1# attach
2
3<!---MARKER_GEN_START-->
4Attach local standard input, output, and error streams to a running container
5
6### Aliases
7
8`docker container attach`, `docker attach`
9
10### Options
11
12| Name | Type | Default | Description |
13|:--------------------------------|:---------|:--------|:----------------------------------------------------|
14| [`--detach-keys`](#detach-keys) | `string` | | Override the key sequence for detaching a container |
15| `--no-stdin` | | | Do not attach STDIN |
16| `--sig-proxy` | `bool` | `true` | Proxy all received signals to the process |
17
18
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20
21## Description
22
23Use `docker attach` to attach your terminal's standard input, output, and error
24(or any combination of the three) to a running container using the container's
25ID or name. This lets you view its output or control it interactively, as
26though the commands were running directly in your terminal.
27
28> **Note**
29>
30> The `attach` command displays the output of the container's `ENTRYPOINT` and
31> `CMD` process. This can appear as if the attach command is hung when in fact
32> the process may simply not be writing any output at that time.
33
34You can attach to the same contained process multiple times simultaneously,
35from different sessions on the Docker host.
36
37To stop a container, use `CTRL-c`. This key sequence sends `SIGKILL` to the
38container. If `--sig-proxy` is true (the default),`CTRL-c` sends a `SIGINT` to
39the container. If the container was run with `-i` and `-t`, you can detach from
40a container and leave it running using the `CTRL-p CTRL-q` key sequence.
41
42> **Note**
43>
44> A process running as PID 1 inside a container is treated specially by
45> Linux: it ignores any signal with the default action. So, the process
46> doesn't terminate on `SIGINT` or `SIGTERM` unless it's coded to do so.
47
48You can't redirect the standard input of a `docker attach` command while
49attaching to a TTY-enabled container (using the `-i` and `-t` options).
50
51While a client is connected to container's `stdio` using `docker attach`,
52Docker uses a ~1MB memory buffer to maximize the throughput of the application.
53Once this buffer is full, the speed of the API connection is affected, and so
54this impacts the output process' writing speed. This is similar to other
55applications like SSH. Because of this, it isn't recommended to run
56performance-critical applications that generate a lot of output in the
57foreground over a slow client connection. Instead, use the `docker logs`
58command to get access to the logs.
59
60## Examples
61
62### Attach to and detach from a running container
63
64The following example starts an Alpine container running `top` in detached mode,
65then attaches to the container;
66
67```console
68$ docker run -d --name topdemo alpine top -b
69
70$ docker attach topdemo
71
72Mem: 2395856K used, 5638884K free, 2328K shrd, 61904K buff, 1524264K cached
73CPU: 0% usr 0% sys 0% nic 99% idle 0% io 0% irq 0% sirq
74Load average: 0.15 0.06 0.01 1/567 6
75 PID PPID USER STAT VSZ %VSZ CPU %CPU COMMAND
76 1 0 root R 1700 0% 3 0% top -b
77```
78
79As the container was started without the `-i`, and `-t` options, signals are
80forwarded to the attached process, which means that the default `CTRL-p CTRL-q`
81detach key sequence produces no effect, but pressing `CTRL-c` terminates the
82container:
83
84```console
85<...>
86 PID PPID USER STAT VSZ %VSZ CPU %CPU COMMAND
87 1 0 root R 1700 0% 7 0% top -b
88^P^Q
89^C
90
91$ docker ps -a --filter name=topdemo
92
93CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
9496254a235bd6 alpine "top -b" 44 seconds ago Exited (130) 8 seconds ago topdemo
95```
96
97Repeating the example above, but this time with the `-i` and `-t` options set;
98
99```console
100$ docker run -dit --name topdemo2 ubuntu:22.04 /usr/bin/top -b
101```
102
103Now, when attaching to the container, and pressing the `CTRL-p CTRL-q` ("read
104escape sequence"), the Docker CLI is handling the detach sequence, and the
105`attach` command is detached from the container. Checking the container's status
106with `docker ps` shows that the container is still running in the background:
107
108```console
109$ docker attach topdemo2
110
111Mem: 2405344K used, 5629396K free, 2512K shrd, 65100K buff, 1524952K cached
112CPU: 0% usr 0% sys 0% nic 99% idle 0% io 0% irq 0% sirq
113Load average: 0.12 0.12 0.05 1/594 6
114 PID PPID USER STAT VSZ %VSZ CPU %CPU COMMAND
115 1 0 root R 1700 0% 3 0% top -b
116read escape sequence
117
118$ docker ps -a --filter name=topdemo2
119
120CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
121fde88b83c2c2 alpine "top -b" 22 seconds ago Up 21 seconds topdemo2
122```
123
124### Get the exit code of the container's command
125
126And in this second example, you can see the exit code returned by the `bash`
127process is returned by the `docker attach` command to its caller too:
128
129```console
130$ docker run --name test -dit alpine
131275c44472aebd77c926d4527885bb09f2f6db21d878c75f0a1c212c03d3bcfab
132
133$ docker attach test
134/# exit 13
135
136$ echo $?
13713
138
139$ docker ps -a --filter name=test
140
141CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
142a2fe3fd886db alpine "/bin/sh" About a minute ago Exited (13) 40 seconds ago test
143```
144
145### <a name="detach-keys"></a> Override the detach sequence (--detach-keys)
146
147Use the `--detach-keys` option to override the Docker key sequence for detach.
148This is useful if the Docker default sequence conflicts with key sequence you
149use for other applications. There are two ways to define your own detach key
150sequence, as a per-container override or as a configuration property on your
151entire configuration.
152
153To override the sequence for an individual container, use the
154`--detach-keys="<sequence>"` flag with the `docker attach` command. The format of
155the `<sequence>` is either a letter [a-Z], or the `ctrl-` combined with any of
156the following:
157
158* `a-z` (a single lowercase alpha character )
159* `@` (at sign)
160* `[` (left bracket)
161* `\\` (two backward slashes)
162* `_` (underscore)
163* `^` (caret)
164
165These `a`, `ctrl-a`, `X`, or `ctrl-\\` values are all examples of valid key
166sequences. To configure a different configuration default key sequence for all
167containers, see [**Configuration file** section](https://docs.docker.com/engine/reference/commandline/cli/#configuration-files).
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