A really simple Caesar Cipher in Bash (or Shell) using `tr`, can also easily be adjusted to encrypt/decrypt ROT13 instead.

caesar-cipher.sh

# Caesar cipher encoding
echo "THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG" | tr '[A-Z]' '[X-ZA-W]'
# output: QEB NRFZH YOLTK CLU GRJMP LSBO QEB IXWV ALD

# Caesar cipher decoding
echo "QEB NRFZH YOLTK CLU GRJMP LSBO QEB IXWV ALD" | tr '[X-ZA-W]' '[A-Z]'
# output: THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG


# Can also be adjusted to ROT13 instead
echo "THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG" | tr '[A-Z]' '[N-ZA-M]'
# output: GUR DHVPX OEBJA SBK WHZCF BIRE GUR YNML QBT

echo "GUR DHVPX OEBJA SBK WHZCF BIRE GUR YNML QBT" | tr '[N-ZA-M]' '[A-Z]'
# output: THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG


# Case-sensitive version of ROT13
tr '[A-Za-z]' '[N-ZA-Mn-za-m]'

Thank you🤘🏼

The Caesar Cipher shifts plaintext three letters to the left to create ciphertext.

In this case, tr '[A-Z]' '[X-ZA-W]', tr translates all occurrences of 'A' to 'X', 'B' to 'Y', 'C' to 'Z', 'D' to 'A', etc.

[X-ZA-W] just means that your output starts with the letter X and continues through the letter Z, then continues with the letter A through the letter W. You could also use [XYZABCDEFGHIJKLMNOPQRSTUVW] instead but tr understands the hyphen '-' to mean 'through'.

Thank you so much. Really helped me in understanding.

so simple and elegant just like it was supposed to be, well done

I came here from a google-search. Thanks for sharing this. However, the brackets are superfluous and the invocation can thus be tr 'A-Za-z' 'X-ZA-Wx-za-w'.