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SDsolar
Asked: 2017-08-11 21:14:00 +0800 CST

Ubuntu 16.04 how to use ping in a bash script to decide whether to skip a block of code

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I have data loggers spread around the solar power system, and a single program that runs at midnight to collect the day's data and put it on a hard drive.

The bash script on the Ubuntu server steps through 7 Raspbian units that have the data collected and puts it in the proper place on the hard drive, in directories named by the units out there. I figured out how to take the generic names today.dat and today.png and save them with the day's date in the archive like 2017-08-10.png and 2017-08-10.dat

--> Occasionally one of the units will not respond due to some problem or other, and it blocks execution of the program.

So I would like to use ping to test whether the system responds before I decide whether to try to gather data from it.

My system names are all in the /etc/hosts file so I reference them by name.

For purposes of this question I am checking SOLAR - the one that monitors the charge controller and inverter behavior.

Here is what I have tried:

if $( ping -c 1 SOLAR | grep icmp* | wc -l ) eq 0
then
   (do stuff like ssh -e "gnuplot makepng" then scp the png and raw data)
fi

(too complicated)

and 

if [ $(ping -c 1 SOLAR >/dev/null) eq 0 ]
    then do the stuff
fi

(looks OK but I don't understand the need for the square brackets)

There must be a better way.

I am hoping to do something more like 

if $(ping -c 1 SOLAR) eq 0
then
   (do stuff)
else
   (then maybe echo "SOLAR didn't respond" | ssmtp [email protected]
fi

I haven't programmed bash in a couple of decades - used C shell most of the time. And I am getting way too much varying advice all over the net about the best way to do this.

So can anybody please suggest a simple way to do this?

bash

1 Answers

  1. Best Answer

    It seems you are only interested in the exit status of ping i.e. whether you are getting ICMP ECHO_RESPONSE from the host to your ECHO_REQUEST; you can just do ping -c 1 ... and ping would exit with status 0 if the host is sending response, 1 if no response and 2 for unknown host.

    You can easily use this with if construct; you don't need to count response, just use ping as if's condition as if would evaluate the exit status of ping to proceed to any defined branch:

    if ping -c 1 SOLAR &>/dev/null; then
    ## Reachable -- do stuffs
else
    ## Unreachable -- do other stuffs
fi

    I am redirecting ping's STDOUT and STDERR streams to /dev/null as we are not interested in those.

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