Docker number of lines in terminal changing inside docker

UPDATE

you can now install goinside command line tool with:

sudo npm install -g goinside

and go inside a docker container with a proper terminal size with:

goinside docker_container_name

---

Logic behind goinside

thanks to @VonC answer we've got a solution for this problem with a simple bash snippet that we put in ~/.profile:

goinside(){
    docker exec -it $1 bash -c "stty cols $COLUMNS rows $LINES && bash";
}
export -f goinside

now you are able to get inside a docker container without terminal size issues with:

$ goinside containername

---

remember to source ~/.profile before using the goinside function.

---

enabling autocompletion in bash

(as it's shared in one of comments below) if you want to enable autocompletion for goinside you can use this snippet in .profile:

goinside(){
    docker exec -it $1 bash -c "stty cols $COLUMNS rows $LINES && bash";
}
_goinside(){
    COMPREPLY=( $(docker ps --format "{{.Names}}" -f name=$2) );
}
complete -F _goinside goinside;
export -f goinside;

---

enabling autocompletion in zsh

if you are using zsh as your default terminal you can use this snippet inside your ~/.zshrc file:

autoload bashcompinit
bashcompinit
goinside(){
    docker exec -it $1 bash -c "stty cols $COLUMNS rows $LINES && bash";
}
_goinside(){
    COMPREPLY=( $(docker ps --format "{{.Names}}" -f name=$2) );
}
complete -F _goinside goinside;
export goinside;

Update Sept. 2018: check if docker 18.06 has the same issue (it should not, after moby/moby issue 33794, and also moby/moby issue 35407 and PR 37172, part of the 18.06 release notes).

---

2016:

The Ubuntu Dockerfile includes:

CMD ["/bin/bash"]

That means the default ENTRYPOINT is sh -c (and I doubt tput line works well in a sh session, since tput uses terminfo database, which might be set only for bash in that image)

You could try overwrite ENTRYPOINT with bash -c and check if that works better.

That does not work from command line though:

docker run --entrypoint /bin/bash --rm  -it ubuntu:16.04 -i -c 'tput lines'
24

I will check the option of defining a custom image.

FROM ubuntu:16.04
ENTRYPOINT ["/bin/bash", "-c"]

The result is the same though:

docker run --rm  -it u 'tput lines'
24

This however "works":

FROM ubuntu:16.04
ENTRYPOINT [ "/bin/bash" ]

With:

docker@default:/c/Users/vonc/prog/testsu$ docker run --rm  -it u -i -c 'ls; tput lines'
bin  boot  dev  etc  home  lib  lib64  media  mnt  opt  proc  root  run  sbin  srv  sys  tmp  usr  var
48

There might be a synchronization issue, as the same command does return 24 from time to time.

Actually, the following always return "not 24" with:

FROM ubuntu:16.04
ENTRYPOINT [ "/bin/bash", "-l", "-i", "-c" ]

docker run --rm  -it u -c 'sleep 0.1; ls; tput lines'
48

The OP silgon proposes in the comments:

docker run --rm -it --entrypoint /bin/bash ubuntu:16.04 -c "sleep 0.1 && tput lines"

---

As BMitch comments below:

Given the success of sleep my suspicion is that docker spins up the container with the running command, and once up, the client attaches to the running container. Typically something that takes milliseconds.

That gave me another idea:

docker@default:/c/Users/vonc/prog/testsu$ 
docker run --entrypoint='/bin/bash' --name ub -d -it ubuntu:16.04
  0d9b8783afbb5e3ff4232da071d3f357985351ea1ce4d142bf6617ac456fb76b
docker@default:/c/Users/vonc/prog/testsu$ 
d attach ub
  root@0d9b8783afbb:/# tput lines
  48
  root@0d9b8783afbb:/# exit
exit
docker@default:/c/Users/vonc/prog/testsu$ drmae
0d9b8783afbb5e3ff4232da071d3f357985351ea1ce4d142bf6617ac456fb76b

A tput lines within an attached session works just fine.
(On the drmae alias, see "How to remove old and unused Docker images")

---

thajeztah adds in the comments:

the container is created, then started with the defaults (80x24), and after that (when -it), a session is attached.
The session is specifying the size of the terminal;

See "Resize a container TTY" API.

 DEBU[0244] Calling POST /v1.25/containers/c42fd5c4eb79c06fd7f9912b8359022f7d93887afbb33b57a67ed8bb7bfee4‌​3a/resize?h=46&w=221 

For more, see docker issue 25450.
It is related to issue 10341 "Container create or start should accept height/width params". Aleksa Sarai (cyphar) adds (Sept. 2016):

This has actually popped up again inside the runtime-spec (opencontainers/runtime-spec PR 563).
Basically, since Windows requires the ability to set the console size on first start, we might end up adding it for all platforms.

---

The OP silgon points out to the code in api/client/container/run.go:

// Telling the Windows daemon the initial size of the tty during start makes
// a far better user experience rather than relying on subsequent resizes
// to cause things to catch up.
if runtime.GOOS == "windows" {
    hostConfig.ConsoleSize[0], hostConfig.ConsoleSize[1] = dockerCli.GetTtySize()
}

With the logical question:

would it make sense to use this property on Linux as well, and set the initial console size using that value?

Kenfe-Mickaël Laventure (mlaventure) is on it, and a new patch could make it to Docker 1.13.