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In a previous tech tip, we saw how to use kill to monitor processes. Another option is to use ps. With both methods, you can check $? for success/failure. However, note that kill -0 may return failure even if the process actually exists. This happens when the current user has no permission to the process in question, for example: kill -0 1.

To check for a process silently (with no output), use:

kill -0 PID 2>/dev/null
ps -p PID >/dev/null

When looking at log files or other files that are compressed and rotated automatically, it's useful to be able to deal with them in a uniform fashion. The following bash function does that:

function data_source ()
{
 local F=$1

 # strip the gz if it's there
 F=$(echo $F | perl -pe 's/.gz$//')

 if [[ -f $F ]] ; then
  cat $F
 elif [[ -f $F.gz ]] ; then
  nice gunzip -c $F
 fi
}

Now, when you want to process the files, you can use:

for file in * ; do
 data_source $file | ...
done

If you have bzip2 files, just modify the data_source function to check for that also.

Using netstat, you can monitor programs that are making connections to remote hosts:

$ netstat -tpe

The -t flag limits the output to show only TCP connections. The -p flag displays the PID and name of the program making the connection. The -e flag displays extra information, such as the user name under which each program is running.