Linux

I have a few possible options but they all rely on other things you might not have or want.

There's a fillable spreadsheet in g∞gle sheets but that requires having an account with the company and to fill it out, you make a copy from the file menu and go from there. It's best to make the copy for your account and leave it blank, making extra copies of that one, in case my link stops working someday (it's been good for at least 3 years so far). Here's the link: https://docs.google.com/spreadsheets/d/1DrSwhSxJdEfO7gAih4UHcd3myJfNh2eW6Kxv0UblAO4/edit?gid=1270711241#gid=1270711241

If you use Discord, there's avrae, a bot that integrates with discord for online d&d sessions. https://avrae.io/

it can be used offline as well but regular pencil and paper would be more convenient in my opinion. it can read character sheets from online sources, including ones made from the spreadsheet link i shared previously, as well as others, the suggestions they give are https://dicecloud.com/ and https://www.dndbeyond.com/ although the latter i found frustrating to use for reasons i cannot recall.

Another website worth checking out is https://www.flapkan.com/ for pdf form fillable character sheets. i have not used this, it was in a folder of bookmarks i have and i forgot about it. it looks promising.

as for linux native programs unfortunately i cannot help, i do not know of anything specific. i've used a few different programs but none were strictly linux based and most used proprietary code so they wouldn't be FLOSS. like for example, I used yEd https://www.yworks.com/products/yed which works on Linux but uses java stuff. I liked it because I could set links to pdf files within the character sheets i'd made, which would open to specific pages of the rules related to what was linked, like rules for spells, how abilities worked and weapon stats and all that.