There are many alternatives to MyTetra for Linux if you are looking for a replacement. The best Linux alternative is Joplin. It's not free, so if you're looking for a free alternative, you could try Standard Notes or CherryTree. If that doesn't suit you, our users have ranked more than 100 alternatives to MyTetra and loads of them are available for Linux so hopefully you can find a suitable replacement. Other interesting Linux alternatives to MyTetra are Zim (Free, Open Source), TiddlyWiki (Free, Open Source), Trilium Notes (Free, Open Source) and Zoho Notebook (Free Personal).
Joplin is a free, open source note taking and to-do application, which can handle a large number of notes organised into notebooks. The notes are searchable, can be copied, tagged and modified either from the applications directly or from your own text editor.
RedNotebook is a graphical diary and journal helping you keep track of notes and thoughts. It includes a calendar navigation, customizable templates, export functionality and word clouds. You can also format, tag and search your entries.