Convert Hex to Decimal in Bash

For reasons I don't fully recall—probably because I was searching for colors to style my web site—I found myself frequently converting hex values to decimal. I quickly grew tired of searching online for a hex converter every time, so I wondered if there was a simpler way to do it directly in the terminal.

Turns out, there is! You can use a simple one-liner in Bash:

$ echo $(( 16#FF ))
255

Neat, simple, and no need to repeat the same search online!

But how does it work?

Bash provides several ways to perform arithmetic operations, but for integer calculations, the recommended approach is to use its built-in Arithmetic Expansion feature. This allows you to evaluate expressions within double parentheses:

$(( expression ))

For example:

$ echo $(( 40 + 2 ))

By default, Bash arithmetic works in base 10. However, you can specify a different base using the syntax base#number. Regardless of the original base, arithmetic expansion always returns the decimal equivalent. This makes it easy to convert hexadecimal values:

$ echo $(( 16#2A ))
42

This method is useful for quick conversions, but be cautious when using it for validation. For example:

$ HEX='F+!P4_'
$ echo $(( 16#$HEX ))
16

This result is misleading because F+!P4_ is not a valid hexadecimal number. Thus, I wouldn’t rely on this method in a script if input validation is required.

An alternative approach is to use printf:

$ printf "%d\n" 0x0F
15

However, it has similar issues:

$ HEX='F+!P4_'
$ printf "%d\n" 0x$HEX
16

So while these methods are handy for quick conversions, they shouldn't be trusted for strict hex validation. Use them wisely!