Add a 'target' build flag, for passing a single custom target on the CLI

morph.go

@@ -33,6 +33,7 @@ var (
 	timeout             int
 	askForSudoPasswd    bool
 	nixBuildArg         []string
+	nixBuildTarget      string
 	nixBuildTargetFile  string
 	build               = buildCmd(app.Command("build", "Build machines"))
 	push                = pushCmd(app.Command("push", "Push machines"))
@@ -89,6 +90,11 @@ func nixBuildArgFlag(cmd *kingpin.CmdClause) {
 		StringsVar(&nixBuildArg)
 }
 
+func nixBuildTargetFlag(cmd *kingpin.CmdClause) {
+	cmd.Flag("target", "A Nix lambda defining the build target to use instead of the default").
+		StringVar(&nixBuildTarget)
+}
+
 func nixBuildTargetFileFlag(cmd *kingpin.CmdClause) {
 	cmd.Flag("target-file", "File containing a Nix attribute set, defining build targets to use instead of the default").
 		HintFiles("nix").
@@ -105,6 +111,7 @@ func skipHealthChecksFlag(cmd *kingpin.CmdClause) {
 func buildCmd(cmd *kingpin.CmdClause) *kingpin.CmdClause {
 	selectorFlags(cmd)
 	nixBuildArgFlag(cmd)
+	nixBuildTargetFlag(cmd)
 	nixBuildTargetFileFlag(cmd)
 	deploymentArg(cmd)
 	return cmd
@@ -477,6 +484,8 @@ func buildHosts(hosts []nix.Host) (resultPath string, err error) {
 		if path, err := filepath.Abs(nixBuildTargetFile); err == nil {
 			nixBuildTargets = fmt.Sprintf("import \"%s\"", path)
 		}
+	} else if nixBuildTarget != "" {
+		nixBuildTargets = fmt.Sprintf("{ \"out\" = %s; }", nixBuildTarget)
 	}
 	resultPath, err = nix.BuildMachines(evalMachinesPath, deploymentPath, hosts, nixBuildArg, nixBuildTargets)
 	if err != nil {