Why do "docker run -t" outputs include \r in the command output?

Question

I'm using Docker client Version: 18.09.2.

When I run start a container interactively and run a date command, then pipe its output to hexdump for inspection, I'm seeing a trailing \n as expected:

$ docker run --rm -i -t alpine
/ # date | hexdump -c
0000000   T   h   u       M   a   r           7       0   0   :   1   5
0000010   :   0   6       U   T   C       2   0   1   9  \n
000001d

However, when I pass the date command as an entrypoint directly and run the container, I get a \r \n every time there's a new line in the output.

$ docker run --rm -i -t --entrypoint=date alpine | hexdump -c
0000000   T   h   u       M   a   r           7       0   0   :   1   6
0000010   :   1   9       U   T   C       2   0   1   9  \r  \n
000001e

This is weird.

It totally doesn't happen when I omit -t (not allocating any TTY):

docker run --rm -i --entrypoint=date alpine | hexdump -c
0000000   T   h   u       M   a   r           7       0   0   :   1   7
0000010   :   3   0       U   T   C       2   0   1   9  \n
000001d

What's happening here?

This sounds dangerous, as I use docker run command in my scripts, and if I forget to omit -t from my scripts, the output I'll collect from docker run command will have invisible/non-printible \r characters which can cause all sorts of issues.

Answer 1

tldr; This is a tty default behaviour and unrelated to docker. Per the ticket filed on github about your exact issue.

---

Quoting the relevant comments in that ticket:

Looks like this is indeed TTY by default translates newlines to CRLF

$ docker run -t --rm debian sh -c "echo -n '\n'" | od -c
0000000   \r  \n
0000002

disabling "translate newline to carriage return-newline" with stty -onlcr correctly gives;

$ docker run -t --rm debian sh -c "stty -onlcr && echo -n '\n'" | od -c
0000000   \n
0000001

Default TTY options seem to be set by the kernel ... On my linux host it contains:

/*
 * Defaults on "first" open.
 */
#define TTYDEF_IFLAG    (BRKINT | ISTRIP | ICRNL | IMAXBEL | IXON | IXANY)
#define TTYDEF_OFLAG    (OPOST | ONLCR | XTABS)
#define TTYDEF_LFLAG    (ECHO | ICANON | ISIG | IEXTEN | ECHOE|ECHOKE|ECHOCTL)
#define TTYDEF_CFLAG    (CREAD | CS7 | PARENB | HUPCL)
#define TTYDEF_SPEED    (B9600)

ONLCR is indeed there.

When we go looking at the ONLCR flag documentation, we can see that:

[-]onlcr: translate newline to carriage return-newline

To again quote the github ticket:

Moral of the story, don't use -t unless you want a TTY.
TTY line endings are CRLF, this is not Docker's doing.