What is the purpose of -i?

The -i flag in the docker run command stands for "interactive." Its primary purpose is to keep the standard input (STDIN) open for the container, allowing you to interact with it. This is particularly useful when you want to run commands inside the container that require user input or when you want to execute an interactive application.

When you use the -i flag, you can send input to the container's process, making it possible to interact with command-line applications or scripts that expect user interaction.

For example, you might run a container with the following command:

docker run -i ubuntu /bin/bash

In this case, the command starts an Ubuntu container and opens a Bash shell. Because of the -i flag, you can type commands directly into the shell, and the container will process them as if you were using a regular terminal.

However, it's common to use the -i flag in combination with the -t flag (-it) to allocate a pseudo-TTY, which provides a better interactive experience.