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

You don't need to use sed or another external utility. Here are a couple of ways Bash can strip the leading zeros for you.

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

The $(()) sets up an arithmetic context and the 10# converts the number from base 10 to base 10 causing any leading zeros to be dropped.

shopt -s extglob
iptables -t nat -I POSTROUTING -s "10.$machinetype.${machinenumber##+(0)}.0/24" -j MASQUERADE

When extglob is turned on, the parameter expansion shown removes all leading zeros. Unfortunately, if the original value is 0, the result is a null string.

Answer 2

you can also do

machinenumber=$(expr $machinenumber + 0)

Answer 3

A pure bash solution:

> N=0001023450 
> [[ $N =~ "0*(.*)" ]] && N=${BASH_REMATCH[1]}
> echo $N 
1023450

Answer 4

I would say you are very close. I do not see a requirement stated for bash, but your nonzero logic is flawed.

nonzero=`echo $machinenumber + 0 | bc`
iptables -t nat -I POSTROUTING -s 10.$machinetype.$nozero.0/24 -j MASQUERADE

Adding 0 is a common method for changing a string number into a non-padded integer. bc is a basic calculator. I use this method for removing space and zero padding from numbers all the time.

While I am not expert in iptables syntax, I am pretty sure the parenthesis are not necessary. Since I already have non-word characters bordering both variables, I do not need special enclosures around them. Word characters are;

[a-zA-z0-9_]

Using this solution, you do not lose zero as a potential value, and should be portable across all shells.

Answer 5

If you are using bash, this looks like the simplest:

nozero=$(bc<<<$machinenumber)

Answer 6

Using sed:

echo 000498 | sed "s/^0*\([1-9]\)/\1/;s/^0*$/0/"
498
echo 000 | sed "s/^0*\([1-9]\)/\1/;s/^0*$/0/"
0