Tee
It is an External command.
It handles the character stream and then split it in the components.
It saves it in the file and the other as output.
It is also a filter.
It is placed any where in the pipeline.
who | tee user.txt
It display the output and also saves it in the file.
who | cmp user.txt
who | tee /dev/tty | wc -l
If a comand is to be substituted the use the comand in the ` `.
example
echo 'There are `ls|wc -l` files in the directory`