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
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:
docker pull hello-world
docker export $(docker create hello-world) > hello-world.tar
tar -C rootfs -xf hello-world.tar
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
⟨https://github.com/opencontainers/runtime-tools⟩ to get more
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