Add Set Session-Specific Environment Variables as a tmux til

tmux/set-session-specific-environment-variables.md

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+# Set Session-Specific Environment Variables
+
+`tmux` allows you to manage separate environments for separate projects. For me
+this usually boils down to arrangements of windows and tabs with different
+servers running.
+
+`tmux` can also provide session-specific environment variables. For anything
+that you use environment variables for.
+
+As an example, let's say I have one project that I always edit with VS Code.
+And another that uses `vim`.
+
+My default editor, as configured in my `~/.zshrc` file is `nvim`.
+
+```bash
+❯ echo $EDITOR
+nvim
+```
+
+If I jump into the first project (`one`), I can set the `EDITOR` to `code` like
+so.
+
+```bash
+❯ tmux setenv EDITOR code
+```
+
+It won't apply to the current pane, but if I open a new one.
+
+```
+❯ echo $EDITOR
+code
+```
+
+I can then jump to the other project (`two`) to set that one to `vim`. This
+time using the tmux command prompt.
+
+
+```bash
+<tmux-prefix>:
+:setenv EDITOR vim
+```
+
+Again, if I open a new pane, the editor will be set.
+
+```
+❯ echo $EDITOR
+vim
+```
+
+All the while, the value of `EDITOR` is preserved as `nvim` for everything
+outside the context of those two tmux sessions.