check_mk_agent: Do not rely on "hostname" cmd on OpenWRT

[stklcode]

Issue:

Built-in BusyBox shell in OpenWRT has no hostname command available by default. While it can be added with additional packages in custom images there are alternative ways to determine the machine's hostname without introducing an additional dependency.

On some default OpenWRT image the "Hostname" attribute is empty:

root@my-openwrt-box:~# hostname
-ash: hostname: not found

root@my-openwrt-box:~# cat /proc/sys/kernel/hostname
my-openwrt-box

root@my-openwrt-box:~# check_mk_agent dump
<<<check_mk>>>
Version: 2.2.0p26
AgentOS: openwrt
Hostname: 
AgentDirectory: /etc/check_mk
...

Well, it doesn't really hurt much, but it's not desired behavior either.

Proposed changes:

Read the hostname from the sysctl parameter kernel.hostname (i.e. read /proc/sys/kernel/hostname) instead which is likely available on every image.

root@my-openwrt-box:~# check_mk_agent dump

<<<check_mk>>>
Version: 2.2.0p26
AgentOS: openwrt
Hostname: my-openwrt-box
AgentDirectory: /etc/check_mk
...

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[stklcode]

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[stklcode]

My first attempt had a conditional function in the preamble

if ! inpath hostname; then
  hostname() {
    cat /proc/sys/kernel/hostname
  }
fi

but I couldn't find any real reasons why a system's hostname command should be preferable in this context.
If desired though, I can update my PR accordingly.

[martinhv]

Thanks for highlighting this.
In OpenWRT hostname isn't there and won't come anytime soon (openwrt/openwrt#11765)

So, your PR is approved from my perspective (product, not dev).
We are pondering as well, if we should not just use /proc/sys/kernel/hostname in all UNIX agents though.
Will forward to a dev.