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.

In reply to Re: how perl do 'envsubst' by NERDVANA
in thread how perl do 'envsubst' by vincentaxhe

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