And that brings us to the question, how do you escape single quotes in the shell? The answer is not putting a backslash in front of a single quote. In the shell, a backslash does not have a special meaning when it's inside single quotes. But then what?

The answer lies in the fact that the shell does concatenation by absense of whitespace. So, if we want to put O'Reilly inside quotes, we break the string into three parts: O, ' and Reilly. The first and last part will be surrounded by single quotes, the single quote will be surrounded by double quotes, and then we stick the three parts together, resulting in: 'O'"'"'Reilly'. This sounds complicated, but we can do this with a single substitution: s/'/'"'"'/g.

#!/usr/bin/perl 
    
use strict;
use warnings;
    
while (<DATA>) {
    chomp;
    my $orig = $_;
    
    s/'/'"'"'/g;
    my $result = `echo '$_' | cat`;
    chomp $result;

    print STDERR "Mismatch [$orig] vs [$result]\n" unless $result eq $
+orig;
}

__DATA__   
foo bar  
'foo bar' 
foo'"'bar 
''''''''''''
'''''''''''
"foo'bar"  
"foo''bar"   
"foo'bar""
'
"'"
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