TIL: $TMPDIR is an environment variable set by macOS (not fish-specific) that points to a per-user temporary directory. Its value looks something like:

/var/folders/xx/xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/T/

The path is unique per user and per boot session — macOS generates it under /var/folders/. Files here are:

• Automatically cleaned up by the system. macOS periodically purges files in /var/folders/ that haven’t been accessed recently (typically after 3 days of inactivity, managed by the periodic daily scripts). A reboot also clears them.
• Not shared across users, since each user gets their own subdirectory.

On Linux, $TMPDIR is often unset; the conventional fallback is /tmp.