docs: fix init-hook script for hostname matching

[michaelhaaf]

The init-hook provided in the useful tips documentations for creating a distrobox with the the same hostname as its host has two typos:

• Command substitution wrapping uname -n suppressing stdout redirect
• trailing "`"

Using this init-hook as suggested causes build errors (see #1164). This fix corrects the suggested script by removing the command substitution and the trailing "`".

To elaborate: I think command substitution here is misplaced since uname -n already prints to stdout and can be redirected directly. Or you can use echo on the provided command substitution. Or you can use the hostname command. I.e.

uname -n > /etc/hostname
echo "$(uname -n)" > /etc/hostname
hostname "$(uname -n)"

Tested on Fedora 39 below:

# Also tested the 'echo "$(uname -n)" > /etc/hostname' with the same result

user@hostname $ distrobox create --image ghcr.io/username/sample-image:latest -n box-name \ 
    --init-hooks 'uname -n > /etc/hostname' && distrobox enter box-name

...

Setting up groups...                    	 [ OK ]
Setting up users...                     	 [ OK ]
Setting up skel...                      	 [ OK ]
Executing init hooks...                 	 [ OK ]

Container Setup Complete!

[email protected] $ cat /etc/hostname 
box-name.hostname

[email protected] $ hostname
box-name.hostname  

[89luca89]

Hi @michaelhaaf
You're right about the wrong command

But I think the correct one would be

user@hostname $ distrobox create --image ghcr.io/username/sample-image:latest -n box-name \ 
    --init-hooks "echo $(uname -n) > /etc/hostname; hostname $(uname -n)" && distrobox enter box-name

Tested and it works

[michaelhaaf]

The following are equivalent and all work:

uname -n > /etc/hostname
echo "$(uname -n)" > /etc/hostname
uname -n > /etc/hostname; hostname "$(uname -n)"
echo "$(uname -n)" > /etc/hostname; hostname "$(uname -n)"

Maybe there's room for preference; personally the first choice makes the most sense to me, the rest have superfluous commands.

(to elaborate: The call to hostname seems redundant, beyond that it is not system-independent and does not make the change permanent, unlike overwriting /etc/hostname. I think the first choice is the most concise: why evaluate uname -n then echo that evaluation if you can just redirect uname -n stdout to begin with.)

[89luca89]

The uname -n > /etc/hostname will result in the container injecting its own hostname in the file

The echo $(uname -n) > /etc/hostname works because the $(uname -n) is evaulated at distrobox-create time, so in the end the result is just one echo foo > /etc/hostname in the init_hook

[michaelhaaf]

The echo $(uname -n) > /etc/hostname works because the $(uname -n) is evaulated at distrobox-create time, so in the end the result is just one echo foo > /etc/hostname in the init_hook

It's unclear to me how that works (in my tests I got the same result using both).

But, in the interest of getting a working version in the documentation/removing the typo/moving on, I've updated my PR to use echo "$(uname -n)" > /etc/hostname instead.

[89luca89]

Thanks a lot @michaelhaaf

[robinhellmers]

Adding a little bit to this conversation as there was a typo in this change. Having the single quotation marks outside of the double quotation marks will prevent $(uname -n) from being evaluated during the distrobox-create command. That does, for me, result in <container-name>.<hostname>, meaning no change at all.

Changing
--init-hooks 'echo "$(uname -n)" > /etc/hostname'
to
--init-hooks "echo '$(uname -n)' > /etc/hostname"
solves that issue.

Other than that, the command
echo '$(uname -n)' > /etc/hostname
is not equivalent to
echo '$(uname -n)' > /etc/hostname; hostname '$(uname -n)'

as the first example requires a reboot for the hostname command to actually use the changed /etc/hostname. Meanwhile if you also execute hostname "$(uname -n)", a reboot isn't required.