Which tool to crop a portion of the screen?

On Gnome Shell (17.10+)

To use the built-in gnome-screenshot capability (by default):

• Ctrl + PrintScreen to copy a screenshot of the whole Desktop.
• Ctrl + Alt + PrintScreen to copy a screenshot of the current active Window.
• Ctrl + Shift + PrintScreen to copy a screenshot of an area you select with your Mouse.

On Unity

To use the built-in Screenshot capability:

• PrintScreen to copy a screenshot of the whole Desktop.
• Alt + PrintScreen to copy a screenshot of the current active Window.
• Shift + PrintScreen to copy a screenshot of an area you select with your Mouse.

Applications > Accessories > Take Screenshot... Select area to grab.

Method 1:

• Go to Applications > Accessories > Take Screenshot

  

• Now you can select the portion of a screen.

Method 2:

• If you use shutter, Applications > Accessories > Shutter

• Open it and click selection in the top of the screen.

  

• Now you can select the portion of the screen.

Method 3:

• Take a screenshot of a whole desktop by hitting printscreen.
• Open the screenshot using GIMP.
• Press SHIFT + C to crop the portion.