Using command line arguments in for loop (bash)

Question

I'm trying to write a bash script that's supposed to perform reverse DNS lookup on a range of IP addresses. The idea is to provide the network (first 3 octets) as $1, the start of the range to be checked as $2 and the end of said range as $3.

So I ended up with:

#!/bin/bash
if [ $# -ne 3 ]
then
    echo "[*] Usage: reverselookup.sh [Network (x.x.x)] [start of range (y)]  [end of range (z)]"
    exit 1
fi

for ip in {$2..$3}
do
    host $1.$ip | grep pointer
done

Now I thought that this would use the normal sequence operation in bash for loops, like in

for i in {2..5}

gives you a loop with 2, 3, 4 and 5. However it doesn't work.

If I echo the $ip inside the loop and run it as e.g.

reverselookup.sh 192.168.10 21 50

it presents me with

{21..50}

Does anyone know whether it's possible to make this work? Or do I have to rethink my appraoch?

Thanks in advance.

See the description of Brace Expansion to see why what you type won't work. — Jonathan Leffler
– Jonathan Leffler, Commented Sep 24, 2013 at 7:27

devnull · Accepted Answer · 2013-09-24 07:30:22Z

6

You can use seq to replace

for ip in {$2..$3}

Say:

for ip in $(seq $2 $3)

answered Sep 24, 2013 at 7:30

devnull

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2 Comments

user000001 Over a year ago

The seq command is probably more portable that the c-style loop. The downside is that it will create an extra process, and that the number of iterations are limited by the size of the command buffer.

fragman Over a year ago

Thanks for the info as well as your approach but since the use case here is a maximum of 255 iterations this shouldn't be an issue. Please do correct me if that's wrong.

user000001 · Accepted Answer · 2013-09-24 07:25:40Z

4

Brace expansion won't work with variables, because it is performed before the variable expansion. You can use a c-style for loop instead:

for ((ip=$2; ip<=$3; ip++))
do
    ....
done

answered Sep 24, 2013 at 7:25

user000001

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Comments

islandsvinur · Accepted Answer · 2013-09-24 07:30:37Z

3

You can use the seq command (part of coreutils) to generate the list; like so

for ip in $(seq $2 $3)
do
    host $1.$ip | grep pointer
done

answered Sep 24, 2013 at 7:30

islandsvinur

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