Removing leading zeros before passing a shell variable to another command

bash strip
Jarmund picture Jarmund · Jun 20, 2012 · Viewed 63.4k times · Source

It turns out that iptables doesn't handle leading zeros too well. As $machinenumber that is used has to have a leading zero in it for other purposes, the idea is simply to create a new variable ($nozero) based on $machinenumber, where leading zeros are stripped away.

$machinenumber is a two-digit number between 01 and 24. Currently it's 09

$machinetype is 74 for now and hasn't caused any problems before.

What I have so far is:

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

While I believe I'm on the right track, the code results in:

ERROR - Unknown string operation

Answer

Dennis Williamson picture Dennis Williamson · Jun 21, 2012

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.

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