Can someone explain the -t option/flag in docker run command?

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I know that -t flag is used along with -i flag for getting an interactive shell. But I cannot digest what the man page for docker run command says about this flag, which is the following,

       -t, --tty=true|false
          Allocate a pseudo-TTY. The default is false.

       When set to true Docker can allocate a pseudo-tty and attach to the standard input of
       any container. This can be used, for example, to run a throwaway interactive shell.
       The default is false.

       The -t option is incompatible with a redirection of the docker client standard input.

Things like pseudo-TTY or “attaching to the standard input”. So if this interactive shell isn’t the standard input for docker container, what is actually the standard input, to which it attaches?

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Was curious about this too so did some googling :blush:

A pseudo TTY is:

A pseudo TTY (or “PTY”) is a pair of devices — a slave and a master — that provide a special sort of communication channel. The slave device behaves much like the device representing the VT100 or ADM-3A “dumb terminal” that we all have on our desks … or that we might have had a few decades ago.

From: Containers, pseudo TTYs, and backward compatibility [LWN.net]

And how it relates to Docker:

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.

Source: Confused about Docker -t option to Allocate a pseudo-TTY - Stack Overflow

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Thank you for the detailed reply! :slight_smile:
Now I know how it works under the hood.

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