Improve version printing

Always detect the date and git hash at build time. This means they will
appear in the version output even if someone builds from source (e.g.
`go get`).

astro/cli/astro/cmd/cmds.go

@@ -113,16 +113,6 @@ var planCmd = &cobra.Command{
 	},
 }
 
-var versionCmd = &cobra.Command{
-	Use:                   "version",
-	DisableFlagsInUseLine: true,
-	Short:                 "Print astro version",
-	RunE: func(cmd *cobra.Command, args []string) error {
-		fmt.Printf("astro version %s (%s) built %s\n", version, commit, date)
-		return nil
-	},
-}
-
 func userVariables() *astro.UserVariables {
 	values := make(map[string]string)
 	filters := make(map[string]bool)

astro/cli/astro/cmd/main.go

@@ -35,10 +35,6 @@ import (
 )
 
 var (
-	version = "DEV"
-	commit  = "none"
-	date    = "unknown"
-
 	userCfgFile string
 	trace       bool
 	userVars    map[string]string

astro/cli/astro/cmd/version.go

@@ -0,0 +1,65 @@
+package cmd
+
+import (
+	"fmt"
+	"os/exec"
+	"strings"
+	"time"
+
+	"github.com/spf13/cobra"
+)
+
+var (
+	// When a release happens, the value of this variable will be overwritten
+	// by the linker to match the release version.
+	version = "dev"
+	commit  = currentGitHash()
+	date    = currentDate()
+)
+
+// currentGitHash returns the git hash of the current working directory or an
+// empty string if the hash can't be determined (i.e. git is not installed).
+func currentGitHash() string {
+	git := exec.Command("git", "rev-parse", "--short", "HEAD")
+
+	output, err := git.Output()
+	if err != nil {
+		return ""
+	}
+
+	return strings.TrimSpace(string(output))
+}
+
+// currentDate returns the current date in a format suitable for printing in
+// the version output.
+func currentDate() string {
+	return time.Now().Format(time.RFC3339)
+}
+
+var versionCmd = &cobra.Command{
+	Use:                   "version",
+	DisableFlagsInUseLine: true,
+	Short:                 "Print astro version",
+	RunE: func(cmd *cobra.Command, args []string) error {
+		versionString := []string{
+			"astro version",
+			version,
+		}
+
+		if commit != "" {
+			versionString = append(versionString, fmt.Sprintf("(%s)", commit))
+		} else {
+			versionString = append(versionString, "(hash unknown)")
+		}
+
+		versionString = append(versionString, fmt.Sprintf("built %s", date))
+
+		println(strings.Join(versionString, " "))
+
+		return nil
+	},
+}
+
+func init() {
+	rootCmd.AddCommand(versionCmd)
+}