stdpath

In nvim:

:h standard-path

• CONFIG DIRECTORY (DEFAULT)
  $XDG_CONFIG_HOME Nvim: stdpath(“config”)
  Unix: ~/.config ~/.config/nvim
  Windows: ~/AppData/Local ~/AppData/Local/nvim

• DATA DIRECTORY (DEFAULT)
  $XDG_DATA_HOME Nvim: stdpath(“data”)
  Unix: ~/.local/share ~/.local/share/nvim
  Windows: ~/AppData/Local ~/AppData/Local/nvim-data

• RUN DIRECTORY (DEFAULT)
  $XDG_RUNTIME_DIR Nvim: stdpath(“run”)
  Unix: /tmp/nvim.user/xxx /tmp/nvim.user/xxx
  Windows: $TMP/nvim.user/xxx $TMP/nvim.user/xxx

• STATE DIRECTORY (DEFAULT)
  $XDG_STATE_HOME Nvim: stdpath(“state”)
  Unix: ~/.local/state ~/.local/state/nvim
  Windows: ~/AppData/Local ~/AppData/Local/nvim-data

Note: Throughout the user manual these defaults are used as placeholders, e.g.
“~/.config” is understood to mean “$XDG_CONFIG_HOME or ~/.config”.

• LOG FILE $NVIM_LOG_FILE E5430
  Besides ‘debug’ and ‘verbose’, Nvim keeps a general log file for internal
  debugging, plugins and RPC clients.
  :echo $NVIM_LOG_FILE
  By default, the file is located at stdpath(“log”)/log unless that path
  is inaccessible or if $NVIM_LOG_FILE was set before |startup|.