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 such programs in Emacs's terminal emulator.
Programs that need a terminal to display output properly are referred
to in this manual as “visual commands,” because they are not simply
line-oriented. You must tell Eshell which commands are visual, by
adding them to
eshell-visual-commands; for commands that are
visual for only certain sub-commands – e.g., ‘git log’ but
not ‘git status’ – use
eshell-visual-subcommands; and for
commands that are visual only when passed certain options, use
Caution: Some tools such as Git use the pager ‘less’ by default to paginate their output but call it with its ‘-F’ option. This option causes ‘less’ to echo the output instead of paginating it if the output is less than one page long. This causes undesirable behavior if, e.g., ‘git diff’, is defined as a visual subcommand. It'll work if the output is big enough and fail if it is less than one page long. If that occurs to you, search for configuration options for calling ‘less’ without the ‘-F’ option. For Git, you can do that using ‘git config --global core.pager 'less -+F'’.
If you want the buffers created by visual programs killed when the
program exits, customize the variable
eshell-destroy-buffer-when-process-dies to a non-
value; the default is
Redirection is mostly the same in Eshell as it is in other command
shells. The output redirection operators
well as pipes are supported, but there is not yet any support for
input redirection. Output can also be redirected to buffers, using
>>> redirection operator, and Elisp functions, using
The buffer redirection operator,
>>>, expects a buffer object
on the right-hand side, into which it inserts the output of the
left-hand side. e.g., ‘echo hello >>> #<buffer *scratch*>’
inserts the string
"hello" into the *scratch* buffer.
The convenience shorthand variant ‘#<buffer-name>’, as in
‘#<*scratch*>’, is also accepted.
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
nil, then the function is the output function; if it is
nil, then the function is passed the redirection mode as a
>>>–and the 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.