In computing, bc is a command in Unix operating systems that can do easy calculations in the shell.
Usually input is passed in using the echo command and pipes. The name stands for Basic Calculator.
echo "(12 / 2 - 3) * 3 + 1.5" | bc
    10.5

Interactive mode

Using the interactive mode lets you do calculations freely in a special shell inside the shell. All you need to do is run the bc command with the -i flag and no parameters.
To exit the interactive mode, write quit. 
bc -i

1+1
    2
(12 / 2 - 3) * 3 + 1.5
    10.2
quit