When you run a program from the shell, usually standard input comes from
your keyboard, and standard output and error both go to your screen.
However, you can ask the shell to change these defaults.

For example, the echo command sends it output to standard output, normally
the screen. But you can send it to a file instead with the output
redirection operator, >. For example, to put the word ``Hello'' in the
file myfile, use this command:

echo Hello > myfile
Use cat or your text file pager (more or less) to view myfile's contents;
see Figure 6.3 on page [*].

Figure 6.3: Redirecting output
\begin{figure}\par\par\begin{list}{}{ \setlength{\rightmargin}{\leftmargin}
\ra... ...llo~>~myfile} \par\$~\textbf{cat~myfile} \par Hello
\par\$\end{list}\end{figure}

You can change the standard input of a command with the input redirection
operator, <. For example, cat < myfile will display the contents of
myfile. This is not useful in practice; for convenience, the cat command
accepts a filename argument. So you can simply say cat myfile, and the
effect will be the same. redirection operators

Under the hood, cat < myfile means that the shell opens myfile and then
feeds its contents to the standard input of cat. cat myfile, without the
redirection operator, means that the cat command receives one argument
(myfile) opens the file itself, and then displays the file.