Is there a renaming tool (for Ubuntu 18.04 Mate) which is able to work on folder names as well?

I am rather new to Linux, I am using Ubuntu 18.04 Mate. When I need to do more complex file renaming tasks I prefer to use KRename (the GUI version that got installed with apt-get). But often I'd also like to rename folders' names (this question is NOT about batch renaming of file names). It seems KRename (GUI) is not capable of doing that. So I wonder whether there's a tool to achieve that (as a beginner I prefer not to write scripts).

rename can rename both files and directories

A command line tool is suitable for this kind of task. I can show how it works with an example:

Make two test directories

$ mkdir rentest1
$ mkdir rentest2

Make a test file

$ touch rentest3

Check that rename does what you expect with a 'dry run' using the option -n

$ rename -n 's/ren/rename-/' rentest*
rename(rentest1, rename-test1)
rename(rentest2, rename-test2)
rename(rentest3, rename-test3)

When it looks good, remove -n and do it

$ rename 's/ren/rename-/' rentest*

Check that it worked for both the file and the directories

$ ls -ld rename*
drwxrwxr-x 2 sudodus sudodus 4096 aug 11 13:47 rename-test1
drwxrwxr-x 2 sudodus sudodus 4096 aug 11 13:47 rename-test2
-rw-rw-r-- 1 sudodus sudodus    0 aug 11 13:48 rename-test3

Learn about all the features in

man rename

My version of rename is installed via the package rename from the repository universe, and it is a Perl script by Larry Wall.

There are quite a few options, both via command line or GUI. Which one suits your needs may depend on the patterns you want to use for renaming.

I am only citing explicitly here one GUI option (since you used KRename), which is thunar -B

You could check other alternatives in sources below.

if using CLI, you could always "wrap" your command with for d in $(find . -type d <other options>) ; do <your operations on "${d}", including renaming> ; done.

Sources:

1. https://www.makeuseof.com/batch-rename-files-in-linux/
2. https://ostechnix.com/how-to-rename-multiple-files-at-once-in-linux/