BASH – The Bourne Again Shell amongst most if not all other shells allows each application to exit with a return code. Some shells and environments have limits on what range this integer can fall into. Something between 0 and 255 inclusive is always a safe bet. In BASH, the variable $? is populated with the return code of the last command to return control back to the shell. It is important to preserve the return code immediately after the application exits that we want to monitor, as subsequent commands will overwrite the variable. The ping tool returns 0 on success:
ping -c1 ${HOST} -q 2>&1 >/dev/null #ping HOST once and do not print any output to the screen
RET=$? #assign the return code to RET so we can preserve it for after the ‘if’
if [ ${RET} -eq 0 ]; then
#we were successful.
echo “We were successful”
else
#we weren’t successful
echo “Host ${HOST} failed ping monitoring on `date`” |mail -s “Uptime Monitoring” admin@example.com
fi
Now of course there are easier ways of achieving the above task, although I’ve laid out the script in this way hoping that the way I have laid it out illustrates capturing the code and preserving it beyond the ‘if’ that follows which would have overwritten it. Just as further illustration, calling ping invalid followed directly by echo $? shows a return code of ‘2’ – obviously the return code for such a failure. Calling echo $? again immediately after shows a return code of ‘0’ as the return code of ping was overwritten by the return code of the first echo statement. Bash builtins return codes to the shell as any other application would.
