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

Question

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

It would help if I could give it as input a range of IP's.

Answer 1

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..

Answer 2

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.

Answer 3

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

Answer 4

There is also fping:

fping -g 192.168.1.0/24

or:

fping -g 192.168.1.0 192.168.1.255

or show only hosts that are alive:

fping -ag 192.168.1.0/24

It pings hosts in parallel so the scan is very fast. I don't know a distribution which includes fping in its default installation but in most distributions you can get it through the package manager.

Answer 5

Also using the "ping the broadcast address" method pointed out by chburd, this pipe should do the trick for you:

ping -c 5 -b 10.11.255.255 | sed -n 's/.* \([0-9]\+\.[0-9]\+\.[0-9]\+\.[0-9]\+\).*/\1/p' | sort | uniq

Of course, you'd have to change the broadcast address to that of your network.

Answer 6

Just for fun, here's an alternate

     #!/bin/bash
     nmap -sP 192.168.1.0/24 > /dev/null 2>&1 && arp -an | grep -v incomplete | awk '{print$2}' | sed -e s,\(,, | sed -e s,\),,

Answer 7

If you're limiting yourself to only having the last octet changing, this script should do it. It should be fairly obvious how to extend it from one to multiple octets.

#! /bin/bash
BASE=$1
START=$2
END=$3

counter=$START

while [ $counter -le $END ]
do
  ip=$BASE.$counter
  if ping -qc 2 $ip
  then
    echo "$ip responds"
  fi
  counter=$(( $counter + 1 ))
done

Answer 8

• ip neighbor
• arp -a
• Arpwatch

Answer 9

As other posters pointed out, nmap is the way to go, but here's how to do the equivalent of a ping scan in bash. I wouldn't use the broadcast ping, as a lot of systems are configured not to respond to broadcast ICMP nowadays.

for i in $(seq 1 254); do
    host="192.168.100.$i"
    ping -c 1 -W 1 $host &> /dev/null
    echo -n "Host $host is "
    test $? -eq 0 && echo "up" || echo "down"
done

Answer 10

#!/bin/bash
#Get the ip address for the range
ip=$(/sbin/ifconfig eth0 | grep 'inet addr:' | cut -d: -f2 | awk '{ print $1}' | cut -d"." -f1,2,3)

# ping test and list the hosts and echo the info

for range in $ip ; do  [ $? -eq 0 ] && ping -c 1 -w 1 $range > /dev/null 2> /dev/null && echo "Node $range is up" 
done

Answer 11

Although an old question, it still seems to be important (at least important enough for me to deal with this). My script relies on nmap too, so nothing special here except that ou can define which interface you want to scan and the IP Range is created automagically (at least kind of).

This is what I came up with

#!/bin/bash
#Script for scanning the (local) network for other computers 

command -v nmap >/dev/null 2>&1 || { echo "I require nmap but it's not installed. Aborting." >&2; exit 1; }

if [ -n ""$@"" ];  then
    ip=$(/sbin/ifconfig $1 | grep 'inet '  | awk '{ print $2}' | cut -d"." -f1,2,3 )
    nmap -sP $ip.1-255
else
    echo -e "\nThis is a script for scanning the (local) network for other computers.\n"
    echo "Enter Interface as parameter like this:"
    echo -e "\t./scannetwork.sh $(ifconfig -lu | awk '{print $2}')\n"

    echo "Possible interfaces which are up are: "   
    for i in $(ifconfig -lu)
    do
        echo -e "\033[32m \t $i \033[39;49m"
    done

    echo "Interfaces which could be used but are down at the moment: "
    for i in $(ifconfig -ld)
    do
        echo -e "\033[31m \t $i \033[39;49m"
    done
    echo
fi

One remark: This script is created on OSX, so there might be some changes to linux environments.

Answer 12

If you want to provide a list of hosts it can be done with nmap, grep and awk.

Install nmap:

$ sudo apt-get install nmap

Create file hostcheck.sh like this:

hostcheck.sh

#!/bin/bash

nmap -sP -iL hostlist -oG pingscan > /dev/null
grep Up pingscan | awk '{print $2}' > uplist
grep Down pingscan | awk '{print $2}' > downlist

-sP: Ping Scan - go no further than determining if host is online

-iL : Input from list of hosts/networks

-oG : Output scan results in Grepable format, to the given filename.

/dev/null : Discards output

Change the access permission:

$ chmod 775 hostcheck.sh

Create file hostlist with the list of hosts to be checked (hostname or IP):

hostlist (Example)

192.168.1.1-5
192.168.1.101
192.168.1.123

192.168.1.1-5 is a range of IPs

Run the script:

./hostcheck.sh hostfile

Will be generated files pingscan with all the information, uplist with the hosts online (Up) and downlist with the hosts offline (Down).

uplist (Example)

192.168.1.1
192.168.1.2
192.168.1.3
192.168.1.4
192.168.1.101

downlist (Example)

192.168.1.5
192.168.1.123

Answer 13

Some machines don't answer pings (e.g. firewalls).

If you only want the local network you can use this command:

(for n in $(seq 1 254);do sudo arping -c1 10.0.0.$n &  done ; wait) | grep reply | grep --color -E '([0-9]+\.){3}[0-9]+'

Explanations part !

1. arping is a command that sends ARP requests. It is present on most of linux.

Example:

sudo arping -c1 10.0.0.14

the sudo is not necessary if you are root ofc.

• 10.0.0.14 : the ip you want to test
• -c1 : send only one request.

---

2. &: the 'I-don't-want-to-wait' character

This is a really useful character that give you the possibility to launch a command in a sub-process without waiting him to finish (like a thread)

---

3. the for loop is here to arping all 255 ip addresses. It uses the seq command to list all numbers.

---

4. wait: after we launched our requests we want to see if there are some replies. To do so we just put wait after the loop. wait looks like the function join() in other languages.

---

5. (): parenthesis are here to interpret all outputs as text so we can give it to grep

---

6. grep: we only want to see replies. the second grep is just here to highlight IPs.

hth

Edit 20150417: Maxi Update !

The bad part of my solution is that it print all results at the end. It is because grep have a big enough buffer to put some lines inside. the solution is to add --line-buffered to the first grep. like so:

(for n in $(seq 1 254);do sudo arping -c1 10.0.0.$n &  done ; wait) | grep --line-buffered reply | grep --color -E '([0-9]+\.){3}[0-9]+'

Answer 14

#!/bin/bash

for ((n=0 ; n < 30 ; n+=1))
do
    ip=10.1.1.$n
    if ping -c 1 -w 1 $ip > /dev/null 2> /dev/null >> /etc/logping.txt; then  
        echo "${ip} is up" # output up
        # sintax >> /etc/logping.txt log with .txt format
    else
        echo "${ip} is down" # output down
    fi
done

Answer 15

The following (evil) code runs more than TWICE as fast as the nmap method

for i in {1..254} ;do (ping 192.168.1.$i -c 1 -w 5  >/dev/null && echo "192.168.1.$i" &) ;done

takes around 10 seconds, where the standard nmap

nmap -sP 192.168.1.1-254

takes 25 seconds...

Answer 16

Well, this is part of a script of mine.

ship.sh 🚢 A simple, handy network addressing 🔎 multitool with plenty of features 🌊

Pings network, displays online hosts on that network with their local IP and MAC address

It doesn't require any edit. Needs root permission to run.

GOOGLE_DNS="8.8.8.8"
ONLINE_INTERFACE=$(ip route get "${GOOGLE_DNS}" | awk -F 'dev ' 'NR == 1 {split($2, a, " "); print a[1]}')
NETWORK_IP=$(ip route | awk "/${ONLINE_INTERFACE}/ && /src/ {print \$1}" | cut --fields=1 --delimiter="/")
NETWORK_IP_CIDR=$(ip route | awk "/${ONLINE_INTERFACE}/ && /src/ {print \$1}")
FILTERED_IP=$(echo "${NETWORK_IP}" | awk 'BEGIN{FS=OFS="."} NF--')

ip -statistics neighbour flush all &>/dev/null

echo -ne "Pinging ${NETWORK_IP_CIDR}, please wait ..."
for HOST in {1..254}; do
  ping "${FILTERED_IP}.${HOST}" -c 1 -w 10 &>/dev/null &
done

for JOB in $(jobs -p); do wait "${JOB}"; done

ip neighbour | \
    awk 'tolower($0) ~ /reachable|stale|delay|probe/{printf ("%5s\t%s\n", $1, $5)}' | \
      sort --version-sort --unique