Bash: while [[ -e /proc/$process_id ]]

Sometimes you need to keep track of the amount of memory a process is taking up (or CPU, or something else). The /proc filesystem contains a subdirectory for each running process; the directory is named with the pid of the process. You can use this fact along with a basic file test and a while loop to track a process for as long as it lives.

> some_interesting_job.py &

> process_id=$(ps -o "%p %c" | grep "some_interesting_job" | cut -f 1 -d ' ');\
 while [[ -e /proc/$process_id ]];\
 do ps -o "%z";\
 sleep 5;\
 done

This will report on the virtual memory size (in KiB - see 'ps' manpage for more details) that the process is taking up. The while loop will terminate when the process completes (or is killed).