ImageMagick

According to Wikipedia:

ImageMagick is a free and open-source software suite for displaying, converting, and editing raster image and vector image files. It can read and write over 200 image file formats.

Installation

Install the imagemagick package. Alternatively install graphicsmagick for GraphicsMagick, a fork of ImageMagick, emphasizing stability of the API and command-line interface.

Usage

See ImageMagick(1), or gm(1) for GraphicsMagick.

Popular operations include -append, -resize, -rotate, -quality and many more. For example, to combine multiple pictures into one:

$ convert -append input.pngs output.png

Note: The sign before an option is important. Opposite operations can be performed by using a plus instead of a minus.

To crop part of multiple images and convert them to another format:

$ mogrify -crop WIDTHxHEIGHT+X+Y -format jpg *.png

Where WIDTH and HEIGHT is the cropped output image size, and X and Y is the offset from the input image size.

Screenshot taking

An easy way to take a screenshot of your current system is using the import(1) command:

$ import -window root screenshot.jpg

import is part of the imagemagick package.

Running import without the -window option allows selecting a window or an arbitrary region interactively. With -pause you can specify a delay in which you can, for example, lower some windows.

Note: If you prefer graphicsmagick alternative, just prepend "gm", e.g. $ gm import -window root screenshot.jpg.

Screenshot of multiple X screens

If you run twinview or dualhead, simply take the screenshot twice and use imagemagick to paste them together:

import -window root -display :0.0 -screen /tmp/0.png
import -window root -display :0.1 -screen /tmp/1.png
convert +append /tmp/0.png /tmp/1.png screenshot.png
rm /tmp/{0,1}.png

Screenshot of individual Xinerama heads

Xinerama-based multi-head setups have only one virtual screen. If the physical screens are different in height, you will find dead space in the screenshot. In this case, you may want to take screenshot of each physical screen individually. As long as Xinerama information is available from the X server, the following will work:

#!/bin/sh
xdpyinfo -ext XINERAMA | sed '/^  head #/!d;s///' |
while IFS=' :x@,' read i w h x y; do
        import -window root -crop ${w}x$h+$x+$y head_$i.png
done

Screenshot of the active/focused window

The following script takes a screenshot of the currently focused window. It works with EWMH/NetWM compatible X Window Managers. To avoid overwriting previous screenshots, the current date is used as the filename.

#!/bin/sh
activeWinLine=$(xprop -root | grep "_NET_ACTIVE_WINDOW(WINDOW)")
activeWinId=${activeWinLine:40}
import -window "$activeWinId" /tmp/$(date +%F_%H%M%S_%N).png

Alternatively, the following should work regardless of EWMH support:

$ import -window "$(xdotool getwindowfocus -f)" /tmp/$(date +%F_%H%M%S_%N).png

Note: If screenshots of some programs (e.g. zathura) appear blank, try appending -frame or removing -f from the xdotool command.