Since Eshell does not communicate with a terminal like most command shells, IO is a little different. If you try to run programs from within Eshell that are not line-oriented, such as programs that use ncurses, you will just get garbage output, since the Eshell buffer is not a terminal emulator. Eshell solves this problem by running specified commands in Emacs's terminal emulator; to let Eshell know which commands need to be run in a terminal, add them to the list eshell-visual-commands.
Redirection is mostly the same in Eshell as it is in other command
shells. The output redirection operators
>> as well
as pipes are supported, but there is not yet any support for input
redirection. Output can also be redirected to Elisp functions, using
eshell-virtual-targets is a list of mappings of virtual device names to functions. Eshell comes with two virtual devices: /dev/kill, which sends the text to the kill ring, and /dev/clip, which sends text to the clipboard.
You can, of course, define your own virtual targets. They are defined
by adding a list of the form
("/dev/name" function mode) to
eshell-virtual-targets. The first element is the device name;
function may be either a lambda or a function name. If
mode is nil, then the function is the output function; if it is
non-nil, then the function is passed the redirection mode as a
function is expected to return the output function.
The output function is called once on each line of output until
nil is passed, indicating end of output.