docker attach

Attach local standard input, output, and error streams to a running container

Usage

$
 
docker attach 
[
OPTIONS] CONTAINER

Refer to the options section for an overview of available OPTIONS for this command.

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 attach command will display the output of the ENTRYPOINT/CMD process. 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 SIGINT or SIGTERM unless 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.

For example uses of this command, refer to the examples section below.

Options

Name, shorthand
Default
Description

--detach-keys

Override the key sequence for detaching a container

--no-stdin

Do not attach STDIN

--sig-proxy
true
Proxy all received signals to the process

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