Standard error and other output redirection

In addition to the standard input and standard output, commands often produce other types of output, such as error or status messages known as diagnostic output. Like standard output, standard error output is written to the screen unless it is redirected.

To redirect standard error or other output, use a file descriptor. A file descriptor is a number associated with each of the I/O files that a command ordinarily uses. File descriptors can also be specified to redirect standard input and standard output. The following numbers are associated with standard input, output, and error:

To redirect standard error output, type the file descriptor number 2 in front of the output or append redirection symbols (> or > >) and a file name after the symbol. For example, the following command takes the standard error output from the cc command where it is used to compile the testfile.c file and appends it to the end of the ERRORS file:

cc testfile.c 2>> ERRORS

Other types of output can also be redirected using the file descriptors from 0 through 9. For example, if the cmd command writes output to file descriptor 9, you can redirect that output to the savedata file with the following command:

cmd 9> savedata

If a command writes to more than one output, you can independently redirect each one. Suppose that a command directs its standard output to file descriptor 1, directs its standard error output to file descriptor 2, and builds a data file on file descriptor 9. The following command line redirects each of these outputs to a different file:

command > standard 2> error 9> data