Open a terminal. Firstly, double check you have the perl rename, by typing man rename. At the bottom, it should say perl v5.18.2 or similar. (I think Ubuntu always ships the perl rename, but other distros ship other versions.)

If it's the correct version, then cd to the directory that contains these files. Then run

 rename 's/^hortstu/hortstu_/' hortstu[!_]*

Explanation

• This will rename all files that match hortstu[!_]*. This means files that start with hortstu, and do not have _ next.


• It will then rename them by
  
  
  
  • finding ^hortstu (i.e. hortstu at the beginning of the string)
  
  
  • replacing it with hortstu_.
  
  


• N.B. the format for hortstu[!_]* is a bash glob. The format for ^hortstu is a regular expression.