Data Streams

standard input, standard output, standard error

stdin e.g. keyboard commands
stdout screen printout
stderr when an error is displayed on screen (failed commands, e.g. command not found)

Distinguishing between stdout and stderr: echo $?
returncode 1: stderr
returncode 0: stdout

example: following command will fail as it is not called with sudo: find /etc -type f. echo $? returns 1

stdin is represented by the number 0, stdout by 1 and stderr by 2.
example: split stdout from stderr and handle them differently:
find /etc -type f 2> /dev/null
/dev/null is a black hole; anything that is sent to /dev/null is never seen again.

example: sent stdout to a file (it is implied):
find /etc -type f > ~/results.txt
displays only stderr, but newly created file has all stdout.

example: send stdout and stderr to two different files (overwrite):
find /etc -type f > ~/results.txt 2> ~/errors.txt

example: append stdout and stderr to two different files:
find /etc -type f >> ~/results.txt 2>> ~/errors.txt