I just want to clarify few things here

1) time command is external as well as internal. To use GNU version of external command use full path:
When you type time command with full path, shell will run internal time command. To verify this use following command:
time is a shell keyword
time is /usr/bin/time

As you see time command is both internal and external to shell. That is why you got an error when you use time –o syntax.

2)
It is nothing but one of the compound command. ( time ls ) is executed in a subshell environment so that you can capture its output. Please see this article for more information: http://www.cyberciti.biz/nixcraft/vi...tand-which.php