Stop using shell just to run commands

There is no need to use shell here, and we should not assume that user's
zshrc configures the PATH correctly. Instead, we should use the
`Process` API to run commands directly, and assume that the PATH in the
process spawn environment is correctly configured.

Sources/WebIDLToSwift/IDLParser.swift

@@ -8,8 +8,8 @@ enum IDLParser {
         let pipe = Pipe()
         task.standardOutput = pipe
         task.standardError = FileHandle.standardError
-        task.arguments = ["-c", "npm start --silent \(modules.joined(separator: " "))"]
-        task.launchPath = "/bin/zsh"
+        task.arguments = ["start", "--silent"] + modules
+        task.launchPath = findExecutable("npm")
         task.currentDirectoryURL = URL(fileURLWithPath: #file)
             .deletingLastPathComponent()
             .appendingPathComponent("../../parse-idl")

Sources/WebIDLToSwift/Shell.swift

@@ -3,7 +3,7 @@ import Foundation
 enum Shell {
     static func format(source: String) {
         print("Formatting generated Swift files...")
-        run(script: "swiftformat --swiftversion 5.5 \(source)")
+        run(executable: "swiftformat", arguments: ["--swiftversion", "5.5", source])
     }
 
     private static let projectRoot = URL(fileURLWithPath: #file)
@@ -18,22 +18,36 @@ enum Shell {
         let patchFile = patchFolder.appendingPathComponent(module.swiftModule).appendingPathExtension("patch").path
         if FileManager.default.fileExists(atPath: patchFile) {
             print("Patching generated Swift files...")
-            run(script: "git apply \(patchFile)")
+            run(executable: "git", arguments: ["apply", patchFile])
         }
     }
 
-    private static func run(script: String) {
+    private static func run(executable: String, arguments: [String]) {
         // print("*** running script: \(script)")
         let task = Process()
         task.standardError = FileHandle.standardError
-        task.arguments = ["-c", script]
-        task.launchPath = "/bin/zsh"
+        task.arguments = arguments
+        task.launchPath = findExecutable(executable)
         task.currentDirectoryURL = projectRoot
         task.launch()
         task.waitUntilExit()
         if task.terminationStatus != 0 {
-            print("Error: \(script) failed with exit code \(task.terminationStatus)")
+            print("Error: \(([executable] + arguments).map { "\"\($0)\"" }.joined(separator: " ")) failed with exit code \(task.terminationStatus)")
             exit(task.terminationStatus)
         }
     }
 }
+
+func findExecutable(_ name: String) -> String? {
+    guard let path = ProcessInfo.processInfo.environment["PATH"] else {
+        return nil
+    }
+    let paths = path.split(separator: ":")
+    for p in paths {
+        let fullPath = URL(fileURLWithPath: String(p)).appendingPathComponent(name).path
+        if FileManager.default.isExecutableFile(atPath: fullPath) {
+            return fullPath
+        }
+    }
+    return nil
+}