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I want to use tr to do some rot13 transformation. I can beautifully understand this command:

tr A-Za-z N-ZA-Mn-za-m <<< "URYC ZR CYRNFR"

which output is HELP ME PLEASE, but I can't figure out how this other command can produce the same rot13 transformation:

tr .............A-Z A-ZA-Z <<< "URYC ZR CYRNFR"

So I have two questions:

1. What's the magic behind the second tr command?


2. How to make the second command work for both lower and upper case, just like the first command?

It works as follows:

SET1-> .............ABCDEFGHIJKLMNOPQRSTUVWXYZ

SET2-> ABCDEFGHIJKLMNOPQRSTUVWXYZABCDEFGHIJKLM

So tr will translate SET1 to SET2.

This is equivalent to first one because it is also shifting by 13 units as there 13 dots.

Ok, so thanks to @Prvt_Yadv I was able to understand the dots. Here's the first question answer:

What's the magic behind the second tr command?

The dots are replaced by a sequence of letters starting from a to the number of dots. So

tr .............A-Z A-ZA-Z will translate to tr A-MA-Z A-ZA-Z

In this case the sets are:

SET1 -> ABCDEFGHIJKLMABCDEFGHIJKLMNOPQRSTUVXWYZ

SET2 -> ABCDEFGHIJKLMNOPQRSTUVXWYZABCDEFGHIJKLMNOPQRSTUVXWYZ

But since the beginning of both sets are identical until letter M, this part is discarded becoming then

SET1 -> ABCDEFGHIJKLMNOPQRSTUVXWYZ

SET2 -> NOPQRSTUVXWYZABCDEFGHIJKLMNOPQRSTUVXWYZ

But since the first set already contains all 26 letters and set2 has repeating trailing letter, those are discarded too, finally becoming

SET1 -> ABCDEFGHIJKLMNOPQRSTUVXWYZ

SET2 -> NOPQRSTUVXWYZABCDEFGHIJKLM

Which is the rot13 substitution and identical to the first command (except for not dealing with lower cases here). The same logic can be applied for the title of the question:

tr ...A-Z A-ZA-Z <<< “JVPQBOV” would become tr A-CA-Z A-ZA-Z <<< “JVPQBOV”

The sets being:

SET1 -> ABCABCDEFGHIJKLMNOPQRSTUVXWYZ

SET2 -> ABCDEFGHIJKLMNOPQRSTUVXWYZABCDEFGHIJKLMNOPQRSTUVXWYZ

Discarding the initial identical sequence and the trailing repeating letters they become:

SET1 -> ABCDEFGHIJKLMNOPQRSTUVXWYZ

SET2 -> DEFGHIJKLMNOPQRSTUVXWYZABC

Which is the rot3 substitution.

Now for the second question:

How to make the second command work for both lower and upper case, just like the first command?

The dots are substituted by a sequence of letter of the same case as the next sequence. That means that tr .....A-Z will translate to tr A-EA-Z whereas tr .....a-z will translate to tr a-ea-z. But the dots only work before the literal sequence, not after. So the immediate solution idea tr .....A-Z.....a-z won't work, because it will not translate to tr A-EA-Za-ea-z. The only reliable way to make it work is to use two tr commands as follow

tr .............A-Z A-ZA-Z <<< "ABJ V hqrefgnaq" | tr .............a-z a-za-z

Now it works for both upper and lower case :)

A caveat to using the dots substitution was gave by @iruvar: this command will not work as expected when the input stings has dots. So the following command won't print T.h.a.n.k.s.

tr .............A-Z A-ZA-Z <<< "G.u.n.a.x.f." | tr .............a-z a-za-z

It will print TMhMaMnMkMsM instead, substituting the dots to M