Caesar Shifting

Pyth, 13 bytes

uXGH.<HQrBG1z

Test suite

Basically, we start with the two strings we want to caesar shift, the lowercase and uppercase alphabets. The list containing both of these is generated by rBG1, bifurcate on uppercase. Then, we reduce over this list, starting with the input string and translating first lowercase, then uppercase letters by the appropriate shift.

Pyth, 16

XXzG.<GQJrG1.<JQ

Try it online or run a Test Suite

Bash + bsd-games package, 21

caesar $[($1+130)%26]

Builtins FTW! Almost feels like Mathematica. Pyth answers are still shorter though.

Input string read from STDIN and integer from command-line. e.g.:

$ ./caesar.sh 13 <<< "Spam spam spam sausage and spam!"
Fcnz fcnz fcnz fnhfntr naq fcnz!
$

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Or if you don't like the builtin:

Bash + coreutils, 63

printf -va %s {a..z}
t=${a:$1%26}${a:0:$1%26}
tr A-Z$a ${t^^}$t