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]'

#!/bin/bash
# rot13.sh: Classic rot13 algorithm,
#           encryption that might fool a 3-year old
#           for about 10 minutes.

# http://tldp.org/LDP/abs/html/textproc.html
# Usage: ./rot13.sh filename
# or     ./rot13.sh <filename
# or     ./rot13.sh and supply keyboard input (stdin)

cat "$@" | tr 'a-zA-Z' 'n-za-mN-ZA-M'   # "a" goes to "n", "b" to "o" ...
#  The   cat "$@"   construct
#+ permits input either from stdin or from files.

exit 0