add login shell flag (#4175)

crates/nu-cli/src/app.rs

@@ -513,11 +513,6 @@ mod tests {
             let args = format!("nu --loglevel={}", level);
             ui.parse(&args)?;
             assert_eq!(ui.loglevel().unwrap(), Ok(level.to_string()));
-
-            let ui = cli_app();
-            let args = format!("nu -l {}", level);
-            ui.parse(&args)?;
-            assert_eq!(ui.loglevel().unwrap(), Ok(level.to_string()));
         }
 
         let ui = cli_app();
@@ -530,6 +525,17 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn can_be_login() -> Result<(), ShellError> {
+        let ui = cli_app();
+        ui.parse("nu -l")?;
+
+        let ui = cli_app();
+        ui.parse("nu --login")?;
+
+        Ok(())
+    }
+
     #[test]
     fn can_be_passed_nu_scripts() -> Result<(), ShellError> {
         let ui = cli_app();

crates/nu-command/src/commands/core_commands/nu_signature.rs

@@ -15,6 +15,7 @@ impl WholeStreamCommand for Command {
             .switch("skip-plugins", "do not load plugins", None)
             .switch("no-history", "don't save history", None)
             .switch("perf", "show startup performance metrics", None)
+            .switch("login", "start Nu as if it was a login shell", Some('l'))
             .named(
                 "commands",
                 SyntaxShape::String,
@@ -33,7 +34,7 @@ impl WholeStreamCommand for Command {
                 "loglevel",
                 SyntaxShape::String,
                 "LEVEL: error, warn, info, debug, trace",
-                Some('l'),
+                None,
             )
             .named(
                 "config-file",