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I have a shell script starting a docker container. Before starting the container, it starts a background loop printing some lines to the terminal. I noticed that these printed lines do not return the cursor to the beginning of the line. What I found out: this happens only during the lifetime of the container. See minimal example:

#!/usr/bin/env bash

cleanup() {
  kill "${BACKGROUND_PROCESS}"
}
trap cleanup EXIT

backgroundLoop() {
  while true; do
    echo .
    sleep 1
  done
}

backgroundLoop &
BACKGROUND_PROCESS=$!

sleep 2
echo Starting docker

docker run --rm -it alpine sleep 5

echo Docker has finished
sleep 2

cleanup

The output is:

.
.
Starting docker
.
.
 .
  .
   .
    .
     Docker has finished
.
.

Can anyone explain why this happens? It seems to be related to this question, I assume that docker reconfigures the current TTY and restores it afterwards. Is that correct?

Removing -t from the docker call actually fixes the problem, which seems to back my theory. However, I need the interactive TTY for my actual use case.

How can I change my code so that all output lines are printed with correct carriage returns?

---

UPDATE:

I got an answer to this question which solves the problem for my minimal example I posted above by calling stty -onocr in the docker container before the actual command.

My real-world scenario, however, uses a custom docker image with a bash script as entry point, so I cannot pass commands as arguments. Minimal example of the entrypoint script entrypoint.sh:

#!/usr/bin/env sh

sleep 5

Then replace the docker call in the test script

docker run --rm -it alpine sleep 5

with

docker run --rm -it -v ./entrypoint.sh:/entrypoint.sh --entrypoint /entrypoint.sh alpine

The output is the same, but the proposed fix does not work in this case. Any ideas how to apply the fix to this slightly different environment?

I have a shell script starting a docker container. Before starting the container, it starts a background loop printing some lines to the terminal. I noticed that these printed lines do not return the cursor to the beginning of the line. What I found out: this happens only during the lifetime of the container. See minimal example:

#!/usr/bin/env bash

cleanup() {
  kill "${BACKGROUND_PROCESS}"
}
trap cleanup EXIT

backgroundLoop() {
  while true; do
    echo .
    sleep 1
  done
}

backgroundLoop &
BACKGROUND_PROCESS=$!

sleep 2
echo Starting docker

docker run --rm -it alpine sleep 5

echo Docker has finished
sleep 2

cleanup

The output is:

.
.
Starting docker
.
.
 .
  .
   .
    .
     Docker has finished
.
.

Can anyone explain why this happens? It seems to be related to this question, I assume that docker reconfigures the current TTY and restores it afterwards. Is that correct?

Removing -t from the docker call actually fixes the problem, which seems to back my theory. However, I need the interactive TTY for my actual use case.

How can I change my code so that all output lines are printed with correct carriage returns?

---

UPDATE:

I got an answer to this question which solves the problem for my minimal example I posted above by calling stty -onocr in the docker container before the actual command.

My real-world scenario, however, uses a custom docker image with a bash script as entry point, so I cannot pass commands as arguments. Minimal example of the entrypoint script entrypoint.sh:

#!/usr/bin/env sh

sleep 5

Then replace the docker call in the test script

docker run --rm -it alpine sleep 5

with

docker run --rm -it -v ./entrypoint.sh:/entrypoint.sh --entrypoint /entrypoint.sh alpine

The output is the same, but the proposed fix does not work in this case. Any ideas how to apply the fix to this slightly different environment?

• bash
• docker
• shell
• tty

Share Improve this question Follow edited Feb 26 at 11:21 carlfriedrich asked Feb 25 at 11:37 carlfriedrichcarlfriedrich 4,09311 gold badge2424 silver badges3838 bronze badges 2

• so why not remove -it and done? – KamilCuk Commented Feb 26 at 12:11
• I need to have ctrl+c forwarded to the container, and that's not possible without -it as far as I know. – carlfriedrich Commented Feb 26 at 12:17

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2 Answers 2

Sorted by: Reset to default 1

Tell the container not to add CR characters to the output by running stty -onocr in your container by changing the docker run command to

docker run --rm -it alpine stty -onocr && sleep 5

Why does my terminal not perform carriage returns while docker is running?

Because docker executable switches the terminal in raw mode when spawning the terminal, as part of the -t option you passed to docker run.

See man stty , https://en.wikipedia./wiki/Terminal_mode .

How can I change my code so that all output lines are printed with correct carriage returns?

Option #1:

You can remove -t flag from docker run.

Option #2:

You can just set NORAW=1 environment variable.

$ ( set -x; ( sleep 0.2; echo .; echo .; echo .; echo .; ) & NORAW=1 docker run -ti --rm alpine sleep 5 ; wait )
+ NORAW=1
+ docker run -ti --rm alpine sleep 5
+ sleep 0.2
+ echo .
.
+ echo .
.
+ echo .
.
+ echo .
.
+ wait

See https://github/docker/cli/blob/068a01ea9470df6494cc92d9e64e240805ae47a7/cli/streams/in.go#L32 .

Option #3:

You can revert from raw mode, for example with stty -raw. Something along:

$ ( set -x; ( sleep 0.2; stty -raw </dev/tty; echo .; echo .; echo .; echo .; ) & docker run -ti --rm alpine sleep 5 ; wait )
+ docker run -ti --rm alpine sleep 5
+ sleep 0.2
+ stty -raw
           + echo .
.
+ echo .
.
+ echo .
.
+ echo .
.
+ wait

The first + echo . gets printed just before stty -raw is able to take place. After -raw, terminal is cooked.

so I cannot pass commands as arguments

If you control command line, you can pass anything as arguments. For example:

docker run --rm -it -v ./entrypoint.sh:/entrypoint.sh \
   --entrypoint /usr/bin/env alpine sh -c 'stty -raw; . entrypoint.sh'

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