from
The Free On-line Dictionary of Computing (8 July 2008)
input/output redirection
I/O redirection
<operating system> In {Unix}, to send ouput from a {process}
to different {file} or {device} or to another process via a
{pipe}, or to have a process read its input from a different
file, device or pipe. Some other {operating systems} have
similar facilities.
To redirect input to come from a file instead of the keyboard,
use "<":
myprog < myfile
Similarly to redirect output to a file instead of the screen:
ls > filelist
A pipe redirects the output of one process directly into the
input of another
who | wc -l
A common misuse by beginners is
cat myfile | myprog
Which is more or less equivalent to "myprog < myfile" except
that it introduces an extra unnecessary cat process and buffer
space for the pipe. Even the "<" is unnecessary with many
standard Unix commands since they accept input file names as
command line arguments anyway.
Unix's concept of {standard input/output} and I/O redirection
make it easy to combine simple processes in powerful ways and
to use the same commands for different purposes.
(1998-04-24)