stderr merged into stdout when using a tty

[bamarni]

Output of docker version:

Client:
 Version:      1.12.0
 API version:  1.24
 Go version:   go1.6.3
 Git commit:   8eab29e
 Built:        Sat Jul 30 03:04:21 UTC 2016
 OS/Arch:      darwin/amd64

Server:
 Version:      1.12.0
 API version:  1.24
 Go version:   go1.6.3
 Git commit:   8eab29e
 Built:        Thu Jul 28 23:54:00 2016
 OS/Arch:      linux/amd64

Output of docker info:

Containers: 12
 Running: 0
 Paused: 0
 Stopped: 12
Images: 211
Server Version: 1.12.0
Storage Driver: aufs
 Root Dir: /mnt/sda1/var/lib/docker/aufs
 Backing Filesystem: extfs
 Dirs: 317
 Dirperm1 Supported: true
Logging Driver: json-file
Cgroup Driver: cgroupfs
Plugins:
 Volume: local
 Network: bridge host null overlay
Swarm: inactive
Runtimes: runc
Default Runtime: runc
Security Options: seccomp
Kernel Version: 4.4.16-boot2docker
Operating System: Boot2Docker 1.12.0 (TCL 7.2); HEAD : e030bab - Fri Jul 29 00:29:14 UTC 2016
OSType: linux
Architecture: x86_64
CPUs: 4
Total Memory: 492.3 MiB
Name: dev
ID: C5GS:YWWS:SOBV:I73L:HHNI:M76S:RXXE:LJ3V:3EEU:UHGY:VSWH:KDNC
Docker Root Dir: /mnt/sda1/var/lib/docker
Debug Mode (client): false
Debug Mode (server): true
 File Descriptors: 13
 Goroutines: 24
 System Time: 2016-08-09T16:45:17.684558238Z
 EventsListeners: 0
Username: bamarni
Registry: https://index.docker.io/v1/
Labels:
 provider=virtualbox
Insecure Registries:
 127.0.0.0/8

Steps to reproduce the issue:

1. docker run -t --rm debian:jessie bash -c "echo stdout && echo stderr >&2" 2>/dev/null

Describe the results you received:

stdout
stderr

Describe the results you expected:

I expected to only see stdout, but stderr seems to be merged with it, because of the tty option?

[cpuguy83]

Yes, the tty multiplexes stdout and stderr onto a single stream.

This is the normal behavior of a tty.

[bamarni]

Thanks for the quick reply. Even though this is the normal behaviour of a tty, I reported it mostly because it felt like an unexpected behaviour of docker. Especially when using cli tools in development environment, it is sometimes useful to distinguish between stdout and stderr, but depending on the -t option docker will either merge stderr into stdout or keep them separate.

Then I don't know if / how this behaviour could be changed in docker, maybe having 2 separate ttys?

[cpuguy83]

@bamarni The solution would really be to not include -t unless you need it... and unless you are going to be physically behind a keyboard typing into a terminal you probably don't need a tty.

[bamarni]

In development environment I always add the -t flag because I'm using helper scripts, for example here (https://github.com/bamarni/dotfiles/blob/master/bin/php-bin).

When running this script indeed I won't always need to type something, nor need the ssh agent forwarding, but it happens often. It'd really be time consuming to type the minimal docker command required just to run a cli tool in development, that's why I opted for full-featured helper scripts.

[dnephin]

How about this:

tty=""
if [ -t 1 ] ; then
  tty="-t"
fi

docker run -i $tty --rm  ...

(http://stackoverflow.com/questions/911168/how-to-detect-if-my-shell-script-is-running-through-a-pipe)

[bamarni]

Thanks @dnephin, very interesting idea. I'll give it a shot and post here again if I find some edge cases, hopefully not 😺