time
time [ -apqvV ] [ -f FORMAT ] [ -o FILE ] [ --append ] [ --verbose ] [ --quiet ] [ --portability ] [ --format=FORMAT ] [ --output=FILE ] [ --version ] [ --help ] COMMAND [ ARGS ]
time runs a command and times how long it takes, summarising system resource usage.
A little known but extremey useful little utility. Just prefix any shell command with the word 'time' and it will tell you how long it took to run.