On Linux, I need to find the currently configured timezone as an Olson location. I want my (C or C++) code to be portable to as many Linux systems as possible.
For example. I live in London, so my current Olson location is "Europe/London". I'm not interested in timezone IDs like "BST", "EST" or whatever.
Debian and Ubuntu have a file
It's hard to get a reliable answer. Relying on things like
/etc/timezone may be the best bet.
tzname and the
tm_zone member of
struct tm, as suggested in other answers, typically contains an abbreviation such as
BST etc, rather than the Olson time string as requested in the question).
/etc/timezoneis a file containing the right answer.
/etc/localtime, which is a symlink to (for example)
However, there are some issues with the above approaches. The
/usr/share/zoneinfo directory also contains files such as
GB, so it's possible the user may configure the symlink to point there.
Also there's nothing to stop the user copying the right timezone file there instead of creating a symlink.
One possibility to get round this (which seems to work on Debian, RedHat and OpenBSD) is to compare the contents of the /etc/localtime file to the files under /usr/share/zoneinfo, and see which ones match:
eta:~% md5sum /etc/localtime 410c65079e6d14f4eedf50c19bd073f8 /etc/localtime eta:~% find /usr/share/zoneinfo -type f | xargs md5sum | grep 410c65079e6d14f4eedf50c19bd073f8 410c65079e6d14f4eedf50c19bd073f8 /usr/share/zoneinfo/Europe/London 410c65079e6d14f4eedf50c19bd073f8 /usr/share/zoneinfo/Europe/Belfast 410c65079e6d14f4eedf50c19bd073f8 /usr/share/zoneinfo/Europe/Guernsey 410c65079e6d14f4eedf50c19bd073f8 /usr/share/zoneinfo/Europe/Jersey 410c65079e6d14f4eedf50c19bd073f8 /usr/share/zoneinfo/Europe/Isle_of_Man ... ...
Of course the disadvantage is that this will tell you all timezones that are identical to the current one. (That means identical in the full sense - not just "currently at the same time", but also "always change their clocks on the same day as far as the system knows".)
Your best bet may be to combine the above methods: use
/etc/timezone if it exists; otherwise try parsing
/etc/localtime as a symlink; if that fails, search for matching timezone definition files; if that fails - give up and go home ;-)
(And I have no idea whether any of the above applies on AIX...)