Removing leading zeros before passing a shell variable to another command

Question

It turns out that iptables doesn\'t handle leading zeros too well. As $machinenumber that is used has to have a leading zero in it for other purposes, the idea

Answer 1

Add the number with 0, it will remove leading zeroes

eg: expr 00010 + 0 #becomes 10

OR

num=00076
num=${num##+(0)}
echo $num

Answer 2

I had to revisit this code the other day due to some unrelated stuff, and due to compatibility with some other software that reads the same script, i found it a lot easiest to rewrite it into this, which should still be valid bash:

iptables -t nat -I POSTROUTING -s 10.($machinetype).($machinenumber + 0).0/24 -j MASQUERADE

Basically, adding 0 forces it to be interpreted as an integer, hence automatically stripping the leading zeros

Answer 3

No, you make all (alomost all) correct. You just must:

• remove spaces around =
• use $() or backticks instead of ()

That would be correct:

 nozero=$(echo $machinenumber | sed 's/^0*//')

Also you must use variables without () around them. You can add "" if you want:

iptables -t nat -I POSTROUTING -s "10.$machinetype.$nozero.0/24" -j MASQUERADE

And of course variables here are not necessary. You can say simply:

iptables -t nat -I POSTROUTING -s "10.$(echo $machinenumber | sed 's/^0*//').$nozero.0/24" -j MASQUERADE

Answer 4

nozero=$(echo $machinenumber | sed 's/^0*//')

Try without the spaces around = and with an additional $ sign.

Answer 5

I can't comment as I don't have sufficient reputation, but I would suggest you accept Dennis's answer (which is really quite neat)

Firstly, I don't think that your answer is valid bash. In my install I get:

> machinetype=74
> machinenumber=05
> iptables -t nat -I POSTROUTING -s 10.($machinetype).($machinenumber + 0).0/24 -j MASQUERADE
-bash: syntax error near unexpected token `('
> echo 10.($machinetype).($machinenumber + 0).0/24
-bash: syntax error near unexpected token `('

If I quote it I get:

> echo "($machinetype).($machinenumber + 0)"
(74).(05 + 0)

I'm assuming you mean:

> echo 10.$(($machinetype)).$(($machinenumber + 0)).0/24
10.74.5.0/24

But, of course it's still a bad solution because of octal:

> machinenumber=09
> echo 10.$(($machinetype)).$(($machinenumber + 0)).0/24
-bash: 09: value too great for base (error token is "09")

I assume that your numbers aren't 08 or 09 at the moment.

Here's Dennis's:

> echo $((10#09))
9
> echo $((10#00))
0
> echo $((10#00005))
5
> echo $((10#))
0

Admittedly, that last one might be an input validation problem for someone.

The sed solution has the problem of:

> echo "0" | sed 's/^0*//'

>

Answer 6

I do it by using

awk '{print $1 + 0}'

I like this better than the sed approach as it still works with numbers like 0, 000, and 001.

So in your example I would replace

nozero=$(echo $machinenumber | sed 's/^0*//')

with

nozero=$(echo $machinenumber | awk '{print $1 + 0}' )