Use command checks if being run as root

Package.swift

@@ -19,6 +19,10 @@ let package = Package(
     ],
     dependencies: [
         // Dependencies declare other packages that this package depends on.
+        .package(
+            url: "https://github.com/onevcat/Rainbow",
+            from: "4.0.0"
+        ),
         .package(
             url: "https://github.com/apple/swift-argument-parser",
             from: "1.5.0"
@@ -36,6 +40,7 @@ let package = Package(
             dependencies: [
                 .product(name: "ArgumentParser", package: "swift-argument-parser"),
                 .product(name: "libunxip", package: "unxip"),
+                .product(name: "Rainbow", package: "Rainbow"),
                 .target(name: "TableKit")
             ]
         ),

Sources/XcodeVersionManager/Commands/UseCommand.swift

@@ -1,5 +1,6 @@
 import ArgumentParser
 import Foundation
+import Rainbow
 
 struct UseCommand: AsyncParsableCommand {
     static let configuration = CommandConfiguration(
@@ -22,7 +23,21 @@ struct UseCommand: AsyncParsableCommand {
         let xcodeVersion = formatter.string(from: xcode)
         print("Switching to \(xcodeVersion)")
 
-        try xcode.use(permissions: xcodeSelectPermissions)
+        do {
+            try xcode.use(permissions: xcodeSelectPermissions)
+        } catch let .sudoRequired(command) {
+            var standardError = StandardErrorOutputStream()
+            print(
+                """
+                Requires superuser permissions to be run.
+                Manually run the following or see `xcvm use --help` for more details.
+                """.red,
+                to: &standardError
+            )
+
+            print(command)
+            throw ExitCode(1)
+        }
     }
 }
 
@@ -36,12 +51,19 @@ extension XcodeApplication.XcodeSelectPermissions: EnumerableFlag {
         case .inherit:
             ArgumentHelp(
                 "xcode-select will be run with same permissions xcvm has.",
-                discussion: #"Calling "sudo xcvm ..." may be required if you do not already have superuser permissions."#
+                discussion: """
+                Use one of the following based on you preference for granting superuser permissions.
+                  - Copy the output commands and paste them into your terminal.
+                  - `xcvm ... | bash` or `eval "$(xcvm ...)"`
+                    Automatically evaluate the output commands. 
+                  - `sudo xcvm ...`
+                    Give xcvm and its subprocesses superuser permissions.
+                """
             )
 
         case .sudoAskpass:
             ArgumentHelp(
-                #"xcvm will call xcode-select with "sudo --askpass ..."."#,
+                #"xcvm will call xcode-select with `sudo --askpass ...`."#,
                 discussion: """
                 The behaviour will be based on the system config.
                   1. If xcode-select has been added to sudoers with NOPASSWD, it won't need to prompt the user.

Sources/XcodeVersionManager/Models/XcodeApplication+Use.swift

@@ -0,0 +1,97 @@
+import Foundation
+import RegexBuilder
+
+extension XcodeApplication {
+    enum XcodeSelectPermissions {
+        case inherit
+        case sudoAskpass
+    }
+
+    func use(permissions: XcodeSelectPermissions) throws(UseError) {
+        let xcodePath = url.absoluteURL.path
+
+        switch permissions {
+        case .inherit:
+            // Get the current user id to determine if being run as root
+            let userId = try execute(
+                "/usr/bin/id",
+                arguments: ["-u"]
+            )
+
+            let command = "/usr/bin/xcode-select"
+            let arguments = [
+                "--switch",
+                xcodePath
+            ]
+
+            // Is root user
+            if String(decoding: userId, as: UTF8.self) == "0" {
+                // Works if xcvm is run with superuser permissions
+                try execute(command, arguments: arguments)
+            } else {
+                var command = "/usr/bin/sudo \(command)"
+                for argument in arguments {
+                    let argument = if argument.contains(.whitespace) {
+                        "'\(argument)'"
+                    } else {
+                        argument
+                    }
+
+                    command += " \(argument)"
+                }
+
+                throw .sudoRequired(command: command)
+            }
+        case .sudoAskpass:
+            // Works if
+            //   xcode-select has NOPASSWD set in sudoers
+            //   touchid is enabled for sudo
+            //   SUDO_ASKPASS environment variable is set with a helper program
+            // otherwise fails to prompt for password
+            try execute(
+                "/usr/bin/sudo",
+                arguments: [
+                    "--askpass",
+                    "/usr/bin/xcode-select",
+                    "--switch",
+                    xcodePath
+                ]
+            )
+        }
+
+        // Register Xcode so plugins work correctly.
+        // https://nshipster.com/xcode-source-extensions/#using-pluginkit
+        try execute(
+            "/System/Library/Frameworks/CoreServices.framework/Frameworks/LaunchServices.framework/Support/lsregister",
+            arguments: [
+                "-f",
+                xcodePath
+            ]
+        )
+    }
+
+    @discardableResult
+    private func execute(_ command: String, arguments: [String]) throws(UseError) -> Data {
+        do {
+            return try Process.execute(command, arguments: arguments)
+        } catch {
+            throw .other(error)
+        }
+    }
+}
+
+extension XcodeApplication {
+    enum UseError: LocalizedError {
+        case sudoRequired(command: String)
+        case other(Error)
+
+        var errorDescription: String? {
+            switch self {
+            case let .sudoRequired(command):
+                return command
+            case let .other(error):
+                return String(describing: error)
+            }
+        }
+    }
+}

Sources/XcodeVersionManager/Models/XcodeApplication.swift

@@ -16,55 +16,6 @@ struct XcodeApplication: Encodable {
     }
 }
 
-// MARK: - Use
-extension XcodeApplication {
-    enum XcodeSelectPermissions {
-        case inherit
-        case sudoAskpass
-    }
-
-    func use(permissions: XcodeSelectPermissions) throws {
-        let xcodePath = url.absoluteURL.path
-
-        switch permissions {
-        case .inherit:
-            // Works if xcvm is run with superuser permissions
-            try Process.execute(
-                "/usr/bin/xcode-select",
-                arguments: [
-                    "--switch",
-                    xcodePath
-                ]
-            )
-        case .sudoAskpass:
-            // Works if
-            //   xcode-select has NOPASSWD set in sudoers
-            //   touchid is enabled for sudo
-            //   SUDO_ASKPASS environment variable is set with a helper program
-            // otherwise fails to prompt for password
-            try Process.execute(
-                "/usr/bin/sudo",
-                arguments: [
-                    "--askpass",
-                    "/usr/bin/xcode-select",
-                    "--switch",
-                    xcodePath
-                ]
-            )
-        }
-
-        // Register Xcode so plugins work correctly.
-        // https://nshipster.com/xcode-source-extensions/#using-pluginkit
-        try Process.execute(
-            "/System/Library/Frameworks/CoreServices.framework/Frameworks/LaunchServices.framework/Support/lsregister",
-            arguments: [
-                "-f",
-                xcodePath
-            ]
-        )
-    }
-}
-
 // MARK: - Comparable
 extension XcodeApplication: Comparable {
     static func < (lhs: XcodeApplication, rhs: XcodeApplication) -> Bool {

Sources/XcodeVersionManager/Utilities/StandardErrorOutputStream.swift

@@ -0,0 +1,10 @@
+import Foundation
+
+public struct StandardErrorOutputStream: TextOutputStream {
+    public init() {}
+
+    public func write(_ string: String) {
+        let data = Data(string.utf8)
+        try? FileHandle.standardError.write(contentsOf: data)
+    }
+}