How can I present a git SHA1 to gnu make?

I put together tarball'd releases of software that include the output from sever开发者_StackOverflowal different projects. These tarballs themselves are considered a release. The released tarball includes a BOM (bill of materials) that itemizes all the projects in it and their associated SHA1 (git) signatures.

To make recreating these tarballs easier, I put together a make system that parses the BOM line-by-line cloning the repository into a subdirectory, checking out the specified version then performing the build. Say a line in my BOM has:

prog-a b5286f27d65ef20eb4508f76de5a1c57d8b21d85 git+ssh://git-user@localhost/~/prog-a

the repository, if not already cloned, would be placed in repos/prog-a, then a checkout is done (cd repos/prog-a; git checkout b5286f27d6) and finally a make (make -C repos/prog-a).

I've not figured out how to let gnu make decide if the checked out version of the code has already built the binary I need. Currently every single sub-project is forced to checkout and rebuild.

How can I present a git repo's SHA1 to GNU make so that it can decide if the project is out of date and needs to be updated (by performing a git checkout)?

[EDIT] This is my pattern file:

REPO_DIR=materials
BOM=$(shell sed -r 's/([^ ]+).+/\1/' bom)
BOM_DIR=$(shell sed -r 's_([^ ]+).+_$(REPO_DIR)/\1_' bom)
BOM_BLD=$(shell sed -r 's_([^ ]+).+_$(REPO_DIR)/\1/\1_' bom)

.PHONY: clean dist-clean

all: $(BOM)

clean:
    @rm $(BOM) $(BOM_BLD) -rf

dist-clean: clean
    @rm $(REPO_DIR)

.SECONDEXPANSION:

$(BOM): % : $(REPO_DIR)/$$*/$$*
    @echo " CP $< $@"
    @cp $< $@

$(BOM_BLD): % :  $$(*D)
    @echo " GIT CHECKOUT"
    @cd $<; git checkout -q $(shell sed -rn '/$(shell echo $@ | sed -r 's_.+/__')/ s/.+ (.+) .+ .+ .+/\1/p' bom)
    @echo " MAKE $@"
    @make -w -C $< $(@F)

$(BOM_DIR): | materials
    @echo " GIT CLONE $@"
    @cd $(REPO_DIR); git clone $(shell sed -rn '/$(shell echo $@ | sed -r 's_.+/__')/ s/.+ (.+) .+ .+/\1/p' bom)

materials:
    @echo " MKDIR $@"
    @mkdir $@

This would be my pattern:

TARGETS=prog-a prog-b prog-c

all: $(TARGETS)

prog-a: SHA1=123fa
prog-b: SHA1=234ab
prog-c: SHA1=345bc

$(TARGETS):
    make -C "$@" -e PARAM=$(SHA1)

In your subdir makefile, imagine something like this:

all:
    git checkout $(PARAM) -- ./
    # ... other build rules

assuming makefiles in the subdirs; you can of course do whatever you want in the make rule.

For even more dynamic make scripts, at least have a look at .SECONDEXPANSION, .PHONY, .PRECIOUS

I ended up doing an unconditional checkout and just took the hit on time if it actually had to manipulate things in the working directory of the bom item.