How can I write a Linux bash script that tells me which computers are ON in my LAN?

I would suggest using nmap's ping-scan flag,

$ nmap -sn 192.168.1.60-70

Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2009-04-09 20:13 BST
Host machine1.home (192.168.1.64) appears to be up.
Host machine2.home (192.168.1.65) appears to be up.
Nmap finished: 11 IP addresses (2 hosts up) scanned in 0.235 seconds

That said, if you want to write it yourself (which is fair enough), this is how I would do it:

for ip in 192.168.1.{1..10}; do ping -c 1 -t 1 $ip > /dev/null && echo "${ip} is up"; done

..and an explanation of each bit of the above command:

Generating list of IP addresses

You can use the {1..10} syntax to generate a list of numbers, for example..

$ echo {1..10}
1 2 3 4 5 6 7 8 9 10

(it's also useful for things like mkdir {dir1,dir2}/{sub1,sub2} - which makes dir1 and dir2, each containing sub1 and sub2)

So, to generate a list of IP's, we'd do something like

$ echo 192.168.1.{1..10}
192.168.1.1 192.168.1.2 [...] 192.168.1.10

Loops

To loop over something in bash, you use for:

$ for thingy in 1 2 3; do echo $thingy; done
1
2
3

Pinging

Next, to ping.. The ping command varies a bit with different operating-systems, different distributions/versions (I'm using OS X currently)

By default (again, on the OS X version of ping) it will ping until interrupted, which isn't going to work for this, so ping -c 1 will only try sending one packet, which should be enough to determine if a machine is up.

Another problem is the timeout value, which seems to be 11 seconds on this version of ping.. It's changed using the -t flag. One second should be enough to see if a machine on the local network is alive or not.

So, the ping command we'll use is..

$ ping -c 1 -t 1 192.168.1.1
PING 192.168.1.1 (192.168.1.1): 56 data bytes

--- 192.168.1.1 ping statistics ---
1 packets transmitted, 0 packets received, 100% packet loss

Checking ping result

Next, we need to know if the machine replied or not..

We can use the && operator to run a command if the first succeeds, for example:

$ echo && echo "It works"

It works
$ nonexistantcommand && echo "This should not echo"
-bash: nonexistantcommand: command not found

Good, so we can do..

ping -c 1 -t 1 192.168.1.1 && echo "192.168.1.1 is up!"

The other way would be to use the exit code from ping.. The ping command will exit with exit-code 0 (success) if it worked, and a non-zero code if it failed. In bash you get the last commands exit code with the variable $?

So, to check if the command worked, we'd do..

ping -c 1 -t 1 192.168.1.1;
if [ $? -eq 0 ]; then
    echo "192.168.1.1 is up";
else 
    echo "ip is down";
fi

Hiding ping output

Last thing, we don't need to see the ping output, so we can redirect stdout to /dev/null with the > redirection, for example:

$ ping -c 1 -t 1 192.168.1.1 > /dev/null && echo "IP is up"
IP is up

And to redirect stderr (to discard the ping: sendto: Host is down messages), you use 2> - for example:

$ errorcausingcommand
-bash: errorcausingcommand: command not found
$ errorcausingcommand 2> /dev/null
$

The script

So, to combine all that..

for ip in 192.168.1.{1..10}; do  # for loop and the {} operator
    ping -c 1 -t 1 192.168.1.1 > /dev/null 2> /dev/null  # ping and discard output
    if [ $? -eq 0 ]; then  # check the exit code
        echo "${ip} is up" # display the output
        # you could send this to a log file by using the >>pinglog.txt redirect
    else
        echo "${ip} is down"
    fi
done

Or, using the && method, in a one-liner:

for ip in 192.168.1.{1..10}; do ping -c 1 -t 1 $ip > /dev/null && echo "${ip} is up"; done

Problem

It's slow.. Each ping command takes about 1 second (since we set the -t timeout flag to 1 second). It can only run one ping command at a time.. The obvious way around this is to use threads, so you can run concurrent commands, but that's beyond what you should use bash for..

"Python threads - a first example" explains how to use the Python threading module to write a multi-threaded ping'er.. Although at that point, I would once again suggest using nmap -sn..


In the real world, you could use nmap to get what you want.

nmap -sn 10.1.1.1-255

This will ping all the addresses in the range 10.1.1.1 to 10.1.1.255 and let you know which ones answer.

Of course, if you in fact want to do this as a bash exercise, you could run ping for each address and parse the output, but that's a whole other story.


Assuming my network is 10.10.0.0/24, if i run a ping on the broadcast address like

ping -b 10.10.0.255

I'll get an answer from all computers on this network that did not block their ICMP ping port.

64 bytes from 10.10.0.6: icmp_seq=1 ttl=64 time=0.000 ms
64 bytes from 10.10.0.12: icmp_seq=1 ttl=64 time=0.000 ms 
64 bytes from 10.10.0.71: icmp_seq=1 ttl=255 time=0.000 ms

So you just have to extract the 4th column, with awk for example:

ping -b 10.10.0.255 | grep 'bytes from' | awk '{ print $4 }'

10.10.0.12:
10.10.0.6:
10.10.0.71:
10.10.0.95:

Well, you will get duplicate, and you may need to remove the ':'.

EDIT from comments : the -c option limits the number of pings since the script will end, we can also limit ourself on unique IPs

ping -c 5 -b 10.10.0.255 | grep 'bytes from' | awk '{ print $4 }' | sort | uniq

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Linux

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