$!: The current number of images in the list.

$> and $<: The increasing/decreasing index of the latest (currently running) repeat...done loop. $> goes from 0 (first loop iteration) to nb_iterations - 1 (last iteration). $< does the opposite.

$/: The current call stack. Stack items are separated by slashes /.

$|: The current value (expressed in seconds) of a millisecond precision timer.

$^: The current verbosity level.

$_cpus: The number of computation cores available on your machine.

$_flags: The list of enabled flags when G'MIC interpreter has been compiled.

$_host: A string telling about the host running the G'MIC interpreter (e.g. cli or gimp).

$_os: A string describing the running operating system.

$_path_rc: The path to the G'MIC folder used to store configuration files (its value is OS-dependent).

$_path_user: The path to the G'MIC user file .gmic or user.gmic (its value is OS-dependent).

$_path_commands: A list of all imported command files (stored as an image list).

$_pid: The current process identifier, as an integer.

$_pixeltype: The type of image pixels (default: float32).

$_prerelease: For pre-releases, the date of the pre-release as yymmdd. For stable releases, this variable is set to 0.

$_version: A 3-digits number telling about the current version of the G'MIC interpreter  (e.g. 350).

$_vt100: Set to 1 if colored text output is allowed on the console. Otherwise, set to 0.

b: The image basename (i.e. filename without the folder path nor extension).

f: The image folder name.

n: The image name or filename (if the image has been read from a file).

t: The text string from the image values regarded as character codes.

x: The image extension (i.e the characters after the last . in the image name).

^: The sequence of all image values, separated by commas ,.

@subset: The sequence of image values corresponding to the specified subset, and separated by commas ,.

Any other feature is considered as a mathematical expression associated to the image [image] and is substituted by the result of its evaluation (float value). For instance, expression {0,w+h} is substituted by the sum of the width and height of the first image (see section Mathematical Expressions for more details). If a mathematical expression starts with an underscore _, the resulting value is truncated to a readable format. For instance, item {_pi} is substituted by 3.14159 (while {pi} is substituted by 3.141592653589793).

A feature delimited by backquotes is replaced by a string whose character codes correspond to the list of values resulting from the evaluation of the specified mathematical expression. For instance, item {`[102,111,111]`} is substituted by foo and item {`vector8(65)`} by AAAAAAAA.

u: screen width (actually independent on the window size).

v: screen height (actually independent on the window size).

uv: screen width*screen height.

d: window width (i.e. width of the window widget).

e: window height (i.e. height of the window widget).

de: window width*window height.

w: display width (i.e. width of the display area managed by the window).

h: display height (i.e. height of the display area managed by the window).

wh: display width*display height.

i: X-coordinate of the display window.

j: Y-coordinate of the display window.

f: current fullscreen state of the instant display.

n: current normalization type of the instant display.

t: window title of the instant display.

x: X-coordinate of the mouse position (or -1, if outside the display area).

y: Y-coordinate of the mouse position (or -1, if outside the display area).

b: state of the mouse buttons { 1:left-but. | 2:right-but. | 4:middle-but. }.

o: state of the mouse wheel.

k: decimal code of the pressed key if any, 0 otherwise.

c: boolean (0 or 1) telling if the instant display has been closed recently.

r: boolean telling if the instant display has been resized recently.

m: boolean telling if the instant display has been moved recently.

Any other feature stands for a keycode name (in capital letters), and is substituted by a boolean describing the current key state { 0:pressed | 1:released }.

You can also prepend a hyphen - to a feature (that supports it) to flush the corresponding event immediately after reading its state (works for keys, mouse and window events).