Install from @latest, retract v0.0.1, print a go-installed binary's version (closes #116)

README "Install" now leads with go install ...@latest: until 1.0 each
release is a v0.0.N tag on main's tip, so @latest gives that tip once
it has passed the build. It also says what @main and v0.0.1 give and
how to check an installed binary. go.mod retracts v0.0.1, the early
build without a database.

sfdupes --version printed "dev" for every go-installed binary, since
only the linker set Version. When the linker sets none, it now prints
the main module's version from runtime/debug.ReadBuildInfo; "dev" is
left only when neither is known. Version is now empty by default.

The retraction takes effect only once a release carrying it is the
newest.

Model: opus-5-5

README.md

@@ -49,15 +49,40 @@ results are as fresh as the last completed scan.
 
 ### Install
 
-With Go installed, this builds and installs the current `main` branch:
+With Go installed, this builds and installs the newest release:
+
+```sh
+go install sneak.berlin/go/sfdupes@latest
+```
+
+Until 1.0, releases are `v0.0.N` tags on the tip of the `main` branch: each time
+`main` moves and its tip passes `script/cibuild`, that tip is tagged with the
+next `v0.0.N`. So `@latest` installs `main`'s tip once it has passed the build.
+`@main` installs the tip before it is tagged:
 
 ```sh
 go install sneak.berlin/go/sfdupes@main
 ```
 
-The binary goes to `$(go env GOPATH)/bin`, or to `$GOBIN` when that is set. A
-binary installed this way reports its version as `dev`; one built from a clone
-or into the Docker image carries the git tag or commit it was built from.
+`v0.0.1` is an early build without a database. It does not work as this README
+describes, and `go.mod` retracts it.
+
+The binary goes to `$(go env GOPATH)/bin`, or to `$GOBIN` when that is set. To
+see which version an installed binary is:
+
+```sh
+sfdupes --version
+go version -m "$(command -v sfdupes)"
+```
+
+`sfdupes --version` prints `sfdupes VERSION`. For a binary from `go install`,
+`VERSION` is the version Go recorded in it: the release tag, such as `v0.0.3`,
+or for `@main` a pseudo-version ending in the first 12 characters of the
+commit's hash. A binary built from a clone or into the Docker image carries the
+git tag or commit it was built from instead, and `VERSION` is `dev` only when
+the binary carries no version at all. `go version -m` prints what Go recorded in
+the binary; its `mod` line names the module's version. Binaries from `v0.0.2`
+and earlier print `dev`, so check those with `go version -m`.
 
 From a clone, `make build` writes the binary to `./sfdupes`:
 
@@ -262,10 +287,10 @@ sfdupes [command] --help
 CPUs), `-x` (`--one-file-system`) keeps the walk of each operand on that
 operand's filesystem, and `--no-default-excludes` lets the walk enter the
 default excludes, a list of paths; all three are described under "`scan` mode".
-`sfdupes --version` (or `-v`) prints one line, `sfdupes VERSION`, to stdout and
-exits 0, writing nothing to stderr. `-h` or `--help`, alone or after a
-subcommand, prints the help text to stderr and exits 0, writing nothing to
-stdout.
+`sfdupes --version` (or `-v`) prints one line, `sfdupes VERSION` (see
+"Install"), to stdout and exits 0, writing nothing to stderr. `-h` or `--help`,
+alone or after a subcommand, prints the help text to stderr and exits 0, writing
+nothing to stdout.
 
 ### Database
 

TODO.md

@@ -29,6 +29,16 @@
 
 # Completed Steps
 
+- releases until 1.0 are `v0.0.N` tags on `main`'s tip, so
+  `go install sneak.berlin/go/sfdupes@latest` installs `main`'s tip once it has
+  passed the build; README "Install" says so, says what `@main` and `v0.0.1`
+  give, and how to check an installed binary. `go.mod` retracts `v0.0.1`, the
+  early build without a database. When the linker set no version,
+  `sfdupes --version` prints the main module's version that Go recorded in the
+  binary, the tag or the pseudo-version of a `go install` build, and `dev` only
+  when neither is known (2026-10-17,
+  https://git.eeqj.de/sneak/sfdupes/issues/116)
+
 - `scan` creates the database's directory where the database path leads: the
   path with its last element cut off as text, never cleaned, so a `..` after a
   symlinked directory is followed through the symlink, as when the file is

go.mod

@@ -24,3 +24,5 @@ require (
 	modernc.org/mathutil v1.7.1 // indirect
 	modernc.org/memory v1.11.0 // indirect
 )
+
+retract v0.0.1 // early build without a database; does not work as the README describes

main.go

@@ -28,6 +28,7 @@ import (
 	"io"
 	"os"
 	"runtime"
+	"runtime/debug"
 
 	"github.com/spf13/cobra"
 )
@@ -58,10 +59,32 @@ var errNoSubcommand = errors.New("no subcommand")
 var errWorkersBelowOne = errors.New("--workers must be at least 1")
 
 // Version is the build version, injected at link time via -ldflags
-// (see the Makefile); "dev" for a plain go build.
+// (see the Makefile); empty when the linker set none, as for a binary
+// built by go install. buildVersion turns it into what --version prints.
 //
 //nolint:gochecknoglobals // written only by the linker
-var Version = "dev"
+var Version string
+
+// buildVersion returns the version that sfdupes --version prints
+// (README "Install"): the version the linker set, when it set one;
+// otherwise the main module's version that Go recorded in the binary,
+// a tag such as v0.0.2 or the pseudo-version of an @main build; and
+// "dev" when neither is known. info is what debug.ReadBuildInfo
+// returns, nil when the binary carries none. Go records "(devel)" for
+// a build that has no version, such as one inside a copy of the source
+// without git.
+func buildVersion(linkerVersion string, info *debug.BuildInfo) string {
+	if linkerVersion != "" {
+		return linkerVersion
+	}
+
+	if info == nil || info.Main.Version == "" ||
+		info.Main.Version == "(devel)" {
+		return "dev"
+	}
+
+	return info.Main.Version
+}
 
 func main() {
 	// Once the reader of a stdout pipe has gone, as in "sfdupes report |
@@ -116,7 +139,10 @@ func newRootCommand(stdout, stderr io.Writer) *cobra.Command {
 	var showVersion bool
 
 	printVersion := runE(func(context.Context, []string) error {
-		_, err := fmt.Fprintf(stdout, "sfdupes %s\n", Version)
+		info, _ := debug.ReadBuildInfo()
+
+		_, err := fmt.Fprintf(stdout, "sfdupes %s\n",
+			buildVersion(Version, info))
 		if err != nil {
 			return fmt.Errorf("write stdout: %w", err)
 		}

main_test.go

@@ -9,6 +9,7 @@ import (
 	"io/fs"
 	"os"
 	"path/filepath"
+	"runtime/debug"
 	"slices"
 	"strconv"
 	"strings"
@@ -714,6 +715,8 @@ func TestRunVersion(t *testing.T) {
 
 	// README §Subcommands: the version is one line on stdout, with
 	// nothing on stderr, and exits 0.
+	info, _ := debug.ReadBuildInfo()
+
 	for _, arg := range []string{"--version", "-v"} {
 		var stdout, stderr bytes.Buffer
 
@@ -722,7 +725,7 @@ func TestRunVersion(t *testing.T) {
 			t.Errorf("run(%s) = %d, want %d", arg, code, exitOK)
 		}
 
-		want := "sfdupes " + Version + "\n"
+		want := "sfdupes " + buildVersion(Version, info) + "\n"
 		if got := stdout.String(); got != want {
 			t.Errorf("run(%s) stdout = %q, want %q", arg, got, want)
 		}
@@ -733,6 +736,40 @@ func TestRunVersion(t *testing.T) {
 	}
 }
 
+func TestBuildVersion(t *testing.T) {
+	t.Parallel()
+
+	// README "Install": the linker's version wins; otherwise the main
+	// module's version that Go recorded; "dev" when neither is known.
+	tagged := &debug.BuildInfo{Main: debug.Module{Version: "v0.0.2"}}
+	pseudo := &debug.BuildInfo{Main: debug.Module{
+		Version: "v1.0.0-rc2.0.20261016120000-4e4a94d0c2a1",
+	}}
+	devel := &debug.BuildInfo{Main: debug.Module{Version: "(devel)"}}
+
+	cases := []struct {
+		name   string
+		linker string
+		info   *debug.BuildInfo
+		want   string
+	}{
+		{"linker set", "v0.0.2-3-g4e4a94d", tagged, "v0.0.2-3-g4e4a94d"},
+		{"linker set, no build info", "4e4a94d", nil, "4e4a94d"},
+		{"build info with a tag", "", tagged, "v0.0.2"},
+		{"build info with a pseudo-version", "", pseudo,
+			"v1.0.0-rc2.0.20261016120000-4e4a94d0c2a1"},
+		{"(devel)", "", devel, "dev"},
+		{"no build info", "", nil, "dev"},
+	}
+
+	for _, c := range cases {
+		if got := buildVersion(c.linker, c.info); got != c.want {
+			t.Errorf("%s: buildVersion(%q, ...) = %q, want %q",
+				c.name, c.linker, got, c.want)
+		}
+	}
+}
+
 func TestRunVersionWriteFailureIsFatal(t *testing.T) {
 	t.Parallel()