LXer: The Linux Time Command: An Overview
 

Published at LXer:

An overview of the Linux Time command, with example on how it can be used to measure command execution

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Time is a builtin, so on different shells time is different. I use tcsh, and time can give a bit more info by default. I tried it on zsh and the same seems to be true.

You can get additional information from your shell man page.