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Report the machine's DNS domain, by running hostname and removing everything up to the first dot.

Version

0.1.0 zef:jmaslak

Depends

none beyond the core

License

Artistic-2.0

Its own test suite

1 file, green

Checked

2026-09-15 against Raku++ 3.28.0 and Rakudo 2026.08

Where it lives

raku.land · source

Install it #

$ rakupp install Sys::Domainname

zef install Sys::Domainname writes the same store; either installer leaves the module usable by both engines.

What it is for #

A program that builds a fully-qualified name, a mail identifier or a default service address wants the machine's DNS domain, and $*KERNEL.hostname gives you the whole thing rather than the domain part.

This distribution is the one sub that separates them.

Asking for the domain #

The answer is specific to the machine, so an example asserts shape:

use Sys::Domainname;

my $d = domainname();
say 'returns a Str        : ', $d ~~ Str;
say 'is defined           : ', $d.defined;
say 'has no trailing newline : ', !$d.contains("\n");

returns a Str        : True
is defined           : True
has no trailing newline : True

That is the entire interface — one exported sub, no arguments.

How it works #

use Sys::Domainname;

say 'this is not a syscall. the implementation is four lines:';
say '  run `hostname -f`, chomp it, delete up to and including the first dot.';
say '';
say 'so the answer depends on what `hostname` is on your PATH,';
say 'and on whether the name it prints contains a dot at all.';
say '';
say 'the sub is declared `sub ... is export` at file scope,';
say 'so it is importable but never qualifiable:';
say '  Sys::Domainname::domainname is reachable : ',
    (try &Sys::Domainname::domainname.defined).defined;

this is not a syscall. the implementation is four lines:
  run `hostname -f`, chomp it, delete up to and including the first dot.

so the answer depends on what `hostname` is on your PATH,
and on whether the name it prints contains a dot at all.

the sub is declared `sub ... is export` at file scope,
so it is importable but never qualifiable:
  Sys::Domainname::domainname is reachable : True

The one thing to know #

It forks a shell, and it never returns an undefined value — so there is no signal that anything went wrong.

A hostname with no dot in it is returned whole, because the stripping is a plain "delete up to the first dot" with no fallback. So a machine with no domain reports its own hostname as its domain.

And when hostname fails, or is not on the PATH at all, the shell writes command not found to stderr and the sub returns the defined empty string. The CATCH { return Str } in the source is unreachable, because a failing external command produces empty output rather than throwing.

.defined is therefore useless as a success test. $d.chars is the only signal, and even that cannot distinguish "this machine has no domain" from "hostname is missing".

Two consequences worth planning for. Because the answer comes from a subprocess resolved through PATH, it can be changed by changing PATH — this is not a trusted source of identity. And every call forks a process, with no caching, so do not put it in a loop.

Where the two engines differ #

Not in this module, but in something it makes visible. qx and qqx do not pass the current %*ENV to the child under Raku++, while run and shell do — so a mutated PATH or any other environment change is invisible to a backtick command on one engine and honoured on the other.

That matters here because the module shells out. It also matters to anything else that sets an environment variable and then shells out expecting the child to see it. Use run or shell with an explicit environment rather than backticks if the child needs to see your changes.

Everything about this module's own behaviour was identical on both engines.