14

I want to continously print dots without newline (waiting behavior).

This bash one-liner works fine on my machine:

$ while true; do sleep 1; printf '.'; done
.......^C

However, when I run it in a Docker container, and when I try to read its output with docker logs, no output is printed:

$ docker run -d --name test_logs ubuntu:14.04 bash -c "while true; do sleep 1; printf '.'; done"
60627015ed0a0d331a26e0c48ccad31c641f2142da55d24e10f7ad5737211a18
$ docker logs test_logs
$ docker logs -f test_logs
^C

I can confirm that the bash loop is executing in the container by using strace on process 1 (bash command):

$ docker exec -t test bash -c 'apt-get install -y strace; strace -p1 -s9999 -e write'
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following NEW packages will be installed:
  strace
0 upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
Need to get 113 kB of archives.
After this operation, 504 kB of additional disk space will be used.
Get:1 http://archive.ubuntu.com/ubuntu/ trusty/main strace amd64 4.8-1ubuntu5 [113 kB]
Fetched 113 kB in 0s (154 kB/s)
debconf: unable to initialize frontend: Dialog
debconf: (TERM is not set, so the dialog frontend is not usable.)
debconf: falling back to frontend: Readline
Selecting previously unselected package strace.
(Reading database ... 11542 files and directories currently installed.)
Preparing to unpack .../strace_4.8-1ubuntu5_amd64.deb ...
Unpacking strace (4.8-1ubuntu5) ...
Setting up strace (4.8-1ubuntu5) ...
Process 1 attached
--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=136, si_status=0, si_utime=0, si_stime=0} ---
write(1, ".", 1)                        = 1
--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=153, si_status=0, si_utime=0, si_stime=0} ---
write(1, ".", 1)                        = 1
--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=154, si_status=0, si_utime=0, si_stime=0} ---
write(1, ".", 1)                        = 1
--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=155, si_status=0, si_utime=0, si_stime=0} ---
write(1, ".", 1)                        = 1

...and so on.

Also, it works fine when directly watching output with option -t (without using docker logs):

$ docker run -t --name test_logs ubuntu:14.04 bash -c "while true; do sleep 1; printf '.'; done"
...........^C

Even weirder, background + pseudo-tty (option -d + option -t) worked once, but then didn't work anymore.

printf is a line-buffered command, and if I add flushing by printing a newline character, it works:

$ docker run -d --name test_logs ubuntu:14.04 bash -c "while true; do sleep 1; printf '.\n'; done"
720e274fcf85f52587b8a2a402465407c5e925c41d80af05ad3a73cebaf7110f
$ docker logs -f test_logs
.
.
.
.
.
.
^C

So I tried to "unbuffer" printf with stdbuf, without any success:

$ docker run -d --name test_logs ubuntu:14.04 bash -c "while true; do sleep 1; stdbuf -o0 printf '.'; done"
2ba2116190c1b510288144dc5a220669f52f701c17f6f102e6bd6af88de4674e
$  docker logs test_logs
$ docker logs -f test_logs
^C

Next I tried to redirect printf to the standard error, still without success:

$ docker run -d --name test_logs ubuntu:14.04 bash -c "while true; do sleep 1; printf '.' >&2; done"
b1645b48bd9afd5b72318fba5296157ce1c0346f6a82fa166e802a979c1b0b0f
$ docker logs test_logs
$ docker logs -f test_logs
^C

...and with both at the same time, still without success:

$  docker run -d --name test_logs ubuntu:14.04 bash -c "while true; do sleep 1; stdbuf -o0 printf '.' >&2; done"

I encountered the same behaviour when using echo -n instead of printf.

My questions: Do I unbuffer correctly? If yes, what makes it not working?

Looking for insight on this :)

4

1 回答 1

8

默认的 docker 日志记录驱动程序（json 文件）不支持无缓冲输出。你最终会看到你的点，但只有在缓冲区被填满之后。

如果您需要无缓冲的输出，可以使用 syslog 日志记录驱动程序。但是，因为它是 syslog，所以我怀疑它会给您想要的东西，因为它将向 syslog 提供无缓冲的输出。在docs.docker.com上记录

由于运行点通常仅在看到更多点意味着工作仍在继续的交互情况下才有用，因此您可能只想用换行符输出周期性状态，以便 docker logs 为您提供更好的信息。

这里的解决方案通常是检查您是否处于交互模式，如果是，则显示点：

#!/bin/bash
while true; do
     if [ -t ]; then
         echo -n .
     elif ((++COUNT % 60 == 0)); then
         echo "$(date) still in progress"
     fi
     sleep 1   # or whatever work is going on
done
于 2018-06-06T23:51:30.443 回答