basenc

# Encode $STRING to base64.
#
echo "$STRING" | basenc --base64
 
# Encode $STRING to URL-safe base64.
#
echo "$STRING" | basenc --base64url
 
# Dencode $BASE64_STRING from base64.
#
echo "$BASE64_STRING" | basenc -d --base64
 
# Dencode $BASE64_STRING from URL-safe base64.
#
echo "$BASE64_STRING" | basenc -d --base64url

The advantage of basenc over base64 is that basenc supports URL-safe base64-encoding/decoding out of the box, which I’ve otherwise found is a pain to work with.