What do 'real', 'user' and 'sys' mean in the output of time(1)?

Question

$ time foo
real        0m0.003s
user        0m0.000s
sys         0m0.004s
$

What do \'real\', \'user\' and \'sys\' mean in the output of time?

Answer 1

In very simple terms, I like to think about it like this:

• real is the actual amount of time it took to run the command (as if you had timed it with a stopwatch)

• user and sys are how much 'work' the CPU had to do to execute the command. This 'work' is expressed in units of time.

Generally speaking:

• user is how much work the CPU did to run to run the command's code
• sys is how much work the CPU had to do to handle 'system overhead' type tasks (such as allocating memory, file I/O, ect.) in order to support the running command

Since these last two times are counting 'work' done, they don't include time a thread might have spent waiting (such as waiting on another process or for disk I/O to finish).

real, however, is a measure of actual runtime and not 'work', so it does include any time spent waiting.