shell - What explains the inaccuracy of `time` on macOS?

Question

Note: I've seen https://stackoverflow.com/a/64315882/21728 and understand that time is not necessarily that precise. However, I'm seeing a 4× difference between the reported time and time it actually took, and I'd like to understand what's causing it on macOS – that's the point of this question.

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I'm trying to compare two ways to run a binary and they report very similar time info:

$ time ../../../node_modules/.bin/quicktype --version
quicktype version 15.0.214
Visit quicktype.io for more info.
../../../node_modules/.bin/quicktype --version  0.46s user 0.06s system 110% cpu 0.474 total

$ time $(yarn bin quicktype) --version
quicktype version 15.0.214
Visit quicktype.io for more info.
$(yarn bin quicktype) --version  0.44s user 0.06s system 110% cpu 0.449 total

However, the latter feels much slower. So I've added timestamps before and after:

$ date +"%T.%3N" && time $(yarn bin quicktype) --version && date +"%T.%3N"
15:11:09.667
quicktype version 15.0.214
Visit quicktype.io for more info.
$(yarn bin quicktype) --version  0.49s user 0.06s system 108% cpu 0.513 total
15:11:11.400

Indeed, the difference between 15:11:09.667 and 15:11:11.400 is almost two seconds but time is reporting about 0.5 second. What explains this rather vast difference?

question from:https://stackoverflow.com/questions/66064977/what-explains-the-inaccuracy-of-time-on-macos

Answer 1

I was using the time wrong.

First, time is different from /usr/bin/time on my Mac:

• time is a shell built-in (I use Zsh)
• /usr/bin/time is BSD time

This gives the expected results:

$ /usr/bin/time bash -c '../../../node_modules/.bin/quicktype --version'
quicktype version 15.0.214
Visit quicktype.io for more info.
        0.49 real         0.47 user         0.06 sys

$ /usr/bin/time bash -c '$(yarn bin quicktype) --version'
quicktype version 15.0.214
Visit quicktype.io for more info.
        2.02 real         1.92 user         0.27 sys