All unix programs have three IO streams by default

0 - input,   formally standard input.  it is referred to as SDTIN
1 - output, formally standard output.  it is referred to as SDTOUT
2 - error,   formally standard error.  it is referred to as SDTERR

by default all input comes from the keyboard and all output (STDOUT and STRERR) goes to the screen.

Example:

cat /tmp/file

if the file exists, the cat command will sent the contents of the file to standard out, the screen.
if the the file does not exist, cat will send an error message to standard error, still the screen

Often is is useful to redirect an output stream into a file. the output redirection operator is >.

1> redirects SDTOUT and 2> redirects STDERR

STDOUT example

cat /tmp/file 1> newfile

if the file exists, the cat command will sent the contents of the file to standard out, newfile.
if the the file does not exist, cat will send an error message to standard error, still the screen since we did not redirect STDERR with 2>

STDERR example

cat /tmp/file 2> newfile

if the file exists, the cat command will sent the contents of the file to standard out, the screen.
if the the file does not exist, cat will send an error message to standard error, newfile

STDERR and STDOUT example

cat /tmp/file 1> newfile 2> errorfile

if the file exists, the cat command will sent the contents of the file to standard out, newfile .
if the the file does not exist, cat will send an error message to standard error, errorfile

STDERR and SDTOUT example (special case, both sent to same file) In this example, we are redirecting both standard out and erro to the same file.

Read the cat command like this: send the contents of /tmp/file to STDOUT (which is newfile) and send STDERR to the same place that STDOUT is going

cat /tmp/file 1> newfile 2>&1

if the file exists, the cat command will sent the contents of the file to standard out, newfile .
if the the file does not exist, cat will send an error message to standard error, which is the same as STDOUT, newfile.

CAREFUL!!! left to right order matters!!!

the following command line is NOT like the previous:

cat /tmp/file 2>&1 1> newfile

read this command left to right....

while running the cat command send STDERR to where STDOUT is going (STDOUT is STILL WRITING TO THE SCREEN!!!!!) and send the contents of the file to STDOUT, which is finally send to newfile.

basically, at the time STDERR was sent to STDOUT, STDOUT was still the screen, only later is STDOUT send to a file. STDERR is not effected by the late redirection of STDOUT </pre>

One more more note. inplace of 1> you can simply use >. > without a number is implicitly STDOUT.

STDIN is a lot simpler. many programs can take file input for STDIN reather then from keyboard. also many programs can take STDIN inplace of command line parms.

back to cat.

everyone knows that this writes the file to STDOUT

cat filename

the exact same thing can be done as

cat  < filename

or you can use cat to copy a file

cat < originalfile > copyOfFile