The curl command line option --write-out or just -w for short, is a powerful and flexible way to extract information from transfers done with the tool. It was introduced already back in version 6.5 in the early 2000.
This option takes an argument in which you can add “variables” that hold all sorts of different information, from time information, to speed, sizes, header content and more.
Some users have right out started to use the -w output for logging of the performed transfer, and when you do that there was a little detail missing: the ability to output the time the transfer completed. After all, most log lines actually feature the time in one way or another.
Starting in curl 8.16.0, curl -w knows the time and now allows the user to specify exactly how to output that time in the output. Suddenly this output is a whole notch better for logging purposes.
%time{format}
Since log files also tend to use different time formats I decided I didn’t want to use a fixed format and risk that a huge portion of users will think it is the wrong one, so I went straight with strftime formatting: the user controls the time format using standard %-flags: different ones for year, month, day, hour, minute, second etc.
Some details to note:
- The time is provided using UTC, not local.
- It also supports %f for microseconds, which is a POSIX extension already used by Python and possible others
- %z and %Z (for time zone offset and name) had to be fixed to become portable and identical across systems and platforms
Example
Here’s a sample command line outputting the time the transfer completed:
curl -w "%time{%a %b %e %Y - %H:%M:%S.%f} %{response_code}\n" https://example.com -o saved
When I ran this command line it gave me this output:
Wed Aug 6 2025 - 12:43:45.160382 200