The problem#

I used to have an occasionally unreliable internet connection. I wanted logs of exactly how unreliable it was and an easy way to have notice when it was back up.

The solution#

Use cron to check online status once a minute and write the result to a file. An easy way to check is to confirm that google.com will reply to a ping (this does give a false negative in the unlikely event that Google is down).

To run a script every minute, put a file in /etc/cron.d containing the line

* * * * * root /root/bin/online-check

where /root/bin/online-check is the following script:

#!/bin/sh

# Check if computer is online by attempting to ping google.com.
PING_RESULT="`ping -c 2 google.com 2>/dev/null`"
if [ $? -eq 0 ] && ! echo "$PING_RESULT" | grep -F '64 bytes from 192.168.' >/dev/null 2>/dev/null
then
    ONLINE="online"
else
    ONLINE="offline"
fi
echo "`date '+%Y-%m-%d %T%z'` $ONLINE" >> /var/log/online.log

The details and pretty printing#

Log details#

That script creates a log in /var/log/online.log like

2010-09-27 05:58:21-0700 offline
2010-09-27 05:59:21-0700 offline
2010-09-27 06:00:02-0700 online
2010-09-27 06:01:02-0700 online

Note the check for a response from 192.168. due to some failures resulting in getting a ping reply from my router. Your network setup may vary and may display different failure modes.

As a little extra redundancy, -c 2 tries two pings.

Generating summaries#

Here are a few scripts I wrote for summarizing the log:

display-offline-ranges:

#!/bin/sh
grep -B1 -A1 offline /var/log/online.log |
    grep -v offline |
    cut -d ' ' -f 1,2

Example output showing the start and end times of two offline periods:

$ display-offline-ranges
2010-09-28 23:28:12-0700
2010-09-29 06:44:02-0700
--
2010-09-29 09:10:02-0700
2010-09-29 09:16:02-0700
--

display-offline-times:

#!/bin/sh
uniq -f2 -cd /var/log/online.log | grep offline

Example output showing offline periods with their length in minutes and starting time:

$ display-offline-times
    117 2010-09-27 04:04:21-0700 offline
     12 2010-09-28 14:46:21-0700 offline
    435 2010-09-28 23:29:12-0700 offline

offline-times-csv (the CSV version of the above):

#!/bin/sh
echo 'Minutes offline,Date,Time'
display-offline-times |
    grep -v '      [2-5] ' |
    sed "s/ offline//" |
    sed 's/\([0-9]\) /\1,/g' |
    sed 's/^ *//'

Example output (for feeding into a spreadsheet program):

$ offline-times-csv
Minutes offline,Date,Time
117,2010-09-27,04:04:21-0700
12,2010-09-28,14:46:21-0700
435,2010-09-28,23:29:12-0700

Pretty printing#

The following just pretty prints the log as it is generated. It colors offline lines with a red background and online lines with a green background so the difference is easily visible across a room. That way you can glance up from whatever else you are doing while waiting for your internet connect to return. The result looks like

2010-09-27 05:58:21-0700 offline
2010-09-27 05:59:21-0700 offline
2010-09-27 06:00:02-0700 online
2010-09-27 06:01:02-0700 online

tail_online_status:

#!/bin/sh
tailf /var/log/online.log | while read line
do
    if (echo $line | grep -Fq online)
    then
        printf "\033[48;5;2m"
    else
        printf "\033[48;5;1m"
    fi
    echo $line
done

tailf is for following log files as they are written. The printf commands are using ANSI escape codes to set the background color of the terminal.