Most UNIX commands invoke a special routine called standard output (or stdout) to display information. They invoke a different routine called standard error (or stderr) to display any error messages. There is a similar routine to get input called standard input (or stdin).
In the following example, mp0094 could not be listed, so an error message went to standard error (stderr):
dtr> ls mp0094 mp0095 ls: mp0094 not found: No such file or directory (error 2) mp0095 dtr>
Here, mp0095 was found and listed to standard output (stdout). Using the standard error routine for error messages allows the shell to redirect the errors from the screen to a file, as in the following example:
dtr> ls mp0094 mp0095 2> myerrors mp0095 dtr> ...