
\S standard input : standard output : standard error output : \S  
\S standard error \S

When a user executes a program, the computer expects input for the
program to come from the user's terminal keyboard, and expects the
program to send the output and "error messages"\S*\S to the terminal
display.  Normal input, output, and error messages are referred to
as standard input, standard output, and standard error output, and
they are implemented as files in UNIX systems.  If the user wants a
command to take input from a file, or send output or error output to
a file, then standard input, standard output, and standard error
output must be redirected (see "redirection"\S*\S). A left arrow (<)
is used to redirect standard input from an ordinary file.  For
example, mail (1) would ordinarily take input that you would
compose at the keyboard.

The following command line, however, redirects  input from the file
"letter" to the mail(1) command.  The contents of the file
are then sent to the person with the login name "sandy",

	mail sandy < letter

Standard output and standard error output are redirected  using a
right arrow (>).  For standard error output, the right arrow is
preceded by the number 2.  In the example, 

	pr memo > temp  2> temperror

standard output of the print command, pr(1) is sent to "temp"
and the error output is sent to "temperror" instead of being 
printed on the display.

\S*\S defined in the \Sglossary\S
