Help with syntax with using For and While Loop To Ping Hosts Till They Are Up

JockVSJock · 01-27-2022, 09:16 AM

I don't script everyday (I would like to)...so these skills are perishable.

I have a script that will ping servers, show that they are up. This is my code that works, which is a for loop.

Code:

#!/bin/bash

for hostname in server{1..4}

do 
  ping -c 1 -t 1 $hostname > /dev/null && echo "${hostname} appears up"; 
done

Output of code below:

Code:

server1 appears up
server2 appears up
server3 appears up
server4 appears up

I want to add a while loop to encompass the for loop, as I want it to continually ping all servers until the servers are up, and once all ping back successfully, then drop out of the loop and and the script.

Code:

#!/bin/bash 

UNREACHABLE=1; 

while [ $UNREACHABLE -ne "0" ];
do
  ping -c 1 -t 1 $hostname > /dev/null && echo "{hostname} appears up";
done

When I run this, I'm getting "syntax error." I'm just not clicking with how to combine the two.

thanks

michaelk · 01-27-2022, 09:29 AM

Something like:

Code:

i=1
sp="/-\|"
echo -n ' '
while true; do
    fping -q server1 server2 server3 server4
    if [ $? -eq 0 ]; then
       echo "All servers are up"
       exit
    fi
    printf "\rWaiting..${sp:i++%${#sp}:1}"
    sleep 1 # might consider adding a sleep statement. 

done

sundialsvcs · 01-27-2022, 11:26 AM

See also the "continuously check processes" thread in this section of LQ. If your actual objective is to query the health of other servers, there are several good subsystems (such as Nagios) that are specifically designed for doing that sort of thing. So, this is a chore that somebody else has already well-solved.

JockVSJock · 01-27-2022, 01:28 PM

That works. Not going to install fping, so I'll keep using ping and add -c and -t flags too. Yup, spot on about adding sleep command too.

But why did you use if statement in the while loop Vs what I was originally trying to do?

thanks

Quote:

Originally Posted by michaelk

Something like:

Code:

i=1
sp="/-\|"
echo -n ' '
while true; do
    fping -q server1 server2 server3 server4
    if [ $? -eq 0 ]; then
       echo "All servers are up"
       exit
    fi
    printf "\rWaiting..${sp:i++%${#sp}:1}"
    sleep 1 # might consider adding a sleep statement. 

done

michaelk · 01-27-2022, 01:49 PM

Its the first thing that popped into my brain...

Lots of ways... Untested

Code:

#!/bin/bash

reached=0
while true; do
  for hostname in server{1..4}
  do 
    ping -c 1 -t 1 $hostname > /dev/null; 
    if [ $? -eq 0 ]; then
       reached=$((reached+1))
    fi
  done
  if [[ $reached == 4 ]]; then
     echo "Servers up"
     exit
  fi
  reached=0
done

or

Code:

#!/bin/bash

arr=(1 0 0 0 0)
while true: do
  for num in {1..4}
  do 
    ping -c 1 -t 1 "server$num" > /dev/null; 
    if [ $? -eq 0 ]; then
      arr[$num]=1
    fi
    if [[ ! " ${arr[*]} " =~ "0" ]]; then
      echo "ready"
      exit
    fi
  done
done

You need to check the return status for each server and then determine if all 4 or up or not.

MadeInGermany · 01-30-2022, 05:55 AM

Try this logic:

Code:

UNREACHABLE=1
while [ "$UNREACHABLE" ]
do
  UNREACHABLE=
  for hostname in server{1..4}
  do 
    if ping -c 1 -t 1 "$hostname" > /dev/null
    then
      echo "${hostname} appears up"
    else
      UNREACHABLE=1
    fi
  done
done

michaelk · 01-30-2022, 07:12 AM

Simpler the mine..

jmgibson1981 · 01-30-2022, 08:54 AM

I've personally never seen anyone try to put ; at the end of a variable declaration. That would be your syntax error I believe.

Code:

UNREACHABLE=1;

should be

Code:

UNREACHABLE=1

boughtonp · 01-30-2022, 09:17 AM

Quote:

Originally Posted by jmgibson1981

I've personally never seen anyone try to put ; at the end of a variable declaration. That would be your syntax error I believe.

Incorrect - the semi-colon simply ends the line/command and so is basically ignored. It is not a syntax error.

You can confirm this for yourself by executing and/or using ShellCheck on:

Code:

#!/bin/sh
UNREACHABLE=1;
echo "$UNREACHABLE"

(If one wanted a literal semi-colon at the end of a value, they would need to quote or escape it.)