First off, selecting the right file to download can be confusing.
There are a bunch of files at plantuml.com/download
You probably want the GPL version. Download the "Compiled jar" link. There are 3 ;links on that row, you may have accidentally downloaded the source instead of the compiled jar. The link to the jar is, at the moment:
https://github.com/plantuml/plantuml/releases/download/v1.2025.4/plantuml-1.2025.4.jar
So you can download that with
wget https://github.com/plantuml/plantuml/releases/download/v1.2025.4/plantuml-1.2025.4.jar
Next, you say it's an older MacBook Pro, but is it an older Mint? I hope it's not too old -- I was using a Ubuntu 10.04 on a virtual machine recently, that I was not allowed to upgrade (don't ask) and the version of Java on that was not able to run PlantUML.
But assuming you have a suitable java on your mint, then `java -jar plantuml-1.2025.4.jar` should run it in interactive mode. That means a window opens, that you tell to open the directory that has your .txt file(s) in it. You can double-click a file in that window, and it should open the image. When you modify your text file and save it, the image updates. Keep modifying and saving until the image is how you like it.
Or, as you say, `java -jar plantuml-1.2025.4.jar sequenceDiagram.txt` can be used, it compiles the .txt to .png (assuming no syntax errors in the .txt file) and exits. I think for developing a diagram, it can be nicer to use the interactive mode
Actually, if the Mint is not too old you might be able to install plantuml using the package manager, or something like `sudo apt-get install plantuml`. Then you can just launch with the command `plantuml` instead of `java -jar plantuml.jar` (although it might be an older version that you get from the repositories, than you would download).
You should NOT have to use sudo to edit the .txt file (nor to run plantuml). Edit the text file in a directory that you have permission to write in, and just use nano instead of sudo nano. Or, if your ancient MacBook is up to it, get a GUI editor like Geany and use that.
Since you used sudo, the file is probably now owned by root. Check with ls -alh sequenceDiagram.txt. You can correct the ownership, i.e. make yourself the owner, with `chown philip:philip sequenceDiagram.txt` (assuming your username is philip) and then you should be able to edit it (assuming you are working in some directory inside your home directory, which you should be).
Frank
