Check args better

scripts/devshell/prettify

@@ -25,11 +25,12 @@ run_format() {
               fourmolu -q -i "$file"
               stylish-haskell -i "$file"
             fi
-
         fi
     done
 }
 
+flag_passed="true"
+
 # Parse command line arguments
 case $1 in
     -t|--tracked)
@@ -50,9 +51,28 @@ case $1 in
         ;;
     *)
         files="$@"
+        flag_passed="false"
         ;;
 esac
 
+if [[ $flag_passed == "true" ]] && [[ $# -gt 1 ]]; then
+  echo "ERROR: only one flag is allowed!"
+  echo -e
+  show_help
+  exit 1
+fi
+
+for file in $files; do
+  if [[ ! -a $file ]]; then
+    echo "ERROR: $file does not exist"
+    exit 1
+    if ![[ -f $file ]]; then
+      echo "ERROR: $file is not a regular file"
+      exit
+    fi
+  fi
+done
+
 for tool in stylish-haskell fourmolu
 do
   if !(which $tool > /dev/null 2>&1); then
@@ -64,7 +84,6 @@ do
   fi
 done
 
-
 if [[ -z $files ]]; then
     echo "No files to format!"
     if [[ -z $1 ]]; then