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The winfo command is used to retrieve information about windows managed by Tk. It can take any of a number of different forms, depending on the option argument. The legal forms are:

winfo atom ?-displayof window? name: Returns a decimal string giving the integer identifier for the atom whose name is name. If no atom exists with the name name then a new one is created. If the -displayof option is given then the atom is looked up on the display of window; otherwise it is looked up on the display of the application's main window.
winfo atomname ?-displayof window? id: Returns the textual name for the atom whose integer identifier is id. If the -displayof option is given then the identifier is looked up on the display of window; otherwise it is looked up on the display of the application's main window. This command is the inverse of the winfo atom command. It generates an error if no such atom exists.
winfo cells window: Returns a decimal string giving the number of cells in the color map for window.
winfo children window: Returns a list containing the path names of all the children of window. Top-level windows are returned as children of their logical parents. The list is in stacking order, with the lowest window first, except for Top-level windows which are not returned in stacking order. Use the wm stackorder command to query the stacking order of Top-level windows.
winfo class window: Returns the class name for window.
winfo colormapfull window: Returns 1 if the colormap for window is known to be full, 0 otherwise. The colormap for a window is “known” to be full if the last attempt to allocate a new color on that window failed and this application has not freed any colors in the colormap since the failed allocation.
winfo containing ?-displayof window? rootX rootY: Returns the path name for the window containing the point given by rootX and rootY. RootX and rootY are specified in screen units (i.e. any form acceptable to Tk_GetPixels) in the coordinate system of the root window (if a virtual-root window manager is in use then the coordinate system of the virtual root window is used). If the -displayof option is given then the coordinates refer to the screen containing window; otherwise they refer to the screen of the application's main window. If no window in this application contains the point then an empty string is returned. In selecting the containing window, children are given higher priority than parents and among siblings the highest one in the stacking order is chosen.
winfo depth window: Returns a decimal string giving the depth of window (number of bits per pixel).
winfo exists window: Returns 1 if there exists a window named window, 0 if no such window exists.
winfo fpixels window number: Returns a floating-point value giving the number of pixels in window corresponding to the distance given by number. Number may be specified in any of the forms acceptable to Tk_GetScreenMM, such as “2.0c” or “1i”. The return value may be fractional; for an integer value, use winfo pixels.
winfo geometry window: Returns the geometry for window, in the form widthxheight+x+y. All dimensions are in pixels.
winfo height window: Returns a decimal string giving window's height in pixels. When a window is first created its height will be 1 pixel; the height will eventually be changed by a geometry manager to fulfil the window's needs. If you need the true height immediately after creating a widget, invoke update to force the geometry manager to arrange it, or use winfo reqheight to get the window's requested height instead of its actual height.
winfo id window: Returns a hexadecimal string giving a low-level platform-specific identifier for window. On Unix platforms, this is the X window identifier. Under Windows, this is the Windows HWND. On the Macintosh the value has no meaning outside Tk.
winfo interps ?-displayof window?: Returns a list whose members are the names of all Tcl interpreters (e.g. all Tk-based applications) currently registered for a particular display. If the -displayof option is given then the return value refers to the display of window; otherwise it refers to the display of the application's main window.
winfo ismapped window: Returns 1 if window is currently mapped, 0 otherwise.
winfo manager window: Returns the name of the geometry manager currently responsible for window, or an empty string if window is not managed by any geometry manager. The name is usually the name of the Tcl command for the geometry manager, such as pack or place. If the geometry manager is a widget, such as canvases or text, the name is the widget's class command, such as canvas.
winfo name window: Returns window's name (i.e. its name within its parent, as opposed to its full path name). The command winfo name . will return the name of the application.
winfo parent window: Returns the path name of window's parent, or an empty string if window is the main window of the application.
winfo pathname ?-displayof window? id: Returns the path name of the window whose X identifier is id. Id must be a decimal, hexadecimal, or octal integer and must correspond to a window in the invoking application. If the -displayof option is given then the identifier is looked up on the display of window; otherwise it is looked up on the display of the application's main window.
winfo pixels window number: Returns the number of pixels in window corresponding to the distance given by number. Number may be specified in any of the forms acceptable to Tk_GetPixels, such as “2.0c” or “1i”. The result is rounded to the nearest integer value; for a fractional result, use winfo fpixels.
winfo pointerx window: If the mouse pointer is on the same screen as window, returns the pointer's x coordinate, measured in pixels in the screen's root window. If a virtual root window is in use on the screen, the position is measured in the virtual root. If the mouse pointer is not on the same screen as window then -1 is returned.
winfo pointerxy window: If the mouse pointer is on the same screen as window, returns a list with two elements, which are the pointer's x and y coordinates measured in pixels in the screen's root window. If a virtual root window is in use on the screen, the position is computed in the virtual root. If the mouse pointer is not on the same screen as window then both of the returned coordinates are -1.
winfo pointery window: If the mouse pointer is on the same screen as window, returns the pointer's y coordinate, measured in pixels in the screen's root window. If a virtual root window is in use on the screen, the position is computed in the virtual root. If the mouse pointer is not on the same screen as window then -1 is returned.
winfo reqheight window: Returns a decimal string giving window's requested height, in pixels. This is the value used by window's geometry manager to compute its geometry.
winfo reqwidth window: Returns a decimal string giving window's requested width, in pixels. This is the value used by window's geometry manager to compute its geometry.
winfo rgb window color: Returns a list containing three decimal values in the range 0 to 65535, which are the red, green, and blue intensities that correspond to color in the window given by window. Color may be specified in any of the forms acceptable for a color option.
winfo rootx window: Returns a decimal string giving the x-coordinate, in the root window of the screen, of the upper-left corner of window's border (or window if it has no border).
winfo rooty window: Returns a decimal string giving the y-coordinate, in the root window of the screen, of the upper-left corner of window's border (or window if it has no border).
winfo screen window: Returns the name of the screen associated with window, in the form displayName.screenIndex.
winfo screencells window: Returns a decimal string giving the number of cells in the default color map for window's screen.
winfo screendepth window: Returns a decimal string giving the depth of the root window of window's screen (number of bits per pixel).
winfo screenheight window: Returns a decimal string giving the height of window's screen, in pixels.
winfo screenmmheight window: Returns a decimal string giving the height of window's screen, in millimeters.
winfo screenmmwidth window: Returns a decimal string giving the width of window's screen, in millimeters.
winfo screenvisual window: Returns one of the following strings to indicate the default visual class for window's screen: directcolor, grayscale, pseudocolor, staticcolor, staticgray, or truecolor.
winfo screenwidth window: Returns a decimal string giving the width of window's screen, in pixels.
winfo server window: Returns a string containing information about the server for window's display. The exact format of this string may vary from platform to platform. For X servers the string has the form “XmajorRminor vendor vendorVersion” where major and minor are the version and revision numbers provided by the server (e.g., X11R5), vendor is the name of the vendor for the server, and vendorRelease is an integer release number provided by the server.
winfo toplevel window: Returns the path name of the top-of-hierarchy window containing window. In standard Tk this will always be a toplevel widget, but extensions may create other kinds of top-of-hierarchy widgets.
winfo viewable window: Returns 1 if window and all of its ancestors up through the nearest toplevel window are mapped. Returns 0 if any of these windows are not mapped.
winfo visual window: Returns one of the following strings to indicate the visual class for window: directcolor, grayscale, pseudocolor, staticcolor, staticgray, or truecolor.
winfo visualid window: Returns the X identifier for the visual for window.
winfo visualsavailable window ?includeids?: Returns a list whose elements describe the visuals available for window's screen. Each element consists of a visual class followed by an integer depth. The class has the same form as returned by winfo visual. The depth gives the number of bits per pixel in the visual. In addition, if the includeids argument is provided, then the depth is followed by the X identifier for the visual.
winfo vrootheight window: Returns the height of the virtual root window associated with window if there is one; otherwise returns the height of window's screen.
winfo vrootwidth window: Returns the width of the virtual root window associated with window if there is one; otherwise returns the width of window's screen.
winfo vrootx window: Returns the x-offset of the virtual root window associated with window, relative to the root window of its screen. This is normally either zero or negative. Returns 0 if there is no virtual root window for window.
winfo vrooty window: Returns the y-offset of the virtual root window associated with window, relative to the root window of its screen. This is normally either zero or negative. Returns 0 if there is no virtual root window for window.
winfo width window: Returns a decimal string giving window's width in pixels. When a window is first created its width will be 1 pixel; the width will eventually be changed by a geometry manager to fulfil the window's needs. If you need the true width immediately after creating a widget, invoke update to force the geometry manager to arrange it, or use winfo reqwidth to get the window's requested width instead of its actual width.
winfo x window: Returns a decimal string giving the x-coordinate, in window's parent, of the upper-left corner of window's border (or window if it has no border).
winfo y window: Returns a decimal string giving the y-coordinate, in window's parent, of the upper-left corner of window's border (or window if it has no border).

Example

Print where the mouse pointer is and what window it is currently over:

lassign [winfo pointerxy .] x y
puts -nonewline "Mouse pointer at ($x,$y) which is "
set win [winfo containing $x $y]
if {$win eq ""} {
    puts "over no window"
} else {
    puts "over $win"
}

Licensed under Tcl/Tk terms
https://www.tcl.tk/man/tcl/TkCmd/winfo.htm