file descriptors

 Data Stream for Input

STDIN – 0

Data Stream for Output

STDOUT – 1

Data Stream for Output that relates to an error occurring.

STDERR – 2

 find /etc/ -name shadow 2>/dev/null

redirect errors to null

Redirect STDOUT to a File

find /etc/ -name shadow 2>/dev/null > results.txt

Redirect STDOUT and STDERR to Separate Files

find /etc/ -name shadow 2> stderr.txt 1> stdout.txt

Redirect STDIN

 cat < stdout.txt

Redirect STDOUT and Append to a File

 find /etc/ -name passwd >> stdout.txt 2>/dev/null

Redirect STDIN Stream to a File

cat << EOF > stream.txt

 End-Of-File (EOF) function of a Linux system file, which defines the input's end. 

Pipes

to use the STDOUT from one program to be processed by another. 

find /etc/ -name *.conf 2>/dev/null | grep systemd

find /etc/ -name *.conf 2>/dev/null | grep systemd | wc -l