Shell Tip: tee

tee

redirections are great tools for sending command output (stdout) to a file (other than the screen).  pipe make the output of one command as input of other command

But if you want, in a single command, to do two things:
display the command output on the screen and write it to a file. In this case you  should use the tee command

Examples:
ls | tee ls.txt
ls | tee -a ls.txt

the -a option will append the file (default is overwrite)

These commands will display the contents of the current directory on the screen, and write the same information to ls.txt file