in reply to how perl do 'envsubst'

use autodie;
use Path::Tiny;

# read the file
my $tpl= path("/home/vincent/.config/kmonad/$config")->slurp;

# perform envsubst
$tpl =~ s/\$ (?| (\w+) | \{(\w+)\} )/$ENV{$1}/gx;

# open a pipe to a child process "kmonad" telling it to read from STDI
+N
open(my $fh, "|-", "kmonad", "/dev/fd/0");

# Write the template to kmonad's stdin
$fh->print($tpl);
$fh->close;

[download]

If you also need to capture the output of kmonad, you'll need IPC::Open3, or IPC::Run.

If you're trying to avoid CPAN modules, you can replace Path::Tiny with my $tpl= do { local $/= undef; open my $fh, "<", "/home/vincent/.config/kmonad/$config"; <$fh> }; which is less elegant and doesn't check for disk errors mid-read.

Also... this is a very simple regex for replacing variables in the template. If your template is using fancy bash notations like ${FOO:-default} in the template then you probably need to shell out to bash for the envsubst.

If you are OK with moving your whole template into the perl script itself, then just do like LanX suggested, or use a  <<END-style here-doc.

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Re^2: how perl do 'envsubst'
by vincentaxhe (Scribe) on Jul 03, 2024 at 09:57 UTC

    thanks for your answer, I didn't know Path::Tiny do the work as File::Slurp, and $fh->print is same as print $fh, I have finished the script, all works great, the process substitution of shell is pipe to /dev/fd/ in essence

[reply]
      Yep, you can see it in action with 
      $ echo <(echo foo)
/dev/fd/63

      [download]
      But, now I just realized there is still a mystery! 
      $ ls /dev/fd
0  1  2  3

      [download]
      So... how can a program open /dev/fd/63 if it doesn't exist?
[reply]
[d/l]
[select]
        So... how can a program open /dev/fd/63 if it doesn't exist?

        Because it does exist at the time the command is run: 

        $ echo <(echo foo) $(ls /dev/fd)
/dev/fd/63 0 1 2 3 63

        [download]

        Greetings,
        🐻

        $gryYup$d0ylprbpriprrYpkJl2xyl~rzg??P~5lp2hyl0p$
[reply]
[d/l]
        if run kmonad <(echo $config), could bind to /dev/fd/61; but use perl only /dev/fd/0 is guaranteed to exist, cann't change to others;
[reply]

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