add --get-login command to podman-login.

Returns user if user is logged-in to the registry.  Returns error
if not logged in with non-zero status code.

Signed-off-by: Theodore Cowan <theodore-cowan@pluralsight.com>

cmd/podman/login.go

@@ -33,6 +33,10 @@ var (
 			Name:  "cert-dir",
 			Usage: "Pathname of a directory containing TLS certificates and keys used to connect to the registry",
 		},
+		cli.BoolTFlag{
+			Name:  "get-login",
+			Usage: "Return the current login user for the registry",
+		},
 		cli.BoolTFlag{
 			Name:  "tls-verify",
 			Usage: "Require HTTPS and verify certificates when contacting registries (default: true)",
@@ -65,6 +69,21 @@ func loginCmd(c *cli.Context) error {
 
 	sc := common.GetSystemContext("", authfile, false)
 
+	if c.IsSet("get-login") {
+		user, err := config.GetUserLoggedIn(sc, server)
+
+		if err != nil {
+			return errors.Wrapf(err, "unable to check for login user")
+		}
+
+		if user == "" {
+			return errors.Errorf("not logged into %s", server)
+		}
+
+		fmt.Printf("%s\n", user)
+		return nil
+	}
+
 	// username of user logged in to server (if one exists)
 	userFromAuthFile, passFromAuthFile, err := config.GetAuthentication(sc, server)
 	if err != nil {

completions/bash/podman

@@ -2217,6 +2217,7 @@ _podman_login() {
      --password
      -p
      --authfile
+     --get-login
      "
      local boolean_options="
 	  --help

docs/podman-login.1.md

@@ -36,6 +36,10 @@ Path of the authentication file. Default is ${XDG_\RUNTIME\_DIR}/containers/auth
 Note: You can also override the default path of the authentication file by setting the REGISTRY\_AUTH\_FILE
 environment variable. `export REGISTRY_AUTH_FILE=path`
 
+**--get-login**
+
+Return the logged-in user for the registry.  Return error if no login is found.
+
 **--cert-dir** *path*
 
 Use certificates at *path* (\*.crt, \*.cert, \*.key) to connect to the registry.