Why does bash think: 016 + 1 = 15?

Question

Can somebody explain to me why a number with a leading 0 gives this funny behaviour?

#!/bin/bash
NUM=016 
SUM=$((NUM + 1)) 
echo "$NUM + 1 = $SUM"

Will print:

016 + 1 = 15

Psst: printf "%03d\n" 10 is completely usable in bash to obtain a leading zero for filenames and such. — Squeezy, Jan 08 '15 at 19:15
@Squeezy Thanks, but that part was already working. The actual problem was not obtaining a filename with a leading 0. It was finding out what the filename was with the highest number and then creating the next-in-sequence, by using printf "prefix-%03d.tif" $SUM. — DeltaLima, Jan 08 '15 at 19:34
Note that you could've figured this out yourself just by doing echo $((016)) — user541686, Jan 09 '15 at 19:59
FYI, this is true in many programming languages: C, C++, Javascript. — Paul Draper, Jan 11 '15 at 23:45

score 128 · Accepted Answer · answered Jan 08 '15 at 15:29

128

The misunderstanding is that the numbers don't mean what you expect.

A leading zero denotes a number with base 8. I.e. 016 is the same as 8#16. If you want to keep the leading zero then you need 10#016.

> num=016
> echo $((num))
14
> echo $((10#$num))
16

answered Jan 08 '15 at 15:29

Hauke Laging

23

There are 10 types of people in the world. Those who understand binary, those who don't, those who weren't expecting a base 8 joke, and 5 other types of people. – Jon Story Jan 12 '15 at 13:36

score 42 · Answer 2 · answered Jan 08 '15 at 15:27

42

Because:

~$ echo $((NUM))
14

if the number begins with 0, it is considered to be an octal value and 16 in octal is 14 in decimal.

answered Jan 08 '15 at 15:27

fredtantini

2

That makes perfectly sence and explains why my script was overwriting my old files :-( – DeltaLima Jan 08 '15 at 15:33

2 Answers2