TLDR
docker exec -it kblog df -> script -c 'docker exec -it kblog df' /dev/null
Explanation
Let's look at a common scenario. We try to create a container using an Ubuntu image and execute something inside.
# Run the container in the background
docker run -d --name kblog ubuntu tail -f /dev/null
# Execute the command inside the container (`-i -t` is shortened to `-it`)
docker exec -it kblog dfBut if you write commands above to file a.sh and run a.sh >a.txt, you will see ... input device is not a TTY ... in a.txt.
According to docs, the -t switch will alloc a pseudo-TTY. What does this mean? Let's start with the phenomenon: without -i, the read command inside the container can't read user input from stdin; without -t, you can't use the Ctrl + C to send SIGINT / SIGTERM to exit the program.
So we often use -it to make running a program inside a container work like running it natively (blocking, able to type in, able to use the Ctrl + some key). But since the docker command is not in a pty environment when using redirect / as a child process, the -t option then causes error.
The most popular solution is removing -t switch, or use some auto-detection to remove, which require huge modifications in existing shell script, bringing a lot of inconsistencies.
I think a better way would be to:
Use the script command, it provides the ability to run commands innner pty. Just script -c 'a.sh' a.txt, then docker exec -it kblog df in a.sh will no longer report errors. If you don't need to save the output, just change the last argument to /dev/null.
TTY in Linux is a long-established thing. The serial port or ttyUSB is the closest to the original usage. After we open Terminal, we use pseudo-TTY or PTY for short, and the mechanism is more complex, so read the documentation if you are interested.
Here is a special point: many people think that puts("\n") will sync stdout's buffer, which is actually wrong. The "refresh on newline encounter" is provided by PTY, stdout is just fd 1 and has no such special feature. If you look at stdout in real time, using a hook or some other methods, you will be able to see this.
But you may find that in some languages, the print statement does immediately sync stdout on newline, such as Golang's fmt.Println("hi"). This is an operation that the language or language std lib does on its own. We should not rely on this language or library specific behavior if not necessary.
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