Don't create a user if one with user ID exists

Programs that do things like getting the home dir end up getting the home dir of the other user by accident.

Use an existing user if the user ID and group ID match. Error out if the group ID matches, or if there are multiple
users with the same user ID.

runas.sh

@@ -42,9 +42,37 @@ function create_user {
   USER="user_${USER_ID}_$GROUP_ID"
   GROUP="group_${USER_ID}_$GROUP_ID"
 
+  if grep -q '^[^:]*:[^:]*:99:100:' /etc/passwd >/dev/null 2>&1
+  then
+    USER=$(grep '^[^:]*:[^:]*:99:100:' /etc/passwd | sed 's/:.*//')
+
+    if [[ $USER == *$'\n'* ]]
+    then
+      echo "$(ts) ERROR: Found multiple users with the proper user ID and group ID. Exiting..."
+      exit 1
+    fi
+
+    echo "$(ts) Found existing user \"$USER\" with the proper user ID and group ID. Skipping creation of user and group..."
+    return
+  fi
+
+  if grep -q '^[^:]*:[^:]*:99:' /etc/passwd >/dev/null 2>&1
+  then
+    USER=$(grep '^[^:]*:[^:]*:99:100:' /etc/passwd | sed 's/:.*//')
+
+    if [[ $USER == *$'\n'* ]]
+    then
+      echo "$(ts) ERROR: Found multiple users with the proper user ID and incorrect group ID. Refusing to modify the group ID. Exiting..."
+    else
+      echo "$(ts) ERROR: Found user \"$USER\" with the proper user ID but incorrect group ID. Refusing to modify the group ID. Exiting..."
+    fi
+
+    exit 1
+  fi
+
   if id -u $USER >/dev/null 2>&1
   then
-    echo "$(ts) User \"$USER\" already exists. Skipping creation of user and group..."
+    echo "$(ts) User \"$USER\" already exists. Skipping creation of new user and group..."
     return
   fi