Standard input redirection
When the notation < filename is
added to the end of a command, the input of the command is read from the specified
file name. The < symbol is known as the input redirection
operator.
Note: Only commands that normally take their input from the keyboard can have
their input redirected.
For example, to send the file letter1 as
a message to user
denise with the mail command,
type the following: mail denise < letter1