root@machine1:/data# echo test|docker exec -i 68b cat  test 

root@machine1:/data# echo test|docker exec -it 68b cat 

cannot enable tty mode on non tty input

But we’ve also passed in two flags: -t and -i.
The -t flag
assigns a pseudo-tty or terminal inside our new container and the -i flag
allows us to make an interactive connection by grabbing the standard in (STDIN)
of the container.

In foreground mode (the default when -d is not specified), docker
run can start the process in the container and attach the console to the process’s standard input, output, and standard error. It can even pretend to be a TTY (this is what most command line executables expect) and pass along signals. All of that is
configurable:

-a=[]           : Attach to `STDIN`, `STDOUT` and/or `STDERR`

-t=false        : Allocate a pseudo-tty

--sig-proxy=true: Proxify all received signal to the process (non-TTY mode only)

-i=false        : Keep STDIN open even if not attached

If you do not specify -a then Docker will attach
all standard streams. You can specify to which of the three standard streams (STDIN, STDOUT, STDERR)
you’d like to connect instead, as in:

$ docker run -a stdin -a stdout -i -t ubuntu /bin/bash

For interactive processes (like a shell), you must use -i -ttogether
in order to allocate a tty for the container process. -i -t is
often written -it as you’ll see in later examples. Specifying -t is
forbidden when the client standard output is redirected or piped, such as in: echo
test | docker run -i busybox cat.