A Bash script for Caesar ciphers. I needed (well, wanted) a tool to do ROT-n ciphers for arbitrary n. So here it is. If n is not provided, it defaults to ROT-13. No ROT-5 for numerals as yet. This is my first shell script for public consumption; feedback is eagerly welcome, but please be gentle.

rot.sh

#! /bin/bash

# Usage: rot [base] string
#
# Base is assumed to be 13 if not specified.
#
# Bases not in 0-25 are taken modulo 26; negatives are increased by
# 26es until they're positive. Thus, any integer should work
# as expected.

function rot() {

	local pattern='^-?[0-9]+$'

	if [[ $1 =~ $pattern ]]; # Is first argument an integer?
	then
		local base="$1"
		local string="$2"
		local origbase="$1" # Save original base for later clarification
	else
		local base="13"
		local string="$1"
		local origbase="null"
	fi

	while (($base < 0))
	do
		base=$(($base + 26)) # Modulo ought to work this way
	done

	local realbase=$(($base % 26))

	local rot0='A-Za-z'
	local rot1='B-ZAb-za'
	local rot2='C-ZA-Bc-za-b'
	local rot3='D-ZA-Cd-za-c'
	local rot4='E-ZA-De-za-d'
	local rot5='F-ZA-Ef-za-e'
	local rot6='G-ZA-Fg-za-f'
	local rot7='H-ZA-Gh-za-g'
	local rot8='I-ZA-Hi-za-h'
	local rot9='J-ZA-Ij-za-i'
	local rot10='K-ZA-Jk-za-j'
	local rot11='L-ZA-Kl-za-k'
	local rot12='M-ZA-Lm-za-l'
	local rot13='N-ZA-Mn-za-m'
	local rot14='O-ZA-No-za-n'
	local rot15='P-ZA-Op-za-o'
	local rot16='Q-ZA-Pq-za-p'
	local rot17='R-ZA-Qr-za-q'
	local rot18='S-ZA-Rs-za-r'
	local rot19='T-ZA-St-za-s'
	local rot20='U-ZA-Tu-za-t'
	local rot21='V-ZA-Uv-za-u'
	local rot22='W-ZA-Vw-za-v'
	local rot23='X-ZA-Wx-za-w'
	local rot24='Y-ZA-Xy-za-x'
	local rot25='ZA-Yza-y'

	local funcname="rot${realbase}"

	# If the base was converted above, send the actual 0-25 base to stderr

	if (($origbase != $realbase))
	then
		echo "(ROT-${realbase})" 1>&2
	fi

	echo "$string" | tr 'A-Za-z' "${!funcname}"

}

rot "$1" "$2"