The U-Boot shell is a powerful tool. It allows modification of the environment, as well as the ability to run commands directly.
Under most circumstances the U-Boot environment is set up to handle most boot situations, however it may be necessary to modify the environment. When using any of the following methods to enter the U-Boot shell, this will cause U-Boot to read a script file from an attached USB mass storage device before dropping to the U-Boot shell. This will happen regardless even if USB boot is disabled for normal boot.