深入探究docker attach的退出方式
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docker attach可以将当前终端的标准输入、标准输出和标准错误连接到指定的容器中，可以显示容器 ENTRYPOINT/CMD 进程的输出，是容器调试的重要方法。docker attach之后退出的方法，主要有CTRL-c和CTRL-p CTRL-q两种。大部分说法中，认为CTRL-c会导致容器退出，推荐使用CTRL-p CTRL-q方法。我在实际使用中，发现这些说法并不准确，这两种方法都会收到docker run和docker attach命令选项的影响。
1. CTRL-c方式
与普通的进程一样，docker attach之后执行CTRL-c会发送SIGINT信号。
docker attach命令有--sig-proxy选项，man docker attach中对该选项描述如下：
--sig-proxy=true|false Proxy all received signals to the process (non-TTY mode only). SIGCHLD, SIGKILL, and SIGSTOP are not proxied. The default is true.
可以看到，该选项仅在non-TTY模式下起作用，即docker run时不指定-t选项。此时，默认--sig-proxy为true，SIGINT信号被发送至docker中PID为1的进程。
[email protected]:~$ docker run -id centos:7 /bin/bash edfe9bc8ae02567ff9309ffb6940a9772f02a18d24e4cf04a9da8c84c01e1f52 [email protected]:~$ docker attach edfe ^C^C^C
[email protected]:~$ docker run -d centos:7 top -b 8700417c309d1b6c8e3a1618f38b232e8eddedae20287e73f2022b0cac716248 [email protected]:~$ docker attach 8700 top - 02:37:24 up 19 days, 1:59, 0 users, load average: 0.94, 1.03, 1.17 Tasks: 1 total, 1 running, 0 sleeping, 0 stopped, 0 zombie %Cpu(s): 10.4 us, 2.0 sy, 0.0 ni, 87.2 id, 0.3 wa, 0.0 hi, 0.0 si, 0.0 st KiB Mem : 8066788 total, 752956 free, 3659576 used, 3654256 buff/cache KiB Swap: 8282108 total, 7576840 free, 705268 used. 3378516 avail Mem PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 1 root 20 0 51780 3576 3132 R 0.0 0.0 0:00.04 top^C [email protected]:~$ docker ps -a CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 8700417c309d centos:7 "top -b" 11 seconds ago Exited (0) 3 seconds ago pensive_brahmagupta
上面两个例子中，docker run没有-t选项，docker attach中--sig-proxy默认为true，将信号发送至容器（SIGCHLD，SIGKILL，SIGSTOP除外）。因此SIGINT发送至PID为1的进程，分别为/bin/bash和top -b。第一个例子，/bin/bash没有处理SIGINT信号（加上-i是为防止进程退出）。第二个例子中，top -b响应了SIGINT信号并退出，因此容器退出。
如果在docker attach时设置--sig-proxy=false，SIGINT信号不会发送到容器中PID为1的进程，而是被docker attach进程响应
[email protected]:~$ docker run -id centos:7 /bin/bash 5a6d97406a068b1a58b02d4b3267a974b8c69e268d37db19b44b2858b5086e0f [email protected]:~$ docker attach --sig-proxy=false 5a6d ^C [email protected]:~$ docker ps -a CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 5a6d97406a06 centos:7 "/bin/bash" 11 seconds ago Up 9 seconds reverent_goldstine
[email protected]:~$ docker run -d centos:7 top -b 37a80db8bf8f8c79bbb910353d04a46697ad54c0bcb4f34f4ef8edd42163d152 [email protected]:~$ docker attach --sig-proxy=false 37a8 top - 02:42:45 up 19 days, 2:04, 0 users, load average: 0.58, 0.74, 0.99 Tasks: 1 total, 1 running, 0 sleeping, 0 stopped, 0 zombie %Cpu(s): 8.2 us, 1.7 sy, 0.0 ni, 90.1 id, 0.1 wa, 0.0 hi, 0.0 si, 0.0 st KiB Mem : 8066788 total, 782360 free, 3642612 used, 3641816 buff/cache KiB Swap: 8282108 total, 7576848 free, 705260 used. 3405820 avail Mem PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 1 root 20 0 51780 3668 3224 R 0.0 0.0 0:00.06 top^C [email protected]:~$ docker ps -a CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 37a80db8bf8f centos:7 "top -b" 15 seconds ago Up 13 seconds tender_kirch
上面两个例子中，CTRL-c都能退出，但退出的是docker attach进程，容器本身不受影响。
以上的例子中，docker run都没有-t选项，如果指定了-t选项呢？
[email protected]:~$ docker run -itd centos:7 /bin/bash 4ff1e8d42537115f46c98eeae6c1554b388cd3322039f8185bb4e3a1848ec519 [email protected]:~$ docker attach 4ff1 [[email protected] /]# ^C [[email protected] /]# ^C [[email protected] /]#
[email protected]:~$ docker run -td centos:7 /bin/bash 7a8f3c2ba41b5b4c047dfe4b9d41774cb6972e1e969b38636affd39920b83b2f [email protected]:~$ docker attach 7a8f [[email protected] /]# ^C [email protected]:~$ docker ps -a CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 7a8f3c2ba41b centos:7 "/bin/bash" 18 seconds ago Up 17 seconds peaceful_bose
[email protected]:~$ docker run -itd centos:7 top -b 829a0f9c9ac70147783d3db9674def5c2179059956923a1d59ccd8a33f339b37 [email protected]:~$ docker attach 829a top - 02:49:10 up 19 days, 2:10, 0 users, load average: 0.55, 0.72, 0.91 Tasks: 1 total, 1 running, 0 sleeping, 0 stopped, 0 zombie %Cpu(s): 11.5 us, 2.3 sy, 0.0 ni, 86.2 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st KiB Mem : 8066788 total, 757280 free, 3659324 used, 3650184 buff/cache KiB Swap: 8282108 total, 7576884 free, 705224 used. 3384468 avail Mem PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 1 root 20 0 51780 3584 3140 R 0.0 0.0 0:00.05 top top - 02:49:10 up 19 days, 2:10, 0 users, load average: 0.55, 0.72, 0.91 Tasks: 1 total, 1 running, 0 sleeping, 0 stopped, 0 zombie %Cpu(s): 0.0 us, 0.0 sy, 0.0 ni,100.0 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st KiB Mem : 8066788 total, 757280 free, 3659324 used, 3650184 buff/cache KiB Swap: 8282108 total, 7576884 free, 705224 used. 3384468 avail Mem PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 1 root 20 0 51780 3584 3140 R 0.0 0.0 0:00.05 top^C [email protected]:~$ docker ps -a CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 829a0f9c9ac7 centos:7 "top -b" 12 seconds ago Exited (0) 2 seconds ago angry_yonath
[email protected]:~$ docker run -td centos:7 top -b 79cfe842cfba1b211dea37fafbc5676c9da2ad6e186f261cccee4098ee1dd02a [email protected]:~$ docker attach 79cf top - 02:50:02 up 19 days, 2:11, 0 users, load average: 0.33, 0.63, 0.87 Tasks: 1 total, 1 running, 0 sleeping, 0 stopped, 0 zombie %Cpu(s): 10.3 us, 2.8 sy, 0.0 ni, 86.8 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st KiB Mem : 8066788 total, 749208 free, 3659560 used, 3658020 buff/cache KiB Swap: 8282108 total, 7576884 free, 705224 used. 3376832 avail Mem PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 1 root 20 0 51780 3636 3192 R 0.0 0.0 0:00.05 top top - 02:50:02 up 19 days, 2:11, 0 users, load average: 0.33, 0.63, 0.87 Tasks: 1 total, 1 running, 0 sleeping, 0 stopped, 0 zombie %Cpu(s): 0.0 us, 0.0 sy, 0.0 ni,100.0 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st KiB Mem : 8066788 total, 749208 free, 3659560 used, 3658020 buff/cache KiB Swap: 8282108 total, 7576884 free, 705224 used. 3376832 avail Mem PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 1 root 20 0 51780 3636 3192 R 0.0 0.0 0:00.05 top^C [email protected]:~$ docker ps -a CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 79cfe842cfba centos:7 "top -b" 9 seconds ago Up 7 seconds loving_ride
可以看到，docker run指定-t选项时，是否指定-i选项也会对docker attach命令有影响。指定-i选项时，SIGINT信号发送到了容器中PID为1的进程，不指定时，信号被docker attach接收。注意，此时docker attach的--sig-proxy选项不起作用。
综上，docker attach后执行CTRL-c会发送SIGINT信号，而信号的接收方会受到docker run的-i、-t选项，和docker attach的--sig-proxy选项影响。
2. CTRL-p CTRL-q方式
CTRL-p CTRL-q方式比较简单，只需要注意docker run时要同时指定-it选项。该方式只会退出docker attach，对容器没有影响
[email protected]:~$ docker run -itd centos:7 /bin/bash e73e04370cee90497783a621fd4a044ca196dc419c06a01bae5a4b116d7afc4e [email protected]:~$ docker attach e73e
#执行CTRL-p CTRL-q [[email protected] /]# [email protected]:~$ [email protected]:~$ docker ps CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES e73e04370cee centos:7 "/bin/bash" 44 seconds ago Up 42 seconds vibrant_archimedes
如果-it选项没有同时指定，CTRL-p CTRL-q无法生效
3. 其他退出方式
上面两种方式在退出时都有限制，有没有其他方法呢？
其实，我们只要另起一个终端，把docker attach进程杀死就可以了
#终端1中执行，没有同时指定-it，CTRL-p CTRL-q无法退出
[email protected]:~$ docker run -id centos:7 /bin/bash
00c031965cb2075643e15a3b88141026e753a79e8b03b960880d505950488014
[email protected]:~$ docker attach 00c0 ^P /bin/bash: line 1: $‘ 20‘: command not found ^P
#终端2中杀死docker attach进程
[email protected]:~/dettach$ pkill -9 -f ‘docker.*attach‘ [email protected]:~/dettach$
#终端1中，docker attach已退出 /bin/bash: line 2: $‘ 20‘: command not found 已杀死 [email protected]:~$ docker ps -a CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 00c031965cb2 centos:7 "/bin/bash" About a minute ago Up About a minute trusting_meitner
参考文档：
1. man docker attach
2. https://docs.docker.com/engine/reference/commandline/attach/
3. https://stackoverflow.com/questions/25267372/correct-way-to-detach-from-a-container-without-stopping-it/45985809#45985809
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