The -t
option goes to how Unix/Linux handles terminal access. In the past, a terminal was a hardline connection, later a modem based connection. These had physical device drivers (they were real pieces of equipment). Once generalized networks came into use, a pseudo-terminal driver was developed. This is because it creates a separation between understanding what terminal capabilities can be used without the need to write it into your program directly (read man pages on stty
, curses
).
So, with that as background, run a container with no options and by default you have a stdout stream (so docker run | <cmd>
works); run with -i
, and you get stdin stream added (so <cmd> | docker run -i
works); use -t
, usually in the combination -it
and you have a terminal driver added, which if you are interacting with the process is likely what you want. It basically makes the container start look like a terminal connection session.
Late answer, but might help someone
docker run/exec -i
will connect the STDIN of the command inside the container to the STDIN of the docker run/exec
itself.
So
docker run -i alpine cat
gives you an empty line waiting for input. Type "hello" you get an echo "hello". The container will not exit until you send CTRL+D because the main process cat
is waiting for input from the infinite stream that is the terminal input of the docker run
.echo "hello" | docker run -i alpine cat
will print "hello" and exit immediately because cat
notices that the input stream has ended and terminates itself.If you try docker ps
after you exit either of the above, you will not find any running containers. In both cases, cat
itself has terminated, thus docker has terminated the container.
Now for "-t", this tells the main process inside docker that its input is a terminal device.
So
docker run -t alpine cat
will give you an empty line, but if you try to type "hello", you will not get any echo. This is because while cat
is connected to a terminal input, this input is not connected to your input. The "hello" that you typed did not reach the input of cat
. cat
is waiting for input that never arrives.echo "hello" | docker run -t alpine cat
will also give you an empty line and will not exit the container on CTRL-D but you will not get an echo "hello" because you didn't pass -i
If you send CTRL+C, you get your shell back, but if you try docker ps
now, you see the cat
container still running. This is because cat
is still waiting on an input stream that was never closed. I have not found any useful use for the -t
alone without being combined with -i
.
Now, for -it
together. This tells cat that its input is a terminal and in the same time connect this terminal to the input of docker run
which is a terminal. docker run/exec
will make sure that its own input is in fact a tty before passing it to cat
. This is why you will get a input device is not a TTY
if you try echo "hello" | docker run -it alpine cat
because in this case, the input of docker run
itself is the pipe from the previous echo and not the terminal where docker run
is executed
Finally, why would you need to pass -t
if -i
will do the trick of connecting your input to cat
's input? This is because commands treat the input differently if it's a terminal. This is also best illustrated by example
docker run -e MYSQL_ROOT_PASSWORD=123 -i mariadb mysql -u root -p
will give you a password prompt. If you type the password, the characters are printed visibly.docker run -i alpine sh
will give you an empty line. If you type a command like ls
you get an output, but you will not get a prompt or colored output.In the last two cases, you get this behavior because mysql
as well as shell
were not treating the input as a tty and thus did not use tty specific behavior like masking the input or coloring the output.
The -t
argument is NOT documented well, or mentioned by many people often, according to a Google search.
It doesn't even show up when you display a list of (what should be) all docker client arguments by typing docker
at the Bash prompt (with the latest version of 1.8.1).
In fact, if you try to get specific help about this argument by typing docker -t --help
if gives this amazingly vague reply:
flag provided but not defined: -t
So, you can't be blamed for being confused about this argument!
There is a mention in the Docker online documentation which says it is to "Allocate a pseudo-tty" and is often used with -i
:
https://docs.docker.com/reference/run/
I saw it used in the documentation for the terrific jwilder/nginx-proxy
docker container in the following way:
docker run -d -p 80:80 --name nginx -v /tmp/nginx:/etc/nginx/conf.d -t nginx
In this case, what it does is send the output to the 'virtual' tty (Bash command prompt/terminal) within this docker container. You can then see this output by running the docker command docker logs CONTAINER
where CONTAINER
is the first couple of characters of this container's ID. This CONTAINER ID can be found by typing docker ps -a
I've seen this -t
argument mentioned briefly in the following link, where it says
The
-t
and-i
flags allocate a pseudo-tty and keep stdin open even if not attached. This will allow you to use the container like a traditional VM as long as the bash prompt is running.
https://coreos.com/os/docs/latest/getting-started-with-docker.html
I hope this helps! I'm not sure why this isn't documented or used much. Maybe it's experimental and will be implemented as a documented feature in upcoming versions.
What I know about the -t
is the following:
docker exec -ti CONTAINER bash
- allows me to "login" in the container. It feels like ssh-ing (it's not).
But the trouble was when I wanted to restore a database.
Usually I dodocker exec -ti mysql.5.7 mysql
- Here I execute the mysql command in the container and get an interactive terminal.
I added <dump.sql
to the previous command so I can restore a db. But it failed with cannot enable tty mode on non tty input
.
Removing the -t
helped. Still don't understand why:
docker exec -i mysql.5.7 mysql < dump.sql
The last one works. Hope this helps people.
The -it
combined options are known as interactive mode.
By default, containers only have an stdout stream (i.e., docker run | CMD
works), to interact with our container, we need these options:
-i
adds an stdin stream (i.e., CMD | docker run
works);-t
allocates a pseudo-TTY master/slave pair with the slave part tied to the running process in the container and the master part tied to your docker command.The stdin stream attaches the container to the stdin of your shell (Docker inherits the stdin stream of your shell) while the TTY line discipline gives you the ability to interact with the container in a keyboard fashion.
The TTY line discipline consists of low-level features provided by the kernel to TTY devices, such as an editing buffer and basic line edition commands.
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