New chapter page collecting the list of special variables (#1860)

* New chapter page collecting the list of special variables

* Added PROCESS_PATH

Author: NotTheDr01ds

.vuepress/configs/sidebar/en.ts

@@ -38,6 +38,7 @@ export const sidebarEn: SidebarConfig = {
         '/book/working_with_records.md',
         '/book/working_with_tables.md',
         '/book/navigating_structured_data.md',
+        '/book/special_variables.md',
       ],
     },
     {

book/special_variables.md

@@ -0,0 +1,166 @@
+# Special Variables
+
+Nushell makes available and uses a number of special variables and constants. Many of these are mentioned or documented in other places in this Book, but this page
+should include _all_ variables for reference.
+
+[[toc]]
+
+## `$nu`
+
+The `$nu` constant is a record containing several useful values:
+
+- `default-config-dir`: The directory where the configuration files are stored and read.
+- `config-path`: The path of the main Nushell config file, normally `config.nu` in the config directory.
+- `env-path`: The optional environment config file, normally `env.nu` in the config directory.
+- `history-path`: The text or SQLite file storing the command history.
+- `loginshell-path`: The optional config file which runs for login shells, normally `login.nu` in the config directory.
+- `plugin-path`: The plugin registry file, normally `plugin.msgpackz` in the config directory.
+- `home-path`: The user's home directory which can be accessed using the shorthand `~`.
+- `data-dir`: The data directory for Nushell, which includes the `./vendor/autoload` directories loaded at startup and other user data.
+- `cache-dir`: A directory for non-essential (cached) data.
+- `vendor-autoload-dirs`: A list of directories where third-party applications should install configuration files that will be auto-loaded during startup.
+- `user-autoload-dirs`: A list of directories where the user may create additional configuration files which will be auto-loaded during startup.
+- `temp-path`: A path for temporary files that should be writeable by the user.
+- `pid`: The PID of the currently running Nushell process.
+- `os-info`: Information about the host operating system.
+- `startup-time`: The amount of time (in duration) that it took for Nushell to start and process all configuration files.
+- `is-interactive`: A boolean indicating whether Nushell was started as an interactive shell (`true`) or is running a script or command-string. For example:
+
+  ```nu
+  $nu.is-interactive
+  # => true
+  nu -c "$nu.is-interactive"
+  # => false
+
+  # Force interactive with --interactive (-i)
+  nu -i -c "$nu.is-interactive"
+  # => true
+  ```
+
+  Note: When started as an interactive shell, startup config files are processed. When started as a non-interactive shell, no config files are read unless explicitly called via flag.
+
+- `is-login`: Indicates whether or not Nushell was started as a login shell.
+- `history-enabled`: History may be disabled via `nu --no-history`, in which case this constant will be `false`.
+- `current-exe`: The full path to the currently-running `nu` binary. Can be combined with `path dirname` (which is constant) to determine the directory where the binary is located.
+
+## `$env`
+
+`$env` is a special mutable variable containing the current environment variables. As with any process, the initial environment is inherited from the parent process which started `nu`.
+
+There are also several environment variables that Nushell uses for specific purposes:
+
+### `$env.config`
+
+`$env.config` is the main configuration record used in Nushell. Settings are documented in `config nu --doc`.
+
+### `$env.PATH`
+
+The search path for executing other applications. It is initially inherited from the parent process as a string, but converted to a Nushell `list` at startup for easy access.
+
+It is converted back to a string before running a child-process.
+
+### `$env.ENV_CONVERSIONS`
+
+Allows users to specify how to convert certain environment variables to Nushell types. See [ENV_CONVERSIONS](./configuration.md#env_conversions).
+
+### `$env.LAST_EXIT_CODE`
+
+The exit code of the last command, usually used for external commands — Equivalent to `$?` from POSIX. Note that this information is also made available to the `catch` block in a `try` expression for external commands. For instance:
+
+```nu
+^ls file-that-does-not-exist e> /dev/null
+$env.LAST_EXIT_CODE
+# => 2
+
+# or
+try {
+  ^ls file-that-does-not-exist e> /dev/null
+} catch {|e|
+  print $e.exit_code
+}
+# => 2
+```
+
+### `env.CMD_DURATION_MS`
+
+The amount of time in milliseconds that the previous command took to run.
+
+### `$env.NU_VERSION`
+
+The current Nushell version. The same as `(version).version`, but, as an environment variable, it is exported to and can be read by child processes.
+
+### `$env.CURRENT_FILE`
+
+Inside a script, module, or sourced-file, this variable holds the fully-qualified filename. Note that this
+information is also available as a constant through the [`path self`](/commands/docs/path_self.md) command.
+
+### `$env.FILE_PWD`
+
+Inside a script, module, or sourced-file, this variable holds the fully qualified name of the directory in which
+the file resides. Note that this value is also available as a constant through:
+
+```nu
+path self | path dirname
+```
+
+### `$env.PROCESS_PATH`
+
+When _executing a script_, this variable represents the name and relative path of the script. Unlike the two variables
+above, it is not present when sourcing a file or importing a module.
+
+Note: Also unlike the two variables above, the exact path (including symlinks) that was used to _invoke_ the file is returned.
+
+### `$env.NU_LIB_DIRS`
+
+A list of directories which will be searched when using the `source`, `use`, or `overlay use` commands. See also:
+
+- The `$NU_LIB_DIRS` constant below
+- [Module Path](./modules/using_modules.md#module-path)
+- [Configuration - `$NU_LIB_DIRS`](./configuration.md#nu_lib_dirs-constant)
+
+### `$env.NU_PLUGIN_DIRS`
+
+A list of directories which will be searched when registering plugins with `plugin add`. See also:
+
+- [Plugin Search Path](./plugins.md#plugin-search-path)
+
+### `$env.PROMPT_*` and `$env.TRANSIENT_PROMPT_*`
+
+A number of variables are available for configuring the Nushell prompt that appears on each commandline. See also:
+
+- [Configuration - Prompt Configuration](./configuration.md#prompt-configuration)
+- `config nu --doc`
+
+### `$env.SHLVL`
+
+`SHLVL` is incremented by most shells when entering a new subshell. It can be used to determine the number of nested shells. For instance,
+if `$env.SHLVL == 2` then typing `exit` should return you to a parent shell.
+
+### `$env.XDG_CONFIG_HOME`
+
+Can be used to optionally override the `$nu.default-config-dir` location. See [Configuration - Startup Variables](./configuration.md#startup-variables).
+
+### `$env.XDG_DATA_DIR`
+
+Can be used to optionally override the `$nu.data-dir` location. See [Configuration - Startup Variables](./configuration.md#startup-variables).
+
+## `$in`
+
+The `$in` variable represents the pipeline input into an expression. See [Pipelines - The Special `$in` Variable](./pipelines.md#pipeline-input-and-the-special-in-variable).
+
+## `$it`
+
+`$it` is a special variable that is _only_ available in a `where` "row condition" — a convenient shorthand which simplifies field access. See `help where` for more information.
+
+## `$NU_LIB_DIRS`
+
+A constant version of `$env.NU_LIB_DIRS` - a list of directories which will be searched when using the `source`, `use`, or `overlay use` commands. See also:
+
+- [Module Path](./modules/using_modules.md#module-path)
+- [Configuration - `$NU_LIB_DIRS`](./configuration.md#nu_lib_dirs-constant)
+
+## `$NU_PLUGIN_DIRS`
+
+A constant version of `$env.NU_PLUGIN_DIRS` - a list of directories which will be searched when registering plugins with `plugin add`. See also:
+
+- [Plugin Search Path](./plugins.md#plugin-search-path)