Base64 Decode and Encode a String from the CLI on macOS – Kevin B. Ridgway's Today I Learned Site

For decoding:

echo -n "bXkgc3RyaW5n" | base64 -D
 
# output
my string

For encoding just drop the flag:

echo -n "my string" | base64
 
# output
bXkgc3RyaW5n