diff --git a/toolflow/build.sbt b/toolflow/build.sbt
index c3f0ab2..99cf425 100644
--- a/toolflow/build.sbt
+++ b/toolflow/build.sbt
@@ -11,5 +11,7 @@ lazy val root = project
 
     libraryDependencies += "org.rogach" %% "scallop" % "5.2.0",
     libraryDependencies += "com.lihaoyi" %% "os-lib" % "0.11.5",
+    libraryDependencies += "com.typesafe.scala-logging" %% "scala-logging" % "3.9.6",
+    libraryDependencies += "ch.qos.logback" % "logback-classic" % "1.5.19",
     libraryDependencies += "org.scalameta" %% "munit" % "1.2.0" % Test
   )
diff --git a/toolflow/src/main/scala/com/minres/tgc/hammer/FileUtils.scala b/toolflow/src/main/scala/com/minres/tgc/hammer/FileUtils.scala
index 237f2dd..26b496f 100644
--- a/toolflow/src/main/scala/com/minres/tgc/hammer/FileUtils.scala
+++ b/toolflow/src/main/scala/com/minres/tgc/hammer/FileUtils.scala
@@ -1,6 +1,6 @@
 package com.minres.tgc.hammer
 
-import org.rogach.scallop.{ValueConverter, listArgConverter, singleArgConverter}
+import org.rogach.scallop.{ScallopOption, Util, ValueConverter, listArgConverter, singleArgConverter}
 import os.*
 import Global.*
 
diff --git a/toolflow/src/main/scala/com/minres/tgc/hammer/Main.scala b/toolflow/src/main/scala/com/minres/tgc/hammer/Main.scala
index e866a8d..6d4392a 100644
--- a/toolflow/src/main/scala/com/minres/tgc/hammer/Main.scala
+++ b/toolflow/src/main/scala/com/minres/tgc/hammer/Main.scala
@@ -1,6 +1,9 @@
 package com.minres.tgc.hammer
 
+import ch.qos.logback.classic.Level
 import com.minres.tgc.hammer.cli.{HammerConf, MySubcommand}
+import com.typesafe.scalalogging.Logger
+import org.slf4j.LoggerFactory
 
 import scala.compiletime.uninitialized
 
@@ -8,8 +11,19 @@ object Main {
   var conf: HammerConf = uninitialized
 
   def main(args: Array[String]): Unit = {
+    var logger = Logger("TGCHammer")
     conf = new HammerConf(args.toIndexedSeq)
 
+    val logLevel = conf.verbose() match {
+      case 0 => Level.WARN
+      case 1 => Level.INFO
+      case 2 => Level.DEBUG
+      case _ => Level.TRACE
+    }
+    LoggerFactory.getLogger(org.slf4j.Logger.ROOT_LOGGER_NAME).asInstanceOf[ch.qos.logback.classic.Logger].setLevel(logLevel)
+
+    os.makeDir(Global.OUT_DIR)
+    os.makeDir(Global.TMP_DIR)
     conf.subcommand match {
       case Some(c: MySubcommand) =>
         val tasks = c.getRequiredTasks
diff --git a/toolflow/src/main/scala/com/minres/tgc/hammer/cli/HammerConf.scala b/toolflow/src/main/scala/com/minres/tgc/hammer/cli/HammerConf.scala
index 7d99bd7..6ab7eb2 100644
--- a/toolflow/src/main/scala/com/minres/tgc/hammer/cli/HammerConf.scala
+++ b/toolflow/src/main/scala/com/minres/tgc/hammer/cli/HammerConf.scala
@@ -6,9 +6,12 @@ import com.minres.tgc.hammer.FileUtils.*
 
 class HammerConf(arguments: Seq[String]) extends ScallopConf(arguments) {
   val outputDirectory: ScallopOption[Path] = opt[Path](default = Some("output".path()), descr = "The base output directory")
+  val verbose: ScallopOption[Int] = tally()
   addSubcommand(new LongnailCommand)
   addSubcommand(new TreenailCommand)
   addSubcommand(new LongnailSchedCommand)
 
+  requireSubcommand()
+
   verify()
 }
diff --git a/toolflow/src/main/scala/com/minres/tgc/hammer/cli/LongnailMergeCommand.scala b/toolflow/src/main/scala/com/minres/tgc/hammer/cli/LongnailMergeCommand.scala
index 9ba9498..ea16913 100644
--- a/toolflow/src/main/scala/com/minres/tgc/hammer/cli/LongnailMergeCommand.scala
+++ b/toolflow/src/main/scala/com/minres/tgc/hammer/cli/LongnailMergeCommand.scala
@@ -10,7 +10,10 @@ import com.minres.tgc.hammer.tasks.longnail.LongnailMergeTask
 class LongnailMergeCommand extends MySubcommand("mergeISAX") {
   val inputFiles: ScallopOption[List[Path]] = trailArg[List[Path]]("inputFiles", group = mainGroup, descr = "One or multiple input files; Both .core_desc as well as .mlir are supported (also mixed)")
   val output: ScallopOption[Path] = opt[Path](short = 'o', default = Some("merged.mlir".path()), descr = "The .mlir file containing the merged ISAXes")
+  val overwrite: ScallopOption[Boolean] = toggle()
 
+  check(inputFiles)(checkPathsIsFile)
+  checkNotIf(overwrite)(output)(checkPathDoesNotExist)
 
   override def getRequiredTasks: Seq[Task] = {
     val (coreDSLFiles, mlirFiles) = inputFiles().partition(_.ext == "core_desc")
diff --git a/toolflow/src/main/scala/com/minres/tgc/hammer/cli/MySubcommand.scala b/toolflow/src/main/scala/com/minres/tgc/hammer/cli/MySubcommand.scala
index 20b05f8..19c60a8 100644
--- a/toolflow/src/main/scala/com/minres/tgc/hammer/cli/MySubcommand.scala
+++ b/toolflow/src/main/scala/com/minres/tgc/hammer/cli/MySubcommand.scala
@@ -4,7 +4,7 @@ import com.minres.tgc.hammer.options.ConfigElement
 import com.minres.tgc.hammer.tasks.Task
 import org.rogach.scallop.{ScallopOption, ScallopOptionGroup, Subcommand, Util}
 
-import java.nio.file.Files
+type Checker[T] = ScallopOption[T] => Either[String,Unit]
 
 abstract class MySubcommand(name: String) extends Subcommand(name) {
   protected val mainGroup: ScallopOptionGroup = group()
@@ -16,12 +16,42 @@ abstract class MySubcommand(name: String) extends Subcommand(name) {
     elem
   }
 
-  def validateOSPathIsDirectory(pathOption: ScallopOption[os.Path]): Unit = addValidation {
-    pathOption.toOption
-      .map {
-        case path if Files.isDirectory(path.toNIO) => Right(())
-        case path => Left(Util.format("File '%s' is not a directory", path))
-      }
-      .getOrElse(Right(()))
+  def checkPred[T](option: ScallopOption[T], predicate: T => Boolean, format: String): Either[String,Unit] = {
+    option.toOption.map {
+      case o if predicate(o) => (Right(()))
+      case o => Left(Util.format(format, o))
+    }.getOrElse(Right(()))
   }
+
+  def checkPredIt[T, CC <: Iterable](option: ScallopOption[CC[T]], predicate: T => Boolean, format: String): Either[String,Unit] = {
+    option.toOption.map(options => {
+      val problems = options.filterNot(predicate)
+      problems match {
+        case Nil => Right(())
+        case problem :: Nil => Left(Util.format(format, problem))
+        case _ => Left(s"Multiple: ${Util.format(format, problems.mkString(", "))}")
+      }
+    }).getOrElse(Right(()))
+  }
+
+  def checkPathExists: Checker[os.Path] = path => checkPred(path, os.exists, "File '%s' does not exist")
+  def checkPathDoesNotExist: Checker[os.Path] = path => checkPred(path, !os.exists(_), "File '%s' already exists")
+  def checkPathIsFile: Checker[os.Path] = path => checkPred(path, os.isFile, "File '%s' is not a file")
+  def checkPathIsDir: Checker[os.Path] = path => checkPred(path, os.isDir, "File '%s' is not a directory")
+
+  
+  def checkPathsExists: Checker[List[os.Path]] = path => checkPredIt(path, os.exists, "File(s) '%s' do not exist")
+  def checkPathsDoesNotExists: Checker[List[os.Path]] = path => checkPredIt(path, !os.exists(_), "File(s) '%s' already exist")
+  def checkPathsIsFile: Checker[List[os.Path]] = path => checkPredIt(path, os.isFile, "File(s) '%s' are not files")
+  def checkPathsIsDir: Checker[List[os.Path]] = path => checkPredIt(path, os.isDir, "File(s) '%s' are not directories")
+
+  def check[T](option: ScallopOption[T])(check: Checker[T]): Unit = addValidation(check(option))
+  def checkPred[T, U](condOpt: ScallopOption[U], cond: U => Boolean)(option: ScallopOption[T])(check: Checker[T]): Unit = addValidation {
+    condOpt.toOption match {
+      case Some(value) if cond(value) => check(option)
+      case _ => Right(())
+    }
+  }
+  def checkIf[T](condOpt: ScallopOption[Boolean])(option: ScallopOption[T])(check: Checker[T]): Unit = checkPred(condOpt, identity)(option)(check)
+  def checkNotIf[T](condOpt: ScallopOption[Boolean])(option: ScallopOption[T])(check: Checker[T]): Unit = checkPred(condOpt, !_)(option)(check)
 }
diff --git a/toolflow/src/main/scala/com/minres/tgc/hammer/cli/TreenailCommand.scala b/toolflow/src/main/scala/com/minres/tgc/hammer/cli/TreenailCommand.scala
index e2c0a90..2879e33 100644
--- a/toolflow/src/main/scala/com/minres/tgc/hammer/cli/TreenailCommand.scala
+++ b/toolflow/src/main/scala/com/minres/tgc/hammer/cli/TreenailCommand.scala
@@ -8,6 +8,10 @@ import os.Path
 class TreenailCommand extends MySubcommand("parseCoreDSL") {
   val coreDSL: ScallopOption[Path] = trailArg[Path]("coreDSL", descr = "The .core_desc input file to be converted")
   val output: ScallopOption[Path] = opt[Path](short = 'o', default = Some("isax.mlir".path()), descr = "The .mlir file containing the converted ISAX")
+  val overwrite: ScallopOption[Boolean] = toggle()
+
+  check(coreDSL)(checkPathIsFile)
+  checkNotIf(overwrite)(output)(checkPathDoesNotExist)
 
   banner(
     """Parse a CoreDSL input using Treenail into the MLIR representation