Save output in multiple file

echo "1st line"
echo "2nd line" >&2
echo "3rd line" >&3
echo "4th line"
echo "5th line" >&2
echo "6th line" >&3

source script.sh >file1.txt 2>file2.txt 3>file3.txt

Redirects stdout to /dev/null

command >/dev/null | grep 'something'

command 1>/dev/null | grep 'something'

Redirects stderr to /dev/null

command 2>/dev/null | grep 'something'

Redirects stderr to /dev/null then redirects stdout to stderr address

command 2>/dev/null 1>&2

command &>/dev/null

Redirect stderr to stdout then redirect stdout to /dev/null

command 2>&1 >/dev/null | grep 'something'