Excercise: use ps to show the running time of a single Linux-process.
Condition: if multiple processes occur, show the oldest.

Solution:

ps -C <processname>  --no-header -o etime --sort=start_time | # get all processes
	tr -d ' ' | # eliminate spaces
	head -1 | # take first line (oldest)
	sed 's/^\([0-9][0-9]:[0-9][0-9]$\)/00:\1/' | # if process is younger than 1 hour add missing hour field (ps shortens the display to min:sec) 
	sed 's/0\([0-9]\)/\1/g' | # eliminate leading zeros
	sed 's/-/ days, /' | # transform dash to days
	awk 'BEGIN{FS=":"} {print " " $1 " hours, " $2 " minutes, " $3 " seconds"}' | # format output
	sed 's/ 1 \(day\|hour\|minute\|second\)s/ 1 \1/g' # correct singular forms

Open question: Is the "days-" field from ps an endless counter or do we have to expect weeks, months, years?
Answer: days- does not explode to weeks or months
.