COPY

COPY has two forms:

  • COPY [--chown=<user>:<group>] <src>... <dest>
  • COPY [--chown=<user>:<group>] ["<src>",... "<dest>"] (this form is required for paths containing whitespace)

Note: The --chown feature is only supported on Dockerfiles used to build Linux containers, and will not work on Windows containers.

Since user and group ownership concepts do not translate between Linux and Windows, the use of /etc/passwd and /etc/group for translating user and group names to IDs restricts this feature to only be viable for Linux OS-based containers.

The COPY instruction copies new files or directories from <src> and adds them to the filesystem of the container at the path <dest>.

Multiple <src> resources may be specified but the paths of files and directories will be interpreted as relative to the source of the context of the build.

Each <src> may contain wildcards and matching will be done using Go’s filepath.Match rules.

For example:

COPY hom* /mydir/        # adds all files starting with "hom"
COPY hom?.txt /mydir/ # ? is replaced with any single character, e.g., "home.txt"

  

The <dest> is an absolute path, or a path relative to WORKDIR, into which the source will be copied inside the destination container.

COPY test relativeDir/   # adds "test" to `WORKDIR`/relativeDir/
COPY test /absoluteDir/ # adds "test" to /absoluteDir/

  

When copying files or directories that contain special characters (such as [ and ]), you need to escape those paths following the Golang rules to prevent them from being treated as a matching pattern.

For example, to copy a file named arr[0].txt, use the following;

COPY arr[[]0].txt /mydir/    # copy a file named "arr[0].txt" to /mydir/

  

All new files and directories are created with a UID and GID of 0, unless the optional --chown flag specifies a given username, groupname, or UID/GID combination to request specific ownership of the copied content.

The format of the --chown flag allows for either username and groupname strings or direct integer UID and GID in any combination.

Providing a username without groupname or a UID without GID will use the same numeric UID as the GID.

If a username or groupname is provided, the container’s root filesystem /etc/passwd and /etc/group files will be used to perform the translation from name to integer UID or GID respectively.

The following examples show valid definitions for the --chown flag:

COPY --chown=55:mygroup files* /somedir/
COPY --chown=bin files* /somedir/
COPY --chown=1 files* /somedir/
COPY --chown=10:11 files* /somedir/

  

If the container root filesystem does not contain either /etc/passwd or /etc/group files and either user or group names are used in the --chown flag, the build will fail on the COPY operation.

Using numeric IDs requires no lookup and will not depend on container root filesystem content.

Note: If you build using STDIN (docker build - < somefile), there is no build context, so COPY can’t be used.

Optionally COPY accepts a flag --from=<name|index> that can be used to set the source location to a previous build stage (created with FROM .. AS <name>) that will be used instead of a build context sent by the user.

The flag also accepts a numeric index assigned for all previous build stages started with FROM instruction.

In case a build stage with a specified name can’t be found an image with the same name is attempted to be used instead.

COPY obeys the following rules:

  • The <src> path must be inside the context of the build; you cannot COPY ../something /something, because the first step of a docker build is to send the context directory (and subdirectories) to the docker daemon.

  • If <src> is a directory, the entire contents of the directory are copied, including filesystem metadata.

Note: The directory itself is not copied, just its contents.

  • If <src> is any other kind of file, it is copied individually along with its metadata. In this case, if <dest> ends with a trailing slash /, it will be considered a directory and the contents of <src> will be written at <dest>/base(<src>).

  • If multiple <src> resources are specified, either directly or due to the use of a wildcard, then <dest> must be a directory, and it must end with a slash /.

  • If <dest> does not end with a trailing slash, it will be considered a regular file and the contents of <src> will be written at <dest>.

  • If <dest> doesn’t exist, it is created along with all missing directories in its path.

ENTRYPOINT

ENTRYPOINT has two forms:

  • ENTRYPOINT ["executable", "param1", "param2"] (exec form, preferred)
  • ENTRYPOINT command param1 param2 (shell form)

An ENTRYPOINT allows you to configure a container that will run as an executable.

For example, the following will start nginx with its default content, listening on port 80:

docker run -i -t --rm -p 80:80 nginx

Command line arguments to docker run <image> will be appended after all elements in an exec form ENTRYPOINT, and will override all elements specified using CMD.

This allows arguments to be passed to the entry point, i.e., docker run <image> -d will pass the -d argument to the entry point.

You can override the ENTRYPOINT instruction using the docker run --entrypoint flag.

The shell form prevents any CMD or run command line arguments from being used, but has the disadvantage that your ENTRYPOINT will be started as a subcommand of /bin/sh -c, which does not pass signals.

This means that the executable will not be the container’s PID 1 - and will not receive Unix signals - so your executable will not receive a SIGTERM from docker stop <container>.

Only the last ENTRYPOINT instruction in the Dockerfile will have an effect.

Exec form ENTRYPOINT example

You can use the exec form of ENTRYPOINT to set fairly stable default commands and arguments and then use either form of CMD to set additional defaults that are more likely to be changed.

FROM ubuntu
ENTRYPOINT ["top", "-b"]
CMD ["-c"]

When you run the container, you can see that top is the only process:

$ docker run -it --rm --name test  top -H
top - 08:25:00 up 7:27, 0 users, load average: 0.00, 0.01, 0.05
Threads: 1 total, 1 running, 0 sleeping, 0 stopped, 0 zombie
%Cpu(s): 0.1 us, 0.1 sy, 0.0 ni, 99.7 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
KiB Mem: 2056668 total, 1616832 used, 439836 free, 99352 buffers
KiB Swap: 1441840 total, 0 used, 1441840 free. 1324440 cached Mem PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
1 root 20 0 19744 2336 2080 R 0.0 0.1 0:00.04 top

  

To examine the result further, you can use docker exec:

$ docker exec -it test ps aux
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
root 1 2.6 0.1 19752 2352 ? Ss+ 08:24 0:00 top -b -H
root 7 0.0 0.1 15572 2164 ? R+ 08:25 0:00 ps aux

And you can gracefully request top to shut down using docker stop test.

The following Dockerfile shows using the ENTRYPOINT to run Apache in the foreground (i.e., as PID 1):

FROM debian:stable
RUN apt-get update && apt-get install -y --force-yes apache2
EXPOSE 80 443
VOLUME ["/var/www", "/var/log/apache2", "/etc/apache2"]
ENTRYPOINT ["/usr/sbin/apache2ctl", "-D", "FOREGROUND"]

 

If you need to write a starter script for a single executable, you can ensure that the final executable receives the Unix signals by using exec and gosu commands: 

#!/usr/bin/env bash
set -e if [ "$1" = 'postgres' ]; then
chown -R postgres "$PGDATA" if [ -z "$(ls -A "$PGDATA")" ]; then
gosu postgres initdb
fi exec gosu postgres "$@"
fi exec "$@"

  

Lastly, if you need to do some extra cleanup (or communicate with other containers) on shutdown, or are co-ordinating more than one executable, you may need to ensure that the ENTRYPOINT script receives the Unix signals, passes them on, and then does some more work:

#!/bin/sh
# Note: I've written this using sh so it works in the busybox container too # USE the trap if you need to also do manual cleanup after the service is stopped,
# or need to start multiple services in the one container
trap "echo TRAPed signal" HUP INT QUIT TERM # start service in background here
/usr/sbin/apachectl start echo "[hit enter key to exit] or run 'docker stop <container>'"
read # stop service and clean up here
echo "stopping apache"
/usr/sbin/apachectl stop echo "exited $0"

If you run this image with docker run -it --rm -p 80:80 --name test apache, you can then examine the container’s processes with docker exec, or docker top, and then ask the script to stop Apache:

$ docker exec -it test ps aux
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
root 1 0.1 0.0 4448 692 ? Ss+ 00:42 0:00 /bin/sh /run.sh 123 cmd cmd2
root 19 0.0 0.2 71304 4440 ? Ss 00:42 0:00 /usr/sbin/apache2 -k start
www-data 20 0.2 0.2 360468 6004 ? Sl 00:42 0:00 /usr/sbin/apache2 -k start
www-data 21 0.2 0.2 360468 6000 ? Sl 00:42 0:00 /usr/sbin/apache2 -k start
root 81 0.0 0.1 15572 2140 ? R+ 00:44 0:00 ps aux
$ docker top test
PID USER COMMAND
10035 root {run.sh} /bin/sh /run.sh 123 cmd cmd2
10054 root /usr/sbin/apache2 -k start
10055 33 /usr/sbin/apache2 -k start
10056 33 /usr/sbin/apache2 -k start
$ /usr/bin/time docker stop test
test
real 0m 0.27s
user 0m 0.03s
sys 0m 0.03s

   

Note: you can override the ENTRYPOINT setting using --entrypoint, but this can only set the binary to exec (no sh -c will be used).

Note: The exec form is parsed as a JSON array, which means that you must use double-quotes (“) around words not single-quotes (‘).

Note: Unlike the shell form, the exec form does not invoke a command shell. This means that normal shell processing does not happen.

For example, ENTRYPOINT [ "echo", "$HOME" ] will not do variable substitution on $HOME.

If you want shell processing then either use the shell form or execute a shell directly, for example: ENTRYPOINT [ "sh", "-c", "echo $HOME" ].

When using the exec form and executing a shell directly, as in the case for the shell form, it is the shell that is doing the environment variable expansion, not docker.

Shell form ENTRYPOINT example

You can specify a plain string for the ENTRYPOINT and it will execute in /bin/sh -c.

This form will use shell processing to substitute shell environment variables, and will ignore any CMD or docker run command line arguments.

To ensure that docker stop will signal any long running ENTRYPOINT executable correctly, you need to remember to start it with exec:

FROM ubuntu
ENTRYPOINT exec top -b

  

When you run this image, you’ll see the single PID 1 process:

$ docker run -it --rm --name test top
Mem: 1704520K used, 352148K free, 0K shrd, 0K buff, 140368121167873K cached
CPU: 5% usr 0% sys 0% nic 94% idle 0% io 0% irq 0% sirq
Load average: 0.08 0.03 0.05 2/98 6
PID PPID USER STAT VSZ %VSZ %CPU COMMAND
1 0 root R 3164 0% 0% top -b

Which will exit cleanly on docker stop:

$ /usr/bin/time docker stop test
test
real 0m 0.20s
user 0m 0.02s
sys 0m 0.04s

  

If you forget to add exec to the beginning of your ENTRYPOINT:

FROM ubuntu
ENTRYPOINT top -b
CMD --ignored-param1

  

You can then run it (giving it a name for the next step):

$ docker run -it --name test top --ignored-param2
Mem: 1704184K used, 352484K free, 0K shrd, 0K buff, 140621524238337K cached
CPU: 9% usr 2% sys 0% nic 88% idle 0% io 0% irq 0% sirq
Load average: 0.01 0.02 0.05 2/101 7
PID PPID USER STAT VSZ %VSZ %CPU COMMAND
1 0 root S 3168 0% 0% /bin/sh -c top -b cmd cmd2
7 1 root R 3164 0% 0% top -b

  

You can see from the output of top that the specified ENTRYPOINT is not PID 1.

If you then run docker stop test, the container will not exit cleanly - the stop command will be forced to send a SIGKILL after the timeout:

$ docker exec -it test ps aux
PID USER COMMAND
1 root /bin/sh -c top -b cmd cmd2
7 root top -b
8 root ps aux
$ /usr/bin/time docker stop test
test
real 0m 10.19s
user 0m 0.04s
sys 0m 0.03s

 

Understand how CMD and ENTRYPOINT interact

Both CMD and ENTRYPOINT instructions define what command gets executed when running a container. There are few rules that describe their co-operation.

  1. Dockerfile should specify at least one of CMD or ENTRYPOINT commands.

  2. ENTRYPOINT should be defined when using the container as an executable.

  3. CMD should be used as a way of defining default arguments for an ENTRYPOINT command or for executing an ad-hoc command in a container.

  4. CMD will be overridden when running the container with alternative arguments.

The table below shows what command is executed for different ENTRYPOINT / CMD combinations:

  No ENTRYPOINT ENTRYPOINT exec_entry p1_entry ENTRYPOINT [“exec_entry”, “p1_entry”]
No CMD error, not allowed /bin/sh -c exec_entry p1_entry exec_entry p1_entry
CMD [“exec_cmd”, “p1_cmd”] exec_cmd p1_cmd /bin/sh -c exec_entry p1_entry exec_entry p1_entry exec_cmd p1_cmd
CMD [“p1_cmd”, “p2_cmd”] p1_cmd p2_cmd /bin/sh -c exec_entry p1_entry exec_entry p1_entry p1_cmd p2_cmd
CMD exec_cmd p1_cmd /bin/sh -c exec_cmd p1_cmd /bin/sh -c exec_entry p1_entry exec_entry p1_entry /bin/sh -c exec_cmd p1_cmd

VOLUME

VOLUME ["/data"]

The VOLUME instruction creates a mount point with the specified name and marks it as holding externally mounted volumes from native host or other containers.

The value can be a JSON array, VOLUME ["/var/log/"], or a plain string with multiple arguments, such as VOLUME /var/log or VOLUME /var/log /var/db.

For more information/examples and mounting instructions via the Docker client, refer to Share Directories via Volumes documentation.

The docker run command initializes the newly created volume with any data that exists at the specified location within the base image. For example, consider the following Dockerfile snippet:

FROM ubuntu
RUN mkdir /myvol
RUN echo "hello world" > /myvol/greeting
VOLUME /myvol

This Dockerfile results in an image that causes docker run to create a new mount point at /myvol and copy the greeting file into the newly created volume.

Notes about specifying volumes

Keep the following things in mind about volumes in the Dockerfile.

  • Volumes on Windows-based containers: When using Windows-based containers, the destination of a volume inside the container must be one of:

    • a non-existing or empty directory
    • a drive other than C:
  • Changing the volume from within the Dockerfile: If any build steps change the data within the volume after it has been declared, those changes will be discarded.

  • JSON formatting: The list is parsed as a JSON array. You must enclose words with double quotes (")rather than single quotes (').

  • The host directory is declared at container run-time: The host directory (the mountpoint) is, by its nature, host-dependent. This is to preserve image portability, since a given host directory can’t be guaranteed to be available on all hosts. For this reason, you can’t mount a host directory from within the Dockerfile. The VOLUME instruction does not support specifying a host-dir parameter. You must specify the mountpoint when you create or run the container.

USER

USER <user>[:<group>] or
USER <UID>[:<GID>]

  

The USER instruction sets the user name (or UID) and optionally the user group (or GID) to use when running the image and for any RUN, CMD and ENTRYPOINT instructions that follow it in the Dockerfile.

Warning: When the user doesn’t have a primary group then the image (or the next instructions) will be run with the root group.

On Windows, the user must be created first if it’s not a built-in account. This can be done with the net user command called as part of a Dockerfile.

    FROM microsoft/windowsservercore
# Create Windows user in the container
RUN net user /add patrick
# Set it for subsequent commands
USER patrick

  


WORKDIR

WORKDIR /path/to/workdir

 

The WORKDIR instruction sets the working directory for any RUN, CMD, ENTRYPOINT, COPY and ADD instructions that follow it in the Dockerfile.

If the WORKDIR doesn’t exist, it will be created even if it’s not used in any subsequent Dockerfile instruction.

The WORKDIR instruction can be used multiple times in a Dockerfile.

If a relative path is provided, it will be relative to the path of the previous WORKDIR instruction.

For example:

WORKDIR /a
WORKDIR b
WORKDIR c
RUN pwd

The output of the final pwd command in this Dockerfile would be /a/b/c.

The WORKDIR instruction can resolve environment variables previously set using ENV.

You can only use environment variables explicitly set in the Dockerfile.

For example:

ENV DIRPATH /path
WORKDIR $DIRPATH/$DIRNAME
RUN pwd

The output of the final pwd command in this Dockerfile would be /path/$DIRNAME  

ARG

ARG <name>[=<default value>]

  

The ARG instruction defines a variable that users can pass at build-time to the builder with the docker build command using the --build-arg <varname>=<value> flag.

If a user specifies a build argument that was not defined in the Dockerfile, the build outputs a warning.

[Warning] One or more build-args [foo] were not consumed.

A Dockerfile may include one or more ARG instructions. For example, the following is a valid Dockerfile:  

FROM busybox
ARG user1
ARG buildno
...

   

Warning: It is not recommended to use build-time variables for passing secrets like github keys, user credentials etc. Build-time variable values are visible to any user of the image with the docker history command.

Default values

An ARG instruction can optionally include a default value:

FROM busybox
ARG user1=someuser
ARG buildno=1
...

If an ARG instruction has a default value and if there is no value passed at build-time, the builder uses the default.  

  

Scope

An ARG variable definition comes into effect from the line on which it is defined in the Dockerfile not from the argument’s use on the command-line or elsewhere. For example, consider this Dockerfile:

1 FROM busybox
2 USER ${user:-some_user}
3 ARG user
4 USER $user
...

A user builds this file by calling: 

$ docker build --build-arg user=what_user .

The USER at line 2 evaluates to some_user as the user variable is defined on the subsequent line 3. The USER at line 4 evaluates to what_user as user is defined and the what_user value was passed on the command line.

Prior to its definition by an ARG instruction, any use of a variable results in an empty string.

An ARG instruction goes out of scope at the end of the build stage where it was defined. To use an arg in multiple stages, each stage must include the ARG instruction.

FROM busybox
ARG SETTINGS
RUN ./run/setup $SETTINGS FROM busybox
ARG SETTINGS
RUN ./run/other $SETTINGS

  

Using ARG variables

You can use an ARG or an ENV instruction to specify variables that are available to the RUN instruction.

Environment variables defined using the ENV instruction always override an ARG instruction of the same name.

Consider this Dockerfile with an ENV and ARG instruction.

1 FROM ubuntu
2 ARG CONT_IMG_VER
3 ENV CONT_IMG_VER v1.0.0
4 RUN echo $CONT_IMG_VER

Then, assume this image is built with this command:

$ docker build --build-arg CONT_IMG_VER=v2.0.1 .

In this case, the RUN instruction uses v1.0.0 instead of the ARG setting passed by the user:v2.0.1 This behavior is similar to a shell script where a locally scoped variable overrides the variables passed as arguments or inherited from environment, from its point of definition.

Using the example above but a different ENV specification you can create more useful interactions between ARG and ENV instructions:

1 FROM ubuntu
2 ARG CONT_IMG_VER
3 ENV CONT_IMG_VER ${CONT_IMG_VER:-v1.0.0}
4 RUN echo $CONT_IMG_VER

Unlike an ARG instruction, ENV values are always persisted in the built image. Consider a docker build without the --build-arg flag:  

$ docker build .

Using this Dockerfile example, CONT_IMG_VER is still persisted in the image but its value would be v1.0.0 as it is the default set in line 3 by the ENV instruction.

The variable expansion technique in this example allows you to pass arguments from the command line and persist them in the final image by leveraging the ENV instruction.

Variable expansion is only supported for a limited set of Dockerfile instructions.

Predefined ARGs

Docker has a set of predefined ARG variables that you can use without a corresponding ARG instruction in the Dockerfile.

  • HTTP_PROXY
  • http_proxy
  • HTTPS_PROXY
  • https_proxy
  • FTP_PROXY
  • ftp_proxy
  • NO_PROXY
  • no_proxy

To use these, simply pass them on the command line using the flag:

--build-arg <varname>=<value>

By default, these pre-defined variables are excluded from the output of docker history.

Excluding them reduces the risk of accidentally leaking sensitive authentication information in an HTTP_PROXY variable.

For example, consider building the following Dockerfile using 

--build-arg HTTP_PROXY=http://user:pass@proxy.lon.example.com
FROM ubuntu
RUN echo "Hello World"

In this case, the value of the HTTP_PROXY variable is not available in the docker history and is not cached.

If you were to change location, and your proxy server changed to http://user:pass@proxy.sfo.example.com, a subsequent build does not result in a cache miss.

If you need to override this behaviour then you may do so by adding an ARG statement in the Dockerfile as follows:

FROM ubuntu
ARG HTTP_PROXY
RUN echo "Hello World"

When building this Dockerfile, the HTTP_PROXY is preserved in the docker history, and changing its value invalidates the build cache.  

 

Impact on build caching

ARG variables are not persisted into the built image as ENV variables are.

However, ARG variables do impact the build cache in similar ways.

If a Dockerfile defines an ARG variable whose value is different from a previous build, then a “cache miss” occurs upon its first usage, not its definition. In particular, all RUN instructions following an ARG instruction use the ARG variable implicitly (as an environment variable), thus can cause a cache miss.

All predefined ARG variables are exempt from caching unless there is a matching ARG statement in the Dockerfile.

For example, consider these two Dockerfile:

1 FROM ubuntu
2 ARG CONT_IMG_VER
3 RUN echo $CONT_IMG_VER

 

1 FROM ubuntu
2 ARG CONT_IMG_VER
3 RUN echo hello

 

If you specify --build-arg CONT_IMG_VER=<value> on the command line, in both cases, the specification on line 2 does not cause a cache miss; line 3 does cause a cache miss.

ARG CONT_IMG_VER causes the RUN line to be identified as the same as running CONT_IMG_VER=<value> echo hello, so if the <value> changes, we get a cache miss.

Consider another example under the same command line:

1 FROM ubuntu
2 ARG CONT_IMG_VER
3 ENV CONT_IMG_VER $CONT_IMG_VER
4 RUN echo $CONT_IMG_VER

In this example, the cache miss occurs on line 3. The miss happens because the variable’s value in the ENV references the ARG variable and that variable is changed through the command line.

In this example, the ENV command causes the image to include the value.  

If an ENV instruction overrides an ARG instruction of the same name, like this Dockerfile:

1 FROM ubuntu
2 ARG CONT_IMG_VER
3 ENV CONT_IMG_VER hello
4 RUN echo $CONT_IMG_VER

Line 3 does not cause a cache miss because the value of CONT_IMG_VER is a constant (hello).

As a result, the environment variables and values used on the RUN (line 4) doesn’t change between builds.  

ONBUILD

ONBUILD [INSTRUCTION]

The ONBUILD instruction adds to the image a trigger instruction to be executed at a later time, when the image is used as the base for another build.

The trigger will be executed in the context of the downstream build, as if it had been inserted immediately after the FROM instruction in the downstream Dockerfile.

Any build instruction can be registered as a trigger.

This is useful if you are building an image which will be used as a base to build other images,

for example an application build environment or a daemon which may be customized with user-specific configuration.

For example, if your image is a reusable Python application builder, it will require application source code to be added in a particular directory, and it might require a build script to be called after that.

You can’t just call ADD and RUN now, because you don’t yet have access to the application source code, and it will be different for each application build.

You could simply provide application developers with a boilerplate Dockerfile to copy-paste into their application, but that is inefficient, error-prone and difficult to update because it mixes with application-specific code.

The solution is to use ONBUILD to register advance instructions to run later, during the next build stage.

Here’s how it works:

  1. When it encounters an ONBUILD instruction, the builder adds a trigger to the metadata of the image being built. The instruction does not otherwise affect the current build.
  2. At the end of the build, a list of all triggers is stored in the image manifest, under the key OnBuild. They can be inspected with the docker inspect command.
  3. Later the image may be used as a base for a new build, using the FROM instruction. As part of processing the FROM instruction, the downstream builder looks for ONBUILD triggers, and executes them in the same order they were registered. If any of the triggers fail, the FROM instruction is aborted which in turn causes the build to fail. If all triggers succeed, the FROM instruction completes and the build continues as usual.
  4. Triggers are cleared from the final image after being executed. In other words they are not inherited by “grand-children” builds.

For example you might add something like this:

[...]
ONBUILD ADD . /app/src
ONBUILD RUN /usr/local/bin/python-build --dir /app/src
[...]
 

Warning: Chaining ONBUILD instructions using ONBUILD ONBUILD isn’t allowed.

Warning: The ONBUILD instruction may not trigger FROM or MAINTAINER instructions.

STOPSIGNAL

STOPSIGNAL signal

The STOPSIGNAL instruction sets the system call signal that will be sent to the container to exit.

This signal can be a valid unsigned number that matches a position in the kernel’s syscall table, for instance 9, or a signal name in the format SIGNAME, for instance SIGKILL.

HEALTHCHECK

The HEALTHCHECK instruction has two forms:

  • HEALTHCHECK [OPTIONS] CMD command (check container health by running a command inside the container)
  • HEALTHCHECK NONE (disable any healthcheck inherited from the base image)

The HEALTHCHECK instruction tells Docker how to test a container to check that it is still working. This can detect cases such as a web server that is stuck in an infinite loop and unable to handle new connections, even though the server process is still running.

When a container has a healthcheck specified, it has a health status in addition to its normal status.

This status is initially starting. Whenever a health check passes, it becomes healthy (whatever state it was previously in). After a certain number of consecutive failures, it becomes unhealthy.

The options that can appear before CMD are:

  • --interval=DURATION (default: 30s)
  • --timeout=DURATION (default: 30s)
  • --start-period=DURATION (default: 0s)
  • --retries=N (default: 3)

The health check will first run interval seconds after the container is started, and then again interval seconds after each previous check completes.

If a single run of the check takes longer than timeout seconds then the check is considered to have failed.

It takes retries consecutive failures of the health check for the container to be considered unhealthy.

start period provides initialization time for containers that need time to bootstrap.

Probe failure during that period will not be counted towards the maximum number of retries.

However, if a health check succeeds during the start period, the container is considered started and all consecutive failures will be counted towards the maximum number of retries.

There can only be one HEALTHCHECK instruction in a Dockerfile. If you list more than one then only the last HEALTHCHECK will take effect.

The command after the CMD keyword can be either a shell command (e.g. HEALTHCHECK CMD /bin/check-running) or an exec array (as with other Dockerfile commands; see e.g. ENTRYPOINT for details).

The command’s exit status indicates the health status of the container. The possible values are:

  • 0: success - the container is healthy and ready for use
  • 1: unhealthy - the container is not working correctly
  • 2: reserved - do not use this exit code

For example, to check every five minutes or so that a web-server is able to serve the site’s main page within three seconds:

HEALTHCHECK --interval=5m --timeout=3s \
CMD curl -f http://localhost/ || exit 1

To help debug failing probes, any output text (UTF-8 encoded) that the command writes on stdout or stderr will be stored in the health status and can be queried with docker inspect. Such output should be kept short (only the first 4096 bytes are stored currently).

When the health status of a container changes, a health_status event is generated with the new status.

The HEALTHCHECK feature was added in Docker 1.12.

SHELL

SHELL ["executable", "parameters"]

The SHELL instruction allows the default shell used for the shell form of commands to be overridden.

The default shell on Linux is ["/bin/sh", "-c"], and on Windows is ["cmd", "/S", "/C"].

The SHELL instruction must be written in JSON form in a Dockerfile.

The SHELL instruction is particularly useful on Windows where there are two commonly used and quite different native shells: cmd and powershell, as well as alternate shells available including sh.

The SHELL instruction can appear multiple times. Each SHELL instruction overrides all previous SHELL instructions, and affects all subsequent instructions. For example:

FROM microsoft/windowsservercore

# Executed as cmd /S /C echo default
RUN echo default # Executed as cmd /S /C powershell -command Write-Host default
RUN powershell -command Write-Host default # Executed as powershell -command Write-Host hello
SHELL ["powershell", "-command"]
RUN Write-Host hello # Executed as cmd /S /C echo hello
SHELL ["cmd", "/S", "/C"]
RUN echo hello

The following instructions can be affected by the SHELL instruction when the shell form of them is used in a Dockerfile: RUN, CMD and ENTRYPOINT.

The following example is a common pattern found on Windows which can be streamlined by using the SHELL instruction:

...
RUN powershell -command Execute-MyCmdlet -param1 "c:\foo.txt"
...

The command invoked by docker will be:  

cmd /S /C powershell -command Execute-MyCmdlet -param1 "c:\foo.txt"

This is inefficient for two reasons.

First, there is an un-necessary cmd.exe command processor (aka shell) being invoked.

Second, each RUN instruction in the shell form requires an extra powershell -command prefixing the command.

To make this more efficient, one of two mechanisms can be employed.

One is to use the JSON form of the RUN command such as:

...
RUN ["powershell", "-command", "Execute-MyCmdlet", "-param1 \"c:\\foo.txt\""]
...

While the JSON form is unambiguous and does not use the un-necessary cmd.exe, it does require more verbosity through double-quoting and escaping.

The alternate mechanism is to use the SHELL instruction and the shell form, making a more natural syntax for Windows users, especially when combined with the escape parser directive: 

# escape=`

FROM microsoft/nanoserver
SHELL ["powershell","-command"]
RUN New-Item -ItemType Directory C:\Example
ADD Execute-MyCmdlet.ps1 c:\example\
RUN c:\example\Execute-MyCmdlet -sample 'hello world'

Resulting in:

PS E:\docker\build\shell> docker build -t shell .
Sending build context to Docker daemon 4.096 kB
Step 1/5 : FROM microsoft/nanoserver
---> 22738ff49c6d
Step 2/5 : SHELL powershell -command
---> Running in 6fcdb6855ae2
---> 6331462d4300
Removing intermediate container 6fcdb6855ae2
Step 3/5 : RUN New-Item -ItemType Directory C:\Example
---> Running in d0eef8386e97 Directory: C:\ Mode LastWriteTime Length Name
---- ------------- ------ ----
d----- 10/28/2016 11:26 AM Example ---> 3f2fbf1395d9
Removing intermediate container d0eef8386e97
Step 4/5 : ADD Execute-MyCmdlet.ps1 c:\example\
---> a955b2621c31
Removing intermediate container b825593d39fc
Step 5/5 : RUN c:\example\Execute-MyCmdlet 'hello world'
---> Running in be6d8e63fe75
hello world
---> 8e559e9bf424
Removing intermediate container be6d8e63fe75
Successfully built 8e559e9bf424
PS E:\docker\build\shell>

The SHELL instruction could also be used to modify the way in which a shell operates. For example, using SHELL cmd /S /C /V:ON|OFF on Windows, delayed environment variable expansion semantics could be modified.

The SHELL instruction can also be used on Linux should an alternate shell be required such as zsh, csh, tcsh and others.

The SHELL feature was added in Docker 1.12.

Dockerfile examples

Below you can see some examples of Dockerfile syntax. If you’re interested in something more realistic, take a look at the list of Dockerization examples.

# Nginx
#
# VERSION 0.0.1 FROM ubuntu
LABEL Description="This image is used to start the foobar executable" Vendor="ACME Products" Version="1.0"
RUN apt-get update && apt-get install -y inotify-tools nginx apache2 openssh-server

 

# Firefox over VNC
#
# VERSION 0.3 FROM ubuntu # Install vnc, xvfb in order to create a 'fake' display and firefox
RUN apt-get update && apt-get install -y x11vnc xvfb firefox
RUN mkdir ~/.vnc
# Setup a password
RUN x11vnc -storepasswd 1234 ~/.vnc/passwd
# Autostart firefox (might not be the best way, but it does the trick)
RUN bash -c 'echo "firefox" >> /.bashrc' EXPOSE 5900
CMD ["x11vnc", "-forever", "-usepw", "-create"]

 

# Multiple images example
#
# VERSION 0.1 FROM ubuntu
RUN echo foo > bar
# Will output something like ===> 907ad6c2736f FROM ubuntu
RUN echo moo > oink
# Will output something like ===> 695d7793cbe4 # You'll now have two images, 907ad6c2736f with /bar, and 695d7793cbe4 with
# /oink.

  

 

 

  

  

  

  

  

  

  

 

 

  

  

  

 

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