Linux Shell Scripting Cookbook - Third Edition

Parallel pings

The previous script tests each address sequentially. The delay for each test is accumulated and becomes large. Running the ping commands in parallel will make this faster. Enclosing the body of the loop in {}& will make the ping commands run in parallel. ( ) encloses a block of commands to run as a subshell, and & sends it to the background:

#!/bin/bash 
#Filename: fast_ping.sh 
# Change base address 192.168.0 according to your network. 

for ip in 192.168.0.{1..255} ; 
do 
   ( 
      ping $ip -c2 &> /dev/null ; 

      if [ $? -eq0 ]; 
      then 
       echo $ip is alive 
      fi 
   )& 
  done 
wait 

In the for loop, we execute many background processes and come out of the loop, terminating the script. The wait command prevents the script from terminating until all its child ...