bitmap image in unix

On windows we can save the screen in a .bmp file format. I sit possible to do the same thing in Unix?

xgrabsc is a screendump program that provides multiple selection styles and several output formats. Selection styles include xwd-style point and click, dragging a rectangle over an arbitrary portion of the screen, timed
snapshots for menu capturing, and keyboard-based selection. Output formats are xwd, XPM (v1 and 2), bitmap, puzzle, and monochrome, greyscale, and color
PostScript. xgrab, part of the package, is an
interactive front-end to xgrabsc.

But I tried Xgrabsc. It did not work.

please see man page for xgrabsc or xgrab