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execute_commands_on_host.md

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Execute a command on the host

It may be needed to execute commands back on the host. Be it the filemanager, an archive manager, a container manager and so on.

Here are a couple of solutions.

The easy one

Install flatpak-spawn inside the container, this example is running on a Fedora Distrobox:

~$ distrobox create --image fedora:35 --name fedora-distrobox
~$ distrobox enter --name fedora-distrobox
user@fedora-distrobox:~$ sudo dnf install -y flatpak-spawn

With flatpak-swpan we can easily execute commands on the host using:

user@fedora-distrobox:~$ flatpak-spawn --host bash -l
~$  # We're back on host!

The not so easy one

Alternatively you may don't have flatpak-spawn in the repository of your container, or simply want an alternative.

We can use chroot to enter back into the host, and execute what we need!

Create an executable file with this content:

#!/bin/sh

result_command="sudo -E chroot --userspec=$(id -u):$(id -g) /run/host/ /usr/bin/env "
for i in $(printenv | grep "=" | grep -Ev ' |"' | grep -Ev "^(_)"); do
 result_command="$result_command $i"
done

exec ${result_command} sh -c " cd ${PWD} && $@"

in ~/.local/bin/host-exec and make it executable with chmod +x ~/.local/bin/host-exec

Now we can simply use this to exec stuff back on the host:

user@fedora-distrobox:~$ host-exec bash -l
~$  # We're back on host!

Integrate host with container seamlessly

Another cool trick we can pull, is to use the handy command_not_found_handle function to try and execute missing commands in the container on the host.

bash / zsh

Place this in your ~/.profile:

command_not_found_handle() {
 # don't run if not in a container
 if [ ! -e /run/.containerenv ] &&
  [ ! -e /.dockerenv ]; then
  exit 127
 fi

 if command -v flatpak-spawn >/dev/null 2>&1; then
  flatpak-spawn --host "${@}"
 elif command -v host-exec >/dev/null 2>&1; then
  host-exec "$@"
 else
  exit 127
 fi
}

if [ -n "${ZSH_VERSION-}" ]; then
 command_not_found_handler() {
  command_not_found_handle "$@"
 }
fi

fish

Place this snippet in a new fish function file (~/.config/fish/functions/fish_command_not_found.fish):

function fish_command_not_found
    # "In a container" check
    if test -e /run/.containerenv -o -e /.dockerenv
        if command -q flatpak-spawn
            flatpak-spawn --host $argv
        else if command -q host-exec
            host-exec "$argv"
        else
            __fish_default_command_not_found_handler $argv
        end
    else
        __fish_default_command_not_found_handler $argv
    end
end

And restart your terminal. Now when a command does not exist on your container, it will be automatically executed back on the host:

user@fedora-distrobox:~$ which podman
/usr/bin/which: no podman in [...]
user@fedora-distrobox:~$ podman version # <-- this is automatically executed on host.
Client:
Version:      3.4.2
API Version:  3.4.2
Go Version:   go1.16.6
Built:        Thu Jan  1 01:00:00 1970
OS/Arch:      linux/amd64

Server:
Version:      3.4.2
API Version:  3.4.2
Go Version:   go1.16.6
Built:        Thu Jan  1 01:00:00 1970
OS/Arch:      linux/amd64

This is also useful to open code, xdg-open, or flatpak from within the container seamlessly.