Question

I’ve a Perl script which does the job of expanding the Unix environment variables. The script is working fine. The script is as below:

# I know this should not be typed here. This is here just for testing.
@ENV{qw(LKUP_DIR DATA_DIR CTRL_DIR MMM)} = qw(/appl1/TSS/lkup /appl1/TSS/data /appl1/TSS/ctrl Oct);
while ( <DATA> )
{
  my $line=$_;
  chomp $line;
  $line =~ s{\$(\w+)}{ exists $ENV{$1} ? $ENV{$1} : '$'.$1 }ge;
  $line =~ s{\$\{(\w+)\}}{ exists $ENV{$1} ? $ENV{$1} : '${'.$1.'}' }ge;
  print "$line\n";
}

__DATA__
${LKUP_DIR}/lookup_file.txt
${CTRL_DIR}/ctrl_file_$MMMM.txt
$CTRL_DIR/ctrl_file_$MMM.txt

I want to convert this script into an one-liner, but I am not sure how handle the single quotes around the $ in the regular expression used in the script mentioned above. I tried the below, but of course it is not working:

perl -lne '$line=$_; $line =~ s{\$(\w+)}{ exists $ENV{$1} ? $ENV{$1} : '$'.$1 }ge; $line =~ s{\$\{(\w+)\}}{ exists $ENV{$1} ? $ENV{$1} : '${'.$1.'}' }ge; print "$line";' DATA.txt

Any suggestions? (I want to do this only in Perl.)

Answer 1

I agree with @Mat: it seems a bad idea. It will be just one line but you lose on readability. Just think that you might edit a year later and still try to understand what the one liner does.

But anyway, in Perl you can use q/STRING/ instead of 'STRING'.

perl -lne '$line=$_; $line =~ s{\$(\w+)}{ exists $ENV{$1} ? $ENV{$1} : q/$/.$1 }ge; $line =~ s{\$\{(\w+)\}}{ exists $ENV{$1} ? $ENV{$1} : q/${/.$1.q/}/ }ge; print "$line";' DATA.txt