6.7 KiB
| title | description | keywords |
|---|---|---|
| attach | The attach command description and usage | attach, running, container |
attach
Usage: docker attach [OPTIONS] CONTAINER
Attach local standard input, output, and error streams to a running container
Options:
--detach-keys string Override the key sequence for detaching a container
--help Print usage
--no-stdin Do not attach STDIN
--sig-proxy Proxy all received signals to the process (default true)
Description
Use docker attach to attach your terminal's standard input, output, and error
(or any combination of the three) to a running container using the container's
ID or name. This allows you to view its ongoing output or to control it
interactively, as though the commands were running directly in your terminal.
Note: The
attachcommand will display the output of theENTRYPOINT/CMDprocess. This can appear as if the attach command is hung when in fact the process may simply not be interacting with the terminal at that time.
You can attach to the same contained process multiple times simultaneously, from different sessions on the Docker host.
To stop a container, use CTRL-c. This key sequence sends SIGKILL to the
container. If --sig-proxy is true (the default),CTRL-c sends a SIGINT to
the container. If the container was run with -i and -t, you can detach from
a container and leave it running using the CTRL-p CTRL-q key sequence.
Note: A process running as PID 1 inside a container is treated specially by Linux: it ignores any signal with the default action. So, the process will not terminate on
SIGINTorSIGTERMunless it is coded to do so.
It is forbidden to redirect the standard input of a docker attach command
while attaching to a TTY-enabled container (using the -i and -t options).
While a client is connected to container's stdio using docker attach, Docker
uses a ~1MB memory buffer to maximize the throughput of the application.
Once this buffer is full, the speed of the API connection is affected, and so
this impacts the output process' writing speed. This is similar to other
applications like SSH. Because of this, it is not recommended to run
performance critical applications that generate a lot of output in the
foreground over a slow client connection. Instead, users should use the
docker logs command to get access to the logs.
Override the detach sequence
If you want, you can configure an override the Docker key sequence for detach. This is useful if the Docker default sequence conflicts with key sequence you use for other applications. There are two ways to define your own detach key sequence, as a per-container override or as a configuration property on your entire configuration.
To override the sequence for an individual container, use the
--detach-keys="<sequence>" flag with the docker attach command. The format of
the <sequence> is either a letter [a-Z], or the ctrl- combined with any of
the following:
a-z(a single lowercase alpha character )@(at sign)[(left bracket)\\(two backward slashes)_(underscore)^(caret)
These a, ctrl-a, X, or ctrl-\\ values are all examples of valid key
sequences. To configure a different configuration default key sequence for all
containers, see Configuration file section.
Examples
Attach to and detach from a running container
The following example starts an ubuntu container running top in detached mode,
then attaches to the container;
$ docker run -d --name topdemo ubuntu:22.04 /usr/bin/top -b
$ docker attach topdemo
top - 12:27:44 up 3 days, 21:54, 0 users, load average: 0.00, 0.00, 0.00
Tasks: 1 total, 1 running, 0 sleeping, 0 stopped, 0 zombie
%Cpu(s): 0.1 us, 0.1 sy, 0.0 ni, 99.8 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
MiB Mem : 3934.3 total, 770.1 free, 674.2 used, 2490.1 buff/cache
MiB Swap: 1024.0 total, 839.3 free, 184.7 used. 2814.0 avail Mem
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
1 root 20 0 7180 2896 2568 R 0.0 0.1 0:00.02 top
As the container was started without the -i, and -t options, signals are
forwarded to the attached process, which means that the default CTRL-p CTRL-q
detach key sequence produces no effect, but pressing CTRL-c terminates the
container:
<...>
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
1 root 20 0 7180 2896 2568 R 0.0 0.1 0:00.02 top^P^Q
^C
$ docker ps -a --filter name=topdemo
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
4cf0d0ebb079 ubuntu:22.04 "/usr/bin/top -b" About a minute ago Exited (0) About a minute ago topdemo
Repeating the example above, but this time with the -i and -t options set;
$ docker run -dit --name topdemo2 ubuntu:22.04 /usr/bin/top -b
Now, when attaching to the container, and pressing the CTRL-p CTRL-q ("read
escape sequence"), the Docker CLI is handling the detach sequence, and the
attach command is detached from the container. Checking the container's status
with docker ps shows that the container is still running in the background:
$ docker attach topdemo2
top - 12:44:32 up 3 days, 22:11, 0 users, load average: 0.00, 0.00, 0.00
Tasks: 1 total, 1 running, 0 sleeping, 0 stopped, 0 zombie
%Cpu(s): 50.0 us, 0.0 sy, 0.0 ni, 50.0 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
MiB Mem : 3934.3 total, 770.6 free, 672.4 used, 2491.4 buff/cache
MiB Swap: 1024.0 total, 839.3 free, 184.7 used. 2815.8 avail Mem
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
1 root 20 0 7180 2776 2452 R 0.0 0.1 0:00.02 topread escape sequence
$ docker ps -a --filter name=topdemo2
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
b1661dce0fc2 ubuntu:22.04 "/usr/bin/top -b" 2 minutes ago Up 2 minutes topdemo2
Get the exit code of the container's command
And in this second example, you can see the exit code returned by the bash
process is returned by the docker attach command to its caller too:
$ docker run --name test -dit alpine
275c44472aebd77c926d4527885bb09f2f6db21d878c75f0a1c212c03d3bcfab
$ docker attach test
/# exit 13
$ echo $?
13
$ docker ps -a --filter name=test
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
a2fe3fd886db alpine "/bin/sh" About a minute ago Exited (13) 40 seconds ago test