Problem

You would like to quickly convert a number from a different base system (i.e. binary, octal, hexadecimal), to decimal.

Solution

Use the echo $(()) shell command to convert it passing it the current base system and the number, as in $((2#101010)) or using the O for octal or 0x for hexadecimal notations.
Examples:

echo $(( 16#FF ))
255
echo $(( 0xFF ))
255
echo $(( 8#21 ))
17
echo $(( 021 ))
17

Taken from Linux Journal’s Work the Shell column (March 2016)