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Redirecting Standard Error
Redirecting standard error lacks the ease of a dedicated redirection operator. To redirect
standard error we must refer to its file descriptor. A program can produce output on any of several numbered file streams. While we have referred to the first three of these file streams as standard input, output and error, the shell references them internally as file de- scriptors 0, 1 and 2, respectively. The shell provides a notation for redirecting files using the file descriptor number. Since standard error is the same as file descriptor number 2, we can redirect standard error with this notation:
[me@linuxbox ~]$ ls -l /bin/usr 2> ls-error.txt
[me@linuxbox ~]$ ls -l /bin/usr 2> ls-error.txt
The file descriptor “2” is placed immediately before the redirection operator to perform the redirection of standard error to the file ls-error.txt.