Question

The following program is in Perl.

cat "test... test... test..." | perl -e '$??s:;s:s;;$?::s;;=]=>%-{<-|}<&|`{;;y; -/:-@[-`{-};`-{/" -;;s;;$_;see'

Can somebody help me to understand how it works?

Answer 1

This bit of code’s already been asked about on the Debian forums.

According to Lacek, the moderator on that thread, what the code originally did is rm -rf /, though they mention they’ve changed the version there so that people trying to figure out how it works don’t delete their entire filesystem. There’s also an explanation there of what the various parts of the Perl code do.

(Did you post this knowing what it did, or were you unaware of it?)

To quote Lacek’s post on it:

Anyway, here is how the script works.

It is basically two regex substitutions and one transliteration.
Piping anything into its standard input makes no difference, the perl
code doesn’t use its input in any way. If you split the long line on
the boundaries of the expressions, you get this:

$??s:;s:s;;$?::
s;;=]=>%-{\\>%<-{;;
y; -/:-@[-`{-};`-{/" -;;
s;;$_;see

The first line is a condition which does nothing save makes the code
look more difficult. If the previous command originated from the perl
code wasn’t successful, it does some substitutions on the standard
input (which the program doesn’t use, so effectively it substitutes
the nothing). Since no previous command exists, $? is always 0, so the
first line never gets executed.

The second line substitutes the
standard input (the nothing) for seemingly meaningless garbage.

The third line is a transliteration operator. It defines 4 ranges, in
which the characters gets substituted to the one range and the 4
characters given in the transliteration replacement. I’d prefer not to
write the whole transliteration table here, because it’s a bit long.
If you are really interested, just write the characters in the defined
ranges (space to ‘/’, ‘:’ to ‘@’, ‘[‘ to ‘(backtick)’, and ‘{‘ to ‘}’), and
write next to them the characters from the replacement range (‘(backtick)’ to
‘{‘), and finally, write the remaining characters (/,”, space and -)
from the replacement pattern. When you have this table, you can see
what character gets replaced to what.

The last line executes the
resulting command by substituting the nothing with the resulted string
(which is ‘xterm’. Originally it was ‘system”rm -rf /”‘, and is held
in $_), evaluates the substitution as an expression and executes it.

(I’ve substituted ‘backtick’ for the actual backtick character here so that the code auto-formatting doesn’t kick in.)