I'm wanting to do some last minute setup on run before passing arguments to the shell entrypoint, to accomplish this I have the following in mind.
ENTRYPOINT ./run_binary ${args}; /bin/sh -c
CMD ./run_binary
However, by doing this, it doesn't seem that any CMD arguments make it to the shell entrypoint. Is there a way around this? I'm just wanting to run a setup step on a binary before handing back control to the shell entrypoint (and then to the USER via CMD).
CMD becomes a list of arguments to send to ENTRYPOINT when both are specified, see the manual, so that's not the way to go
but you could use a .sh script as ENTRYPOINT, that first executes your binary command and then forwards the received arguments to a shell
I haven't tried it but something along the lines of
#!/bin/sh
./run_binary
/bin/sh -c "$#"
You could use an intermediate build image that triggers an ONBUILD statement from your original Dockerfile, see: https://docs.docker.com/engine/reference/builder/#/onbuild
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.
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.
Regarding CMD and ENTRYPOINT, see: https://docs.docker.com/engine/reference/builder/#understand-how-cmd-and-entrypoint-interact
Dockerfile should specify at least one of CMD or ENTRYPOINT
commands.
ENTRYPOINT should be defined when using the container as an
executable.
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.
CMD will be overridden when running the container with alternative
arguments.
Related
How can I run a command against a container and tell docker not to run the entry point? e.g.
docker-compose run foo bash
The above will run the entry point of the foo machine. How to prevent it temporarily without modifying Dockerfile?
docker-compose run --entrypoint=bash foo bash
It'll run a nested bash, a bit useless, but you'll have your prompt.
If you control the image, consider moving the entire default command line into the CMD instruction. Docker concatenates the ENTRYPOINT and CMD together when running a container, so you can do this yourself at build time.
# Bad: prevents operators from running any non-Python command
ENTRYPOINT ["python"]
CMD ["myapp.py"]
# Better: allows overriding command at runtime
CMD ["python", "myapp.py"]
This is technically "modifying the Dockerfile" but it won't change the default operation of your container: if you don't specify entrypoint: or command: in the docker-compose.yml then it will run the exact same command, but it also allows running things like debug shells in the way you're trying to.
I tend to reserve ENTRYPOINT for two cases. There's a common pattern of using an ENTRYPOINT to do some first-time setup (e.g., running database migrations) and then exec "$#" to run whatever was passed as CMD. This preserves the semantics of CMD (your docker-compose run bash will still work, but migrations will happen first). If I'm building a FROM scratch or other "distroless" image where it's actually impossible to run other commands (there isn't a /bin/sh at all) then making the single thing in the image be the ENTRYPOINT makes sense.
I'm confused about when should I use CMD vs RUN. For example, to execute bash/shell commands (i.e. ls -la) I would always use CMD or is there a situation where I would use RUN? Trying to understand the best practices about these two similar Dockerfile directives.
RUN is an image build step, the state of the container after a RUN command will be committed to the container image. A Dockerfile can have many RUN steps that layer on top of one another to build the image.
CMD is the command the container executes by default when you launch the built image. A Dockerfile will only use the final CMD defined. The CMD can be overridden when starting a container with docker run $image $other_command.
ENTRYPOINT is also closely related to CMD and can modify the way a container is started from an image.
RUN - command triggers while we build the docker image.
CMD - command triggers while we launch the created docker image.
I found this article very helpful to understand the difference between them:
RUN -
RUN instruction allows you to install your application and packages
required for it. It executes any commands on top of the current image
and creates a new layer by committing the results. Often you will find
multiple RUN instructions in a Dockerfile.
CMD -
CMD instruction allows you to set a default command, which will be
executed only when you run container without specifying a command.
If Docker container runs with a command, the default command will be
ignored. If Dockerfile has more than one CMD instruction, all but last
CMD instructions are ignored.
The existing answers cover most of what anyone looking at this question would need. So I'll just cover some niche areas for CMD and RUN.
CMD: Duplicates are Allowed but Wasteful
GingerBeer makes an important point: you won't get any errors if you put in more than one CMD - but it's wasteful to do so. I'd like to elaborate with an example:
FROM busybox
CMD echo "Executing CMD"
CMD echo "Executing CMD 2"
If you build this into an image and run a container in this image, then as GingerBeer states, only the last CMD will be heeded. So the output of that container will be:
Executing CMD 2
The way I think of it is that "CMD" is setting a single global variable for the entire image that is being built, so successive "CMD" statements simply overwrite any previous writes to that global variable, and in the final image that's built the last one to write wins. Since a Dockerfile executes in order from top to bottom, we know that the bottom-most CMD is the one gets this final "write" (metaphorically speaking).
RUN: Commands May not Execute if Images are Cached
A subtle point to notice about RUN is that it's treated as a pure function even if there are side-effects, and is thus cached. What this means is that if RUN had some side effects that don't change the resultant image, and that image has already been cached, the RUN won't be executed again and so the side effects won't happen on subsequent builds. For example, take this Dockerfile:
FROM busybox
RUN echo "Just echo while you work"
First time you run it, you'll get output such as this, with different alphanumeric IDs:
docker build -t example/run-echo .
Sending build context to Docker daemon 9.216kB
Step 1/2 : FROM busybox
---> be5888e67be6
Step 2/2 : RUN echo "Just echo while you work"
---> Running in ed37d558c505
Just echo while you work
Removing intermediate container ed37d558c505
---> 6f46f7a393d8
Successfully built 6f46f7a393d8
Successfully tagged example/run-echo:latest
Notice that the echo statement was executed in the above. Second time you run it, it uses the cache, and you won't see any echo in the output of the build:
docker build -t example/run-echo .
Sending build context to Docker daemon 9.216kB
Step 1/2 : FROM busybox
---> be5888e67be6
Step 2/2 : RUN echo "Just echo while you work"
---> Using cache
---> 6f46f7a393d8
Successfully built 6f46f7a393d8
Successfully tagged example/run-echo:latest
RUN - Install Python , your container now has python burnt into its image
CMD - python hello.py , run your favourite script
Note: Don’t confuse RUN with CMD. RUN actually runs a command and
commits the result; CMD does not execute anything at build time, but
specifies the intended command for the image.
from docker file reference
https://docs.docker.com/engine/reference/builder/#cmd
RUN Command:
RUN command will basically, execute the default command, when we are building the image. It also will commit the image changes for next step.
There can be more than 1 RUN command, to aid in process of building a new image.
CMD Command:
CMD commands will just set the default command for the new container. This will not be executed at build time.
If a docker file has more than 1 CMD commands then all of them are ignored except the last one. As this command will not execute anything but just set the default command.
RUN: Can be many, and it is used in build process, e.g. install multiple libraries
CMD: Can only have 1, which is your execute start point (e.g. ["npm", "start"], ["node", "app.js"])
There has been enough answers on RUN and CMD. I just want to add a few words on ENTRYPOINT. CMD arguments can be overwritten by command line arguments, while ENTRYPOINT arguments are always used.
This article is a good source of information.
I am trying to figure out how to get the CMD command in dockerfile to run a script on startup for docker run I know that using the RUN command will get the image to prerun that script when building the image but I want it to run the script everytime I run a new container using that image. The script is just a simple script that outputs the current date/time to a file.
Here is the dockerfile that works if I use RUN
# Pull base image
FROM alpine:latest
# gcr.io/dev-ihm-analytics-platform/practice_docker:ulta
WORKDIR /root/
RUN apk --update upgrade && apk add bash
ADD ./script.sh ./
RUN ./script.sh
Here is the same dockerfile that doesnt work with CMD
# Pull base image
FROM alpine:latest
# gcr.io/dev-ihm-analytics-platform/practice_docker:ulta
WORKDIR /root/
RUN apk --update upgrade && apk add bash
ADD ./script.sh ./
CMD ["./script.sh"]
I have tried all sorts of things after the CMD command like ["/script.sh"], ["bash script.sh"], ["bash", "./script.sh"], bash script.sh but I always get an error and I don't know what I am doing wrong. All I want is to
docker run -it name_of_container bash
and then find that the script has executed be seeing there is an output file with the run information in the container once I am inside
There’s three basic ways to do this:
You can RUN ./script.sh. It will happen once, at docker build time, and be baked into your image.
You can CMD ./script.sh. It will happen once, and be the single command the container runs. If you provide some alternate command (docker run ... bash for instance) that runs instead of this CMD.
You can write a custom entrypoint script that does this first-time setup, then runs the CMD or whatever got passed on the command line. The main container process is the entrypoint, and it gets passed the command as arguments. This script (and whatever it does inside) will get run on every startup. This script can look something like
#!/bin/sh
./script.sh
exec "$#"
It needs to be separately COPYd into the image, and then you’d set something like ENTRYPOINT ["./entrypoint.sh"].
(Given the problem as you’ve actually described it — you have a shell script and you want to run it and inspect the file output in an interactive shell — I’d just run it at your local command prompt and not involve Docker at all. This avoids all of these sequencing and filesystem mapping issues.)
There are multiple ways to achieve what you want, but your first attempt, with the RUN ./script.sh line is probably the best.
The CMD and ENTRYPOINT commands are overridable on the command-line as flags to the container run command. So, if you want to ensure that this is run every time you start the container, then it shouldn't be part of the CMD or ENTRYPOINT commands.
Well, iam using the CMD command to start my Java applications and when the container is inside the WORKDIR iam executing the following:
CMD ["/usr/bin/java", "-jar", "-Dspring.profiles.active=default", "/app.jar"]
Have you tried to remove the "." in the CMD command so it looks like that:
CMD ["/script.sh"]
There might be a different syntax when using RUN or CMD.
I have a Dockerfile with the following CMD as the last line
CMD ["/usr/local/myapp/bin/startup.sh", "docker"]
Part of a script that is executed against the docker image during startup is as follows
# find directory of cacerts file in DOCKER_JAVA_HOME of container
DOCKER_CACERTS_DIR=$(dirname "$(docker run "$DOCKER_IMAGE_ID" find "$DOCKER_JAVA_HOME" -name cacerts)")
However, this still executes the CMD line from my Dockerfile.
I have found that I can alter this behaviour by changing the line in the script as follows.
# find directory of cacerts file in DOCKER_JAVA_HOME of container
DOCKER_CACERTS_DIR=$(dirname "$(docker run --entrypoint find "$DOCKER_IMAGE_ID" "$DOCKER_JAVA_HOME" -name cacerts)")
However, I didn't think this would be necessary. Is it normal for docker to execute the CMD when overridden in the docker run command? I thought this was supposed to be one of the differences between using CMD and ENTRYPOINT, that you could easily override CMD without using the --entrypoint flag.
In case it's important, this is using docker version 17.03.0-ce
The image being run has an ENTRYPOINT defined somewhere. Probably in the image you are building FROM if there isn't one in your Dockerfile.
When ENTRYPOINT and CMD are defined, Docker will pass the CMD to the ENTRYPOINT as arguments. From there, it's up to the ENTRYPOINT executable to decide what to do.
The arguments could be ignored completely, modified as the entry point sees fit or it can pass the complete command on to be run. That behaviour is image specific.
I'm confused about when should I use CMD vs RUN. For example, to execute bash/shell commands (i.e. ls -la) I would always use CMD or is there a situation where I would use RUN? Trying to understand the best practices about these two similar Dockerfile directives.
RUN is an image build step, the state of the container after a RUN command will be committed to the container image. A Dockerfile can have many RUN steps that layer on top of one another to build the image.
CMD is the command the container executes by default when you launch the built image. A Dockerfile will only use the final CMD defined. The CMD can be overridden when starting a container with docker run $image $other_command.
ENTRYPOINT is also closely related to CMD and can modify the way a container is started from an image.
RUN - command triggers while we build the docker image.
CMD - command triggers while we launch the created docker image.
I found this article very helpful to understand the difference between them:
RUN -
RUN instruction allows you to install your application and packages
required for it. It executes any commands on top of the current image
and creates a new layer by committing the results. Often you will find
multiple RUN instructions in a Dockerfile.
CMD -
CMD instruction allows you to set a default command, which will be
executed only when you run container without specifying a command.
If Docker container runs with a command, the default command will be
ignored. If Dockerfile has more than one CMD instruction, all but last
CMD instructions are ignored.
The existing answers cover most of what anyone looking at this question would need. So I'll just cover some niche areas for CMD and RUN.
CMD: Duplicates are Allowed but Wasteful
GingerBeer makes an important point: you won't get any errors if you put in more than one CMD - but it's wasteful to do so. I'd like to elaborate with an example:
FROM busybox
CMD echo "Executing CMD"
CMD echo "Executing CMD 2"
If you build this into an image and run a container in this image, then as GingerBeer states, only the last CMD will be heeded. So the output of that container will be:
Executing CMD 2
The way I think of it is that "CMD" is setting a single global variable for the entire image that is being built, so successive "CMD" statements simply overwrite any previous writes to that global variable, and in the final image that's built the last one to write wins. Since a Dockerfile executes in order from top to bottom, we know that the bottom-most CMD is the one gets this final "write" (metaphorically speaking).
RUN: Commands May not Execute if Images are Cached
A subtle point to notice about RUN is that it's treated as a pure function even if there are side-effects, and is thus cached. What this means is that if RUN had some side effects that don't change the resultant image, and that image has already been cached, the RUN won't be executed again and so the side effects won't happen on subsequent builds. For example, take this Dockerfile:
FROM busybox
RUN echo "Just echo while you work"
First time you run it, you'll get output such as this, with different alphanumeric IDs:
docker build -t example/run-echo .
Sending build context to Docker daemon 9.216kB
Step 1/2 : FROM busybox
---> be5888e67be6
Step 2/2 : RUN echo "Just echo while you work"
---> Running in ed37d558c505
Just echo while you work
Removing intermediate container ed37d558c505
---> 6f46f7a393d8
Successfully built 6f46f7a393d8
Successfully tagged example/run-echo:latest
Notice that the echo statement was executed in the above. Second time you run it, it uses the cache, and you won't see any echo in the output of the build:
docker build -t example/run-echo .
Sending build context to Docker daemon 9.216kB
Step 1/2 : FROM busybox
---> be5888e67be6
Step 2/2 : RUN echo "Just echo while you work"
---> Using cache
---> 6f46f7a393d8
Successfully built 6f46f7a393d8
Successfully tagged example/run-echo:latest
RUN - Install Python , your container now has python burnt into its image
CMD - python hello.py , run your favourite script
Note: Don’t confuse RUN with CMD. RUN actually runs a command and
commits the result; CMD does not execute anything at build time, but
specifies the intended command for the image.
from docker file reference
https://docs.docker.com/engine/reference/builder/#cmd
RUN Command:
RUN command will basically, execute the default command, when we are building the image. It also will commit the image changes for next step.
There can be more than 1 RUN command, to aid in process of building a new image.
CMD Command:
CMD commands will just set the default command for the new container. This will not be executed at build time.
If a docker file has more than 1 CMD commands then all of them are ignored except the last one. As this command will not execute anything but just set the default command.
RUN: Can be many, and it is used in build process, e.g. install multiple libraries
CMD: Can only have 1, which is your execute start point (e.g. ["npm", "start"], ["node", "app.js"])
There has been enough answers on RUN and CMD. I just want to add a few words on ENTRYPOINT. CMD arguments can be overwritten by command line arguments, while ENTRYPOINT arguments are always used.
This article is a good source of information.