/bin/bash isn't necessarily everyone's shell and thus any environment variables they set up upon login won't necessarily be set by bash.

$command = sprintf('exec - %s -c env', (getpwuid($>))[8]);
$ENV{'HOME'} = (getpwuid($>))[7];
$env = `$command`;
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This should run someone's shell as a login shell regardless of what shell it is, and $env will contain the output of env which should be easy to parse.