_4

tool_1:
    # commands for tool 1

tool_2: | tool_1
    # commands for tool 2

tool_3: | tool_1
    # commands for tool 3

tool_4: | tool_1
    # commands for tool 4

If I do make -j on this guy, is what I have here correct to ensure that the commands for tool_1 are executed exactly once, and before make tries to build any of tool_[234]?

What I'm looking for is to have make -j cause tool_1 to be built first, then to开发者_如何转开发ol_[234] to be built in parallel, but without executing the commands for tool_1 three times. I hope that makes sense. Thanks for any suggestions or ideas!

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make -j behaves exactly as you expect in your question. It does not make dependencies multiple times.

What does that pipe (|) character do in your dependency list?