Fake Git (for Go)

Ever done go build inside a Dockerfile with .git appropriately in .dockerignore, and wished Go's -buildvcs=true in Go 1.24+ could still embed a useful version number?

Well, if you've got an alternate way to get what the version number should be (embedded in a file, in the code, an environment variable / Dockerfile build ARG, etc), this might be the project for you!

Install fake-git.sh as git in PATH before your real git binary (or instead of installing git), make sure .git exists (as a directory specifically, even if empty), and set the following environment variables before invoking go build -buildvcs=true (or =auto):

• FAKEGIT_GO_SEMVER
  the actual semantic version number you want embedded in the build metadata; must match Go's vMAJOR[.MINOR[.PATCH[-PRERELEASE][+BUILD]]]
  additionally, Go tends to be even pickier about this for VCS metadata, and insists there be no +BUILD (which is sane) and also that the version include all three of MAJOR, MINOR, and PATCH explicitly
  (if you see Go invoking git for-each-ref, you've probably got a bad semver that Go doesn't like)

• FAKEGIT_GO_REVISION (optional)
  defaults to FAKEGIT_GO_SEMVER; controls vcs.revision

• FAKEGIT_GO_TIMESTAMP (optional)
  defaults to 0; controls vcs.time and must be a Unix timestamp (ie, seconds since 1970-01-01T00:00:00Z)

• FAKEGIT_GO_MODIFIED (optional)
  defaults to ""; controls vcs.modified, set to non-empty to set to true

After setting those, you can use git --fake to verify that they're probably correct and will likely work (and that you've properly shadowed/provided git in PATH).

See Dockerfile.test in this repository for a full working example (that also tests/verifies the result).

Caveat

The most important caveat here is that this is all really hacky, and prone to breakage at any/every turn. The Go project's implementation has been reasonably stable, but they could completely change how they implement this at any time and fully break this project (and I obviously cannot control that).

Example

$ go version -m ./faked
./faked: go1.25.1
	path	github.com/tianon/fake-git/test-go
	mod	github.com/tianon/fake-git/test-go	v1.2.3-4.5.6.7.8.9.0	
	build	-buildmode=exe
	build	-compiler=gc
	build	DefaultGODEBUG=containermaxprocs=0,decoratemappings=0,tlssha1=1,updatemaxprocs=0,x509sha256skid=0
	build	CGO_ENABLED=1
	build	CGO_CFLAGS=
	build	CGO_CPPFLAGS=
	build	CGO_CXXFLAGS=
	build	CGO_LDFLAGS=
	build	GOARCH=amd64
	build	GOOS=linux
	build	GOAMD64=v1
	build	vcs=git
	build	vcs.revision=1.2.3.4.5.6.7.8.9.0
	build	vcs.time=2009-02-13T23:31:30Z
	build	vcs.modified=false