watch 'ps aux | grep [n]ginx' # default: rerun every 2 seconds. Quote pipelines!
watch -n 1 'ss -s' # every second
watch -d 'ip -s link show eth0' # -d highlights what changed between runs
watch -n 5 -t 'df -h /' # -t hides the header
watch -g 'command' # exit when the output changes
watch -e 'command' # stop if the command fails
[n]ginx in the grep pattern stops grep from matching its own process. Without quotes, the pipe applies to watch itself. Alternatives: while sleep 2; do clear; command; done, or the tool's own interval option (vmstat 1, iostat 1, ping).