runc-spec man page

runc spec - create a new specification file


runc spec [command options] [arguments...]


The spec command creates the new specification file named "config.json" for the bundle.

The spec generated is just a starter file. Editing of the spec is required to achieve desired results. For example, the newly generated spec includes an args parameter that is initially set to call the "sh" command when the container is started. Calling "sh" may work for an ubuntu container or busybox, but will not work for containers that do not include the "sh" program.


To run docker's hello-world container one needs to set the args parameter in the spec to call hello. This can be done using the sed command or a text editor. The following commands create a bundle for hello-world, change the default args parameter in the spec from "sh" to "/hello", then run the hello command in a new hello-world container named container1:

mkdir hello
cd hello
docker pull hello-world
docker export $(docker create hello-world) > hello-world.tar
mkdir rootfs
tar -C rootfs -xf hello-world.tar
runc spec
sed -i 's;"sh";"/hello";' config.json
runc start container1

In the start command above, "container1" is the name for the instance of the container that you are starting. The name you provide for the container instance must be unique on your host.

An alternative for generating a customized spec config is to use "oci-runtime-tool", the sub-command "oci-runtime-tool generate" has lots of options that can be used to do any customizations as you want, see runtime-tools ⟨⟩ to get more information.

When starting a container through runc, runc needs root privilege. If not already running as root, you can use sudo to give runc root privilege. For example: "sudo runc start container1" will give runc root privilege to start the container on your host.

Alternatively, you can start a rootless container, which has the ability to run without root privileges. For this to work, the specification file needs to be adjusted accordingly. You can pass the parameter --rootless to this command to generate a proper rootless spec file.


--bundle value, -b value     path to the root of the bundle directory

--rootless                   generate a configuration for a rootless container