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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 myname@gmail.com
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?

  • You know to write foo eq 0, but don't know why [ ] is needed or that it's -eq? You need a bash tutorial. – muru Aug 11 '17 at 5:24
  • What can I say? I'm flattered you took the time to go through my 2500+ answers on SE to conclude that I couldn't write such an answer. – muru Aug 12 '17 at 14:12
  • Good point. Shifting from C Shell to bash (when I used to use korn long ago) is requiring some tutoring. And yes I have seen a lot of examples where you do so quite thoroughly, @muru, so I take that back. --> In any case, while you were weighing in about my lack of ability (the reason I had to ask the question), the answer was simultaneously being written and within a couple of hours was incorporated into my program, where it will now save me time every day. – SDsolar Aug 12 '17 at 23:15
  • it wasn't the lack of ability so much as the danger - say you wrote something that looks it works, but that was actually due to a quirk of syntax, and you deploy it somewhere where it can cause actual damage due to some edge case you couldn't test... – muru Aug 13 '17 at 0:06
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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.

  • So the if statement will evaluate success or failure and I don't need to check the numerical return value, eh? This is short and sweet - just what I have been looking for. Thank you. – SDsolar Aug 11 '17 at 5:34
  • 1
    @SDsolar Read it as if true (exit status is not like the return value BTW). $() is called command substitution, it has specific use cases but not needed here. – heemayl Aug 11 '17 at 5:35
  • Thank you again, @heemayl. You have helped me a lot. I was having the midnight program email me a log file and needed to look at it carefully each day to catch anomalies. Now I get email alerts regarding only those sensors in need of a touch of TLC. That's a HUGE improvement. TNX AGN. – SDsolar Aug 12 '17 at 3:29
  • 1
    @SDsolar Great. Glad I could help :) – heemayl Aug 12 '17 at 8:02

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