Through the command shell we can redirect the three standard file streams so that we can get input from either a file or another command instead of from our keyboard, and we can write output and errors to files or send them as input for subsequent commands.

For example, if we have a program called do_something that reads from stdin and writes to stdout and stderr, we can change its input source by using the less-than sign ( < ) followed by the name of the file to be consumed for input data:

do_something < input-file

If you want to send the output to a file, use the greater-than sign ( > ) as in:

do_something > output-file

Because stderr is not the same as stdout, error messages will still be seen on the terminal windows in the above example.

If you want to redirect stderr to a separate file, you use stderr’s file descriptor number (2), the greater-than sign ( > ), followed by the name of the file you want to hold everything the running command writes to stderr:

do_something 2> error-file

A special shorthand notation can be used to put anything written to file descriptor 2 (stderr) in the same place as file descriptor 1 (stdout): 2>&1

do_something > all-output-file 2>&1

bash permits an easier syntax for the above:

do_something >& all-output-file

~~DISCUSSION~~