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How can I pass an optional argument to a bash script that will replace an existing variable in the script? For example:

#!/bin/bash
#hostfinder.sh
#Find hosts in current /24 network

subnet=$(hostname -i | cut -d. -f1,2,3)

for j in \
    $(for i in $subnet.{1..255}; do host $i; done | grep -v not | cut -d" " -f5)
do ping -c1 -w 1 $j; done | grep -i from | cut -d" " -f3,4,5 | tr ':' ' ' | \
sed -e 's/from/Alive:/'

This will grab the IP of the current host, run a reverse lookup against possible neighbors, ping test any hostnames it finds, and display an output similar to this:

Alive: host1.domain (10.64.17.23)
Alive: host2.domain (10.64.17.24)
...

Call me crazy, but it's way faster than nmap and spits out a nice list.

Anyway, I'd like to optionally pass the first three octets of any class C network address to the $subnet variable as the $1 argument when executing the script. For example:

./hostfinder.sh 10.20.0

My first thought was to try something like $subnet=$1, but I assume that will not work. I'm not really interested in re-writing the script to be more elegant or anything, I am mostly just curious about what I put in the subject line.

share|improve this question
nmap is meant to do a completely different thing. – LtWorf Mar 13 at 9:41
nmap can do a lot of things, though. :) – gNU.be Mar 13 at 9:47
Also, I think a ping sweep with nmap goes something like this: nmap -sP 192.168.1.1-254 – gNU.be Mar 13 at 9:50
Did subnet=$1 not work for you? – fbynite Mar 13 at 10:22

2 Answers

up vote 1 down vote accepted

How about replace: 

subnet=$(hostname -i | cut -d. -f1,2,3)

with:

case $# in  
  0) subnet=$(hostname -i | cut -d. -f1,2,3);;
  1) subnet="${1}";;
  *) echo "To many arguments" >&2; exit 1;;
esac
  • $# is the number of command line arguments
  • This is less elegant as getopt but easy to understand and extend.
share|improve this answer
Thank you. I will play with this and respond. – gNU.be Mar 13 at 10:40
That worked like a champ. Thanks a lot! I can extend it with more options for class A or B now. This will turn into a nice portable sweeper where I don't have nmap installed or when nmap -sP just doesn't seem to work on certain subnets with hosts I need to inventory. – gNU.be Mar 13 at 11:21
up vote 0 down vote

Try using getopt, to read from the options in the command line, and then set your variable.

share|improve this answer
Thanks for the suggestion. Would you mind sharing a bit of an example, please? – gNU.be Mar 13 at 9:48

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