xmessage

The xmessage program displays a window containing a message from the command line, a file, or standard input. Along the lower edge of the message is row of buttons; clicking the left mouse button on any of these buttons will cause xmessage to exit. Which button was pressed is returned in the exit status and, optionally, by writing the label of the button to standard output.

The program is typically used by shell scripts to display information to the user or to ask the user to make a choice.

Unless a size is specified, xmessage sizes itself to fit the message, up to a maximum size. If the message is too big for the window, xmessage will display scroll bars.