I want to know does react the ENTRYPOINT instruction from Dockerfiles when heritage happens :
Let's say for example I have an image called : jenkins
FROM java:8-jdk
RUN ...
ENTRYPOINT ["/bin/tini", "--", "/usr/local/bin/jenkins.sh"]
By running this image, the ENTRYPOINT directive will start and install the application as expected
Let's say now that I want to extend this image with a new Dockerfile, I call it : jenkins-custom
FROM jenkins
# enable start tls
RUN echo "JENKINS_JAVA_OPTIONS=\"-Dmail.smtp.starttls.enable=true\"" >> /etc/default/jenkins
RUN chown jenkins:docker /etc/default/jenkins
Should I consider that :
the jenkins entrypoint is triggered after my new lines.
entrypoint will be trigered before my new lines.
entrypoint will not be triggered.
In my example, I am trying to activate STARTTLS in default Jenkins Docker image, should I just restart the process in the second image ?
The commands in ENTRYPOINT run when you execute docker run. However, commands in RUN are executed when you run docker build.
In your case, what's going to happen is that when you docker build the image, a new Jenkins configuration file is generated, and then when you docker run it, tini is launched, and in turns execute the jenkins-entrypoint.sh.
If what you're trying to do is change the Jenkins configuration and nothing else, what you have here is good.
Related
I have read the docs about CMD and ENTRYPOINT
https://docs.docker.com/engine/reference/builder/#entrypoint
Here, they have mentioned in the table that "NO CMD and NO ENTYRPOINT is not allowed", But I created a Dockerfile without CMD and ENTRYPOINT and the image was built successfully.
Download alpine tar from here Alpine Tar
Dockerfile
from scratch
ADD alpine-minirootfs-3.11.2-x86_64.tar.gz /
COPY . /
Building the image:
docker build -t test:1 .
Sending build context to Docker daemon 2.724MB
Step 1/3 : from scratch
-----
Successfully tagged test:1
docker run -ti test:1 /bin/sh
/ #
It worked!! So why in the docs it's mentioned that either CMD or ENTRYPOINT is necessary?
Specifying a command at the end of the docker run command line supplies (or overrides) CMD; similarly, the docker run --entrypoint option supplies (or overrides) ENTRYPOINT. In your example you gave a command /bin/sh so there's something for the container to do; if you leave it off, you'll get an error.
As a matter of style your Dockerfiles should almost always declare a CMD, unless you're extending a base image that's already running the application automatically (nginx, tomcat). That will let you docker run the image and launch the application embedded in it without having to remember a more specific command-line invocation.
The following line from documentation is incorrect.
Dockerfile should specify at least one of CMD or ENTRYPOINT commands.
It should probably say -
CMD or ENTRYPOINT is necessary for running a container.
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 built my docker image using openjdk.
# config Dockerfile
FROM openjdk:8
COPY . /usr/src/myapp
WORKDIR /usr/src/myapp
# build image
docker build -t shantanuo/dbt .
It is working as expected using this command...
docker run -p 8081:8080 -it shantanuo/dbt
Once I log-in, I have to run this command...
sh bin/startup.sh
My Question: Is it possible to add the startup command to dockerfile? I tried adding this line in my dockerfile.
CMD ["sh", "bin/startup.sh"]
But after building the image, I can not use -d parameter to start the container.
You can use the entrypoint to run the startup script. In the entrypoint you can specify your custom script, and then run catlina.sh.
Example:
ENTRYPOINT "bin/startup.sh && catalina.sh run"
This will run your startup script and then start your tomcat server, and it won't exit the container.
This is addressed in the documentation here: https://docs.docker.com/config/containers/multi-service_container/
If one of your processes depends on the main process, then start your helper process FIRST with a script like wait-for-it, then start the main process SECOND and remove the fg %1 line.
#!/bin/bash
# turn on bash's job control
set -m
# Start the primary process and put it in the background
./my_main_process &
# Start the helper process
./my_helper_process
# the my_helper_process might need to know how to wait on the
# primary process to start before it does its work and returns
# now we bring the primary process back into the foreground
# and leave it there
fg %1
A docker container must have a dedicated task. It is important that this task/startup script does not terminate. When it does, the task is done and everything for docker is done right.
It makes no sense to start a container only with the JDK. You have to put your application in it.
I think it would help when you will post what you exactly want to do.
The Docker reference is always a good place to look at: https://docs.docker.com/engine/reference/builder/#entrypoint
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.