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Ping inside FOR loop

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# 1  
Old 12-05-2010
Question Ping inside FOR loop
Hello,

I'm trying to write (my own primitive) traceroute command that would write out all IPs that packets are hoping through. Smilie

So I've put the ping command inside a for loop

PHP Code:
#!/bin/bash
domain=${1-www.google.com}; #If no argument, then do www.google.com.

for ((i=1; i<=30; i++)) # max. 30 hops
do 
    ping -t $i $domain # I would later add "grep" to extract IPs
done

exit 0; 
What is wrong with this script? All I get is 30 "Usage: ping [...]" replies. Smilie

EDIT: I see it: I've written $1 instead of $i. Smilie But don't you worry, I will most surely come back for help with grep! Smilie
Last edited by courteous; 12-05-2010 at 11:23 AM.. Reason: novice-like error
# 2  
Old 12-05-2010
Hi.

I think that should be $i, not $1 in your ping command. But -t is used to set a ping timeout.

According to the man page on my system -R is used to record the route. You might also want to use -c?
This User Gave Thanks to Scott For This Post:
courteous
# 3  
Old 12-05-2010
Quote:
Originally Posted by scottn
You might also want to use -c?
First, thank you for the reply.
  1. I've seen other scripts that used, say, -c 2, but I don't (really) understand why (yes, I looked at man ping).

    In ICMP's first 8 bits I'll be getting #11 (TTL expired) and not #0 (ECHO_REPLY), which is what switch -c is referring to. Well, what is wrong with my "logic"?
    ADD: Should I use -c 1 for each hop on the way, is that right?
  2. How to find the right maximum TTL number? Smilie for example, pinging google.com it takes 14 hops. So, if I do the for loop 30 times, the last 16 IPs are all (almost) the same.
Last edited by courteous; 12-05-2010 at 11:46 AM..
# 4  
Old 12-05-2010
You miss a colon :

Code:
domain=${1:-www.google.com}

By the way you should check the ping option available for your OS version
(look at the -c option that let you ping a <count> times so the ping isn't "endless")
# 5  
Old 12-05-2010
Hi ctsgnb,
the "default option" works also without the colon. Smilie

I'd be most grateful for any help on 2. question (on choosing max. TTL) ...
and of course any correction on my "thinking" in 1. point (on -c switch). Smilie

ADD: I think I should let ping run inside the for loop, until I'd get ECHO_REPLY, and then break the for loop.
Code:
PING www.l.google.com (209.85.135.99) 56(84) bytes of data.
From 209.85.253.26 icmp_seq=1 Time to live exceeded

--- www.l.google.com ping statistics ---
1 packets transmitted, 0 received, +1 errors, 100% packet loss, time 0ms
Here should be the break-like statement: above is TTL_EXPIRED, below ECHO_REPLY. Is that right?
PING www.l.google.com (209.85.135.99) 56(84) bytes of data.
64 bytes from mu-in-f99.1e100.net (209.85.135.99): icmp_req=1 ttl=250 time=45.0 ms

--- www.l.google.com ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 45.052/45.052/45.052/0.000 ms

Last edited by courteous; 12-05-2010 at 11:55 AM..
# 6  
Old 12-05-2010
The colon isn't really a requirement in this case:

Code:
/ $ set -- www.bbc.co.uk    
               
/ $ domain=${1-www.google.com}
/ $ echo $domain
www.bbc.co.uk

/ $ domain=${2-www.google.com}
/ $ echo $domain              
www.google.com

O/P: -c has nothing to do with hops, but how many times you want to attempt a ping before stopping.

There is no "right TTL" number. Quote from the ping man page:
Quote:
In normal operation ping prints the ttl value from the packet it receives.
# 7  
Old 12-05-2010
Quote:
Originally Posted by courteous
Hi ctsgnb,
the "default option" works also without the colon. Smilie
I'll sleep a little bit less ignorant tonight Smilie
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