Encode a string

$ echo -n 'Softwaretester.info' | openssl base64 -e

The -n after the echo command, prevents echo from appending a newline character on the end of the string.

Decode a string

$ echo 'U29mdHdhcmV0ZXN0ZXIuaW5mbw==' | openssl base64 -d

Encode content of file

$ openssl base64 -e -in example.txt -out example.b64