Running containers (original) (raw)

Docker runs processes in isolated containers. A container is a process which runs on a host. The host may be local or remote. When you execute docker run, the container process that runs is isolated in that it has its own file system, its own networking, and its own isolated process tree separate from the host.

This page details how to use the docker run command to run containers.

A docker run command takes the following form:

The docker run command must specify animage referenceto create the container from.

Image references

The image reference is the name and version of the image. You can use the image reference to create or run a container based on an image.

• docker run IMAGE[:TAG][@DIGEST]
• docker create IMAGE[:TAG][@DIGEST]

An image tag is the image version, which defaults to latest when omitted. Use the tag to run a container from specific version of an image. For example, to run version 24.04 of the ubuntu image: docker run ubuntu:24.04.

Image digests

Images using the v2 or later image format have a content-addressable identifier called a digest. As long as the input used to generate the image is unchanged, the digest value is predictable.

The following example runs a container from the alpine image with thesha256:9cacb71397b640eca97488cf08582ae4e4068513101088e9f96c9814bfda95e0 digest:

Options

[OPTIONS] let you configure options for the container. For example, you can give the container a name (--name), or run it as a background process (-d). You can also set options to control things like resource constraints and networking.

Commands and arguments

You can use the [COMMAND] and [ARG...] positional arguments to specify commands and arguments for the container to run when it starts up. For example, you can specify sh as the [COMMAND], combined with the -i and -t flags, to start an interactive shell in the container (if the image you select has ansh executable on PATH).

Note

Depending on your Docker system configuration, you may be required to preface the docker run command with sudo. To avoid having to use sudo with the docker command, your system administrator can create a Unix group called docker and add users to it. For more information about this configuration, refer to the Docker installation documentation for your operating system.

When you start a container, the container runs in the foreground by default. If you want to run the container in the background instead, you can use the--detach (or -d) flag. This starts the container without occupying your terminal window.

While the container runs in the background, you can interact with the container using other CLI commands. For example, docker logs lets you view the logs for the container, and docker attach brings it to the foreground.

For more information about docker run flags related to foreground and background modes, see:

• docker run --detach: run container in background
• docker run --attach: attach to stdin, stdout, and stderr
• docker run --tty: allocate a pseudo-tty
• docker run --interactive: keep stdin open even if not attached

For more information about re-attaching to a background container, seedocker attach.

You can identify a container in three ways:

The UUID identifier is a random ID assigned to the container by the daemon.

The daemon generates a random string name for containers automatically. You can also define a custom name usingthe --name flag. Defining a name can be a handy way to add meaning to a container. If you specify a name, you can use it when referring to the container in a user-defined network. This works for both background and foreground Docker containers.

A container identifier is not the same thing as an image reference. The image reference specifies which image to use when you run a container. You can't rundocker exec nginx:alpine sh to open a shell in a container based on thenginx:alpine image, because docker exec expects a container identifier (name or ID), not an image.

While the image used by a container is not an identifier for the container, you find out the IDs of containers using an image by using the --filter flag. For example, the following docker ps command gets the IDs of all running containers based on the nginx:alpine image:

For more information about using filters, seeFiltering.

Containers have networking enabled by default, and they can make outgoing connections. If you're running multiple containers that need to communicate with each other, you can create a custom network and attach the containers to the network.

When multiple containers are attached to the same custom network, they can communicate with each other using the container names as a DNS hostname. The following example creates a custom network named my-net, and runs two containers that attach to the network.

For more information about container networking, seeNetworking overview

By default, the data in a container is stored in an ephemeral, writable container layer. Removing the container also removes its data. If you want to use persistent data with containers, you can use filesystem mounts to store the data persistently on the host system. Filesystem mounts can also let you share data between containers and the host.

Docker supports two main categories of mounts:

• Volume mounts
• Bind mounts

Volume mounts are great for persistently storing data for containers, and for sharing data between containers. Bind mounts, on the other hand, are for sharing data between a container and the host.

You can add a filesystem mount to a container using the --mount flag for thedocker run command.

The following sections show basic examples of how to create volumes and bind mounts. For more in-depth examples and descriptions, refer to the section of thestorage section in the documentation.

Volume mounts

To create a volume mount:

The --mount flag takes two parameters in this case: source and target. The value for the source parameter is the name of the volume. The value oftarget is the mount location of the volume inside the container. Once you've created the volume, any data you write to the volume is persisted, even if you stop or remove the container:

The target must always be an absolute path, such as /src/docs. An absolute path starts with a / (forward slash). Volume names must start with an alphanumeric character, followed by a-z0-9, _ (underscore), . (period) or- (hyphen).

Bind mounts

To create a bind mount:

In this case, the --mount flag takes three parameters. A type (bind), and two paths. The source path is a the location on the host that you want to bind mount into the container. The target path is the mount destination inside the container.

Bind mounts are read-write by default, meaning that you can both read and write files to and from the mounted location from the container. Changes that you make, such as adding or editing files, are reflected on the host filesystem:

The exit code from docker run gives information about why the container failed to run or why it exited. The following sections describe the meanings of different container exit codes values.

125

Exit code 125 indicates that the error is with Docker daemon itself.

126

Exit code 126 indicates that the specified contained command can't be invoked. The container command in the following example is: /etc.

127

Exit code 127 indicates that the contained command can't be found.

Other exit codes

Any exit code other than 125, 126, and 127 represent the exit code of the provided container command.

The operator can also adjust the performance parameters of the container:

User memory constraints

We have four ways to set user memory usage:

Examples:

We set nothing about memory, this means the processes in the container can use as much memory and swap memory as they need.

We set memory limit and disabled swap memory limit, this means the processes in the container can use 300M memory and as much swap memory as they need (if the host supports swap memory).

We set memory limit only, this means the processes in the container can use 300M memory and 300M swap memory, by default, the total virtual memory size (--memory-swap) will be set as double of memory, in this case, memory + swap would be 2*300M, so processes can use 300M swap memory as well.

We set both memory and swap memory, so the processes in the container can use 300M memory and 700M swap memory.

Memory reservation is a kind of memory soft limit that allows for greater sharing of memory. Under normal circumstances, containers can use as much of the memory as needed and are constrained only by the hard limits set with the-m/--memory option. When memory reservation is set, Docker detects memory contention or low memory and forces containers to restrict their consumption to a reservation limit.

Always set the memory reservation value below the hard limit, otherwise the hard limit takes precedence. A reservation of 0 is the same as setting no reservation. By default (without reservation set), memory reservation is the same as the hard memory limit.

Memory reservation is a soft-limit feature and does not guarantee the limit won't be exceeded. Instead, the feature attempts to ensure that, when memory is heavily contended for, memory is allocated based on the reservation hints/setup.

The following example limits the memory (-m) to 500M and sets the memory reservation to 200M.

Under this configuration, when the container consumes memory more than 200M and less than 500M, the next system memory reclaim attempts to shrink container memory below 200M.

The following example set memory reservation to 1G without a hard memory limit.

The container can use as much memory as it needs. The memory reservation setting ensures the container doesn't consume too much memory for long time, because every memory reclaim shrinks the container's consumption to the reservation.

By default, kernel kills processes in a container if an out-of-memory (OOM) error occurs. To change this behaviour, use the --oom-kill-disable option. Only disable the OOM killer on containers where you have also set the-m/--memory option. If the -m flag is not set, this can result in the host running out of memory and require killing the host's system processes to free memory.

The following example limits the memory to 100M and disables the OOM killer for this container:

The following example, illustrates a dangerous way to use the flag:

The container has unlimited memory which can cause the host to run out memory and require killing system processes to free memory. The --oom-score-adjparameter can be changed to select the priority of which containers will be killed when the system is out of memory, with negative scores making them less likely to be killed, and positive scores more likely.

Kernel memory constraints

Kernel memory is fundamentally different than user memory as kernel memory can't be swapped out. The inability to swap makes it possible for the container to block system services by consuming too much kernel memory. Kernel memory includes：

• stack pages
• slab pages
• sockets memory pressure
• tcp memory pressure

You can setup kernel memory limit to constrain these kinds of memory. For example, every process consumes some stack pages. By limiting kernel memory, you can prevent new processes from being created when the kernel memory usage is too high.

Kernel memory is never completely independent of user memory. Instead, you limit kernel memory in the context of the user memory limit. Assume "U" is the user memory limit and "K" the kernel limit. There are three possible ways to set limits:

Examples:

We set memory and kernel memory, so the processes in the container can use 500M memory in total, in this 500M memory, it can be 50M kernel memory tops.

We set kernel memory without -m, so the processes in the container can use as much memory as they want, but they can only use 50M kernel memory.

Swappiness constraint

By default, a container's kernel can swap out a percentage of anonymous pages. To set this percentage for a container, specify a --memory-swappiness value between 0 and 100. A value of 0 turns off anonymous page swapping. A value of 100 sets all anonymous pages as swappable. By default, if you are not using--memory-swappiness, memory swappiness value will be inherited from the parent.

For example, you can set:

Setting the --memory-swappiness option is helpful when you want to retain the container's working set and to avoid swapping performance penalties.

By default, all containers get the same proportion of CPU cycles. This proportion can be modified by changing the container's CPU share weighting relative to the weighting of all other running containers.

To modify the proportion from the default of 1024, use the -c or --cpu-sharesflag to set the weighting to 2 or higher. If 0 is set, the system will ignore the value and use the default of 1024.

The proportion will only apply when CPU-intensive processes are running. When tasks in one container are idle, other containers can use the left-over CPU time. The actual amount of CPU time will vary depending on the number of containers running on the system.

For example, consider three containers, one has a cpu-share of 1024 and two others have a cpu-share setting of 512. When processes in all three containers attempt to use 100% of CPU, the first container would receive 50% of the total CPU time. If you add a fourth container with a cpu-share of 1024, the first container only gets 33% of the CPU. The remaining containers receive 16.5%, 16.5% and 33% of the CPU.

On a multi-core system, the shares of CPU time are distributed over all CPU cores. Even if a container is limited to less than 100% of CPU time, it can use 100% of each individual CPU core.

For example, consider a system with more than three cores. If you start one container {C0} with -c=512 running one process, and another container{C1} with -c=1024 running two processes, this can result in the following division of CPU shares:

PID    container	CPU	CPU share
100    {C0}		0	100% of CPU0
101    {C1}		1	100% of CPU1
102    {C1}		2	100% of CPU2

CPU period constraint

The default CPU CFS (Completely Fair Scheduler) period is 100ms. We can use--cpu-period to set the period of CPUs to limit the container's CPU usage. And usually --cpu-period should work with --cpu-quota.

Examples:

If there is 1 CPU, this means the container can get 50% CPU worth of run-time every 50ms.

In addition to use --cpu-period and --cpu-quota for setting CPU period constraints, it is possible to specify --cpus with a float number to achieve the same purpose. For example, if there is 1 CPU, then --cpus=0.5 will achieve the same result as setting --cpu-period=50000 and --cpu-quota=25000 (50% CPU).

The default value for --cpus is 0.000, which means there is no limit.

For more information, see theCFS documentation on bandwidth limiting.

Cpuset constraint

We can set cpus in which to allow execution for containers.

Examples:

This means processes in container can be executed on cpu 1 and cpu 3.

This means processes in container can be executed on cpu 0, cpu 1 and cpu 2.

We can set mems in which to allow execution for containers. Only effective on NUMA systems.

Examples:

This example restricts the processes in the container to only use memory from memory nodes 1 and 3.

This example restricts the processes in the container to only use memory from memory nodes 0, 1 and 2.

CPU quota constraint

The --cpu-quota flag limits the container's CPU usage. The default 0 value allows the container to take 100% of a CPU resource (1 CPU). The CFS (Completely Fair Scheduler) handles resource allocation for executing processes and is default Linux Scheduler used by the kernel. Set this value to 50000 to limit the container to 50% of a CPU resource. For multiple CPUs, adjust the --cpu-quota as necessary. For more information, see theCFS documentation on bandwidth limiting.

Block IO bandwidth (Blkio) constraint

By default, all containers get the same proportion of block IO bandwidth (blkio). This proportion is 500. To modify this proportion, change the container's blkio weight relative to the weighting of all other running containers using the --blkio-weight flag.

Note

The blkio weight setting is only available for direct IO. Buffered IO is not currently supported.

The --blkio-weight flag can set the weighting to a value between 10 to 1000. For example, the commands below create two containers with different blkio weight:

If you do block IO in the two containers at the same time, by, for example:

You'll find that the proportion of time is the same as the proportion of blkio weights of the two containers.

The --blkio-weight-device="DEVICE_NAME:WEIGHT" flag sets a specific device weight. The DEVICE_NAME:WEIGHT is a string containing a colon-separated device name and weight. For example, to set /dev/sda device weight to 200:

If you specify both the --blkio-weight and --blkio-weight-device, Docker uses the --blkio-weight as the default weight and uses --blkio-weight-deviceto override this default with a new value on a specific device. The following example uses a default weight of 300 and overrides this default on /dev/sda setting that weight to 200:

The --device-read-bps flag limits the read rate (bytes per second) from a device. For example, this command creates a container and limits the read rate to 1mbper second from /dev/sda:

The --device-write-bps flag limits the write rate (bytes per second) to a device. For example, this command creates a container and limits the write rate to 1mbper second for /dev/sda:

Both flags take limits in the <device-path>:<limit>[unit] format. Both read and write rates must be a positive integer. You can specify the rate in kb(kilobytes), mb (megabytes), or gb (gigabytes).

The --device-read-iops flag limits read rate (IO per second) from a device. For example, this command creates a container and limits the read rate to1000 IO per second from /dev/sda:

The --device-write-iops flag limits write rate (IO per second) to a device. For example, this command creates a container and limits the write rate to1000 IO per second to /dev/sda:

Both flags take limits in the <device-path>:<limit> format. Both read and write rates must be a positive integer.

By default, the docker container process runs with the supplementary groups looked up for the specified user. If one wants to add more to that list of groups, then one can use this flag:

By default, Docker containers are "unprivileged" and cannot, for example, run a Docker daemon inside a Docker container. This is because by default a container is not allowed to access any devices, but a "privileged" container is given access to all devices (see the documentation oncgroups devices).

The --privileged flag gives all capabilities to the container. When the operator executes docker run --privileged, Docker enables access to all devices on the host, and reconfigures AppArmor or SELinux to allow the container nearly all the same access to the host as processes running outside containers on the host. Use this flag with caution. For more information about the --privileged flag, see thedocker run reference.

If you want to limit access to a specific device or devices you can use the --device flag. It allows you to specify one or more devices that will be accessible within the container.

By default, the container will be able to read, write, and mknod these devices. This can be overridden using a third :rwm set of options to each --device flag:

In addition to --privileged, the operator can have fine grain control over the capabilities using --cap-add and --cap-drop. By default, Docker has a default list of capabilities that are kept. The following table lists the Linux capability options which are allowed by default and can be dropped.

The next table shows the capabilities which are not granted by default and may be added.

Further reference information is available on thecapabilities(7) - Linux man page, and in theLinux kernel source code.

Both flags support the value ALL, so to allow a container to use all capabilities except for MKNOD:

The --cap-add and --cap-drop flags accept capabilities to be specified with a CAP_ prefix. The following examples are therefore equivalent:

For interacting with the network stack, instead of using --privileged they should use --cap-add=NET_ADMIN to modify the network interfaces.

To mount a FUSE based filesystem, you need to combine both --cap-add and--device:

The default seccomp profile will adjust to the selected capabilities, in order to allow use of facilities allowed by the capabilities, so you should not have to adjust this.

When you build an image from aDockerfile, or when committing it, you can set a number of default parameters that take effect when the image starts up as a container. When you run an image, you can override those defaults using flags for the docker run command.

• Default entrypoint
• Default command and options
• Expose ports
• Environment variables
• Healthcheck
• User
• Working directory

Default command and options

The command syntax for docker run supports optionally specifying commands and arguments to the container's entrypoint, represented as [COMMAND] and[ARG...] in the following synopsis example:

This command is optional because whoever created the IMAGE may have already provided a default COMMAND, using the Dockerfile CMD instruction. When you run a container, you can override that CMD instruction just by specifying a new COMMAND.

If the image also specifies an ENTRYPOINT then the CMD or COMMANDget appended as arguments to the ENTRYPOINT.

Default entrypoint

The entrypoint refers to the default executable that's invoked when you run a container. A container's entrypoint is defined using the DockerfileENTRYPOINT instruction. It's similar to specifying a default command because it specifies, but the difference is that you need to pass an explicit flag to override the entrypoint, whereas you can override default commands with positional arguments. The defines a container's default behavior, with the idea that when you set an entrypoint you can run the container as if it were that binary, complete with default options, and you can pass in more options as commands. But there are cases where you may want to run something else inside the container. This is when overriding the default entrypoint at runtime comes in handy, using the --entrypoint flag for the docker run command.

The --entrypoint flag expects a string value, representing the name or path of the binary that you want to invoke when the container starts. The following example shows you how to run a Bash shell in a container that has been set up to automatically run some other binary (like /usr/bin/redis-server):

The following examples show how to pass additional parameters to the custom entrypoint, using the positional command arguments:

You can reset a containers entrypoint by passing an empty string, for example:

Note

Passing --entrypoint clears out any default command set on the image. That is, any CMD instruction in the Dockerfile used to build it.

Exposed ports

By default, when you run a container, none of the container's ports are exposed to the host. This means you won't be able to access any ports that the container might be listening on. To make a container's ports accessible from the host, you need to publish the ports.

You can start the container with the -P or -p flags to expose its ports:

• The -P (or --publish-all) flag publishes all the exposed ports to the host. Docker binds each exposed port to a random port on the host.
  The -P flag only publishes port numbers that are explicitly flagged as exposed, either using the Dockerfile EXPOSE instruction or the --exposeflag for the docker run command.
• The -p (or --publish) flag lets you explicitly map a single port or range of ports in the container to the host.

The port number inside the container (where the service listens) doesn't need to match the port number published on the outside of the container (where clients connect). For example, inside the container an HTTP service might be listening on port 80. At runtime, the port might be bound to 42800 on the host. To find the mapping between the host ports and the exposed ports, use thedocker port command.

Environment variables

Docker automatically sets some environment variables when creating a Linux container. Docker doesn't set any environment variables when creating a Windows container.

The following environment variables are set for Linux containers:

Additionally, you can set any environment variable in the container by using one or more -e flags. You can even override the variables mentioned above, or variables defined using a Dockerfile ENV instruction when building the image.

If the you name an environment variable without specifying a value, the current value of the named variable on the host is propagated into the container's environment:

Healthchecks

The following flags for the docker run command let you control the parameters for container healthchecks:

Example:

The health status is also displayed in the docker ps output.

User

The default user within a container is root (uid = 0). You can set a default user to run the first process with the Dockerfile USER instruction. When starting a container, you can override the USER instruction by passing the-u option.

The followings examples are all valid:

Note

If you pass a numeric user ID, it must be in the range of 0-2147483647. If you pass a username, the user must exist in the container.

Working directory

The default working directory for running binaries within a container is the root directory (/). The default working directory of an image is set using the Dockerfile WORKDIR command. You can override the default working directory for an image using the -w (or --workdir) flag for the docker run command:

If the directory doesn't already exist in the container, it's created.