Add Set The asdf Package Version For A Single Shell as a unix til

unix/set-the-asdf-package-version-for-a-single-shell.md

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+# Set The asdf Package Version For A Single Shell
+
+Generally when using [`asdf`](https://asdf-vm.com/), I set the global version
+for a package/language to a good up-to-date default. And then I set `local`
+versions for specific projects which get written to the `.tool-versions` file.
+
+There is another option that `asdf` supports for when you want to use a version
+without it being so _sticky_.
+
+```bash
+$ asdf shell postgres 12.3
+
+$ psql --version
+psql (PostgreSQL) 12.3
+
+$ postgres --version
+postgres (PostgreSQL) 12.3
+```
+
+With the
+[`shell`](https://asdf-vm.com/#/core-manage-versions?id=set-current-version)
+command, I tell `asdf` to shim the specified package version just for this
+shell instance. If I switch back to another tab or open a new one, neither of
+them will have been impacted by this shell-constrained setting.
+
+This is handy for a one-off situation where you want to try something at a
+specific version without impact the rest of your terminal session.