TZ variable

[ippipp]

Hi!

I found this post and tested it on my laptop: https://blog.packagecloud.io/eng/2017/02/21/set-environment-variable-save-thousands-of-system-calls/

I compared "./test" and "TZ=:/etc/localtime strace -ttT ./test" and this gave me similar results as in that blog post.

Questions:
1) Is it safe to use this variable in gentoo globally?
and
2) What is best way to set it system wide?

[NeddySeagoon]

ippipp,

Maybe. On Gentoo, /etc/localtime should be a file. With some use cases, a symlink is a very bad thing.
One of the OpenRC startup services uses /etc/timezone to make a new copy of the timezone file every boot.
This means that your timezone file in use, is only updated every boot regardless of what emerge installs.
Setting TZ will change nothing there.

With a /etc/localtime symlink, the timezone file will change as soon as emerge updates /usr/share/zoneinfo/ (if your timezone happens to change)
With TZ set, you would not get the change until you updated TZ.

Its only going to catch you out if you rarely reboot or go a long time between updates and your timezone file in use gets changed.
Its always safe for UTC as that never changes.
TZ should be set by whatever service does the /etc/localtime copy, so that if you restart it, TZ gets updated too.
_________________
Regards,

NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.

[cboldt]

Neddy:

TZ is just an environment variable that points to /etc/localtime. /etc/localtime remains a regular file. If /etc/localtime changes, TZ still points to the same (now changed) file.

Not to say there aren't any hidden issues, there may be. But the way I read your concern, it (your concern) doesn't come into play.

ippipp: Set environment variables from files in /etc/env.d ... I use /etc/env.d/00Local for any environment variables not assigned to a /etc/env.d file by package installation. After you make a change anywhere in /etc/env.d, you have to run `env-update` to cause those changes to be consolidated into /etc/profile.env (which, for bash, is read/sourced by /etc/profile) and into /etc/csh.env

Bear in mind that those changes aren't automatically picked up by any running shell.

[NeddySeagoon]

cboldt,

/etc/localtime is only changed by a service startup script.
As a regular file it can be out of date with the file in /usr/share/zoneinfo/ until the service is restarted.
Its certainly a corner case.

On a desktop that's rebooted every day, or even every six weeks to keep the kernel current, its a non issue.
_________________
Regards,

NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.

[cboldt]

Neddy:

/usr/share/zoneinfo/ only changes when sys-libs/timezone-data is updated. As you point out, this can be more frequent than rebooting. My recent history of updating timezone-data:

[khayyam]

[cboldt]

khayyam:

Thanks for that. LOL at ...

[John R. Graham]

Long ago (ca. 8 years ago) it was a symlink. However, some systems don't have /usr mounted before the very first usage of the time zone info, so it was made into a copy.

- John
_________________
I can confirm that I have received between 0 and 499 National Security Letters.

[NeddySeagoon]

khayyam,

Thank you.

I'm sure it used to be a service but I remember /etc/localtime being a symlink too and having a separate /usr, so the symlink was broken until /usr was mounted.
_________________
Regards,

NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.

[khayyam]

[cboldt]

Researching a little, for education, I was pointed to `man 3 tzset`

Seems the system will seek info first from $TZ, then from /etc/localtime

The only reference to plaintext "EST5EDT" on the configuration part of my system is in /etc/timezone. Nothing in the /etc branch includes the string "timezone," which cause me to conclude that /etc/timezone is used by the ebuild you cited, to copy the correct file from /usr/share/zoneinfo, into /etc/localtime. And by golly, looking at timezone-data-2016h.ebuild, there it is, in the get_TIMEZONE routine.

At this point, I am still not sure of the better/best setting for the TZ variable. And to make the point more precisely, that variable does not exist in my environment - which is different from it exists but is empty, which is what I implied before.

[khayyam]

[cboldt]

Now I wonder if there is a way to get the string so there is no need to open the file, if TZ is defined.

[khayyam]

[cboldt]

The variable string is pretty easy to figure out, and even if it changes, I suspect I'd get plenty of notice.

TZ="EST+5EDT,M3.2.0/2,M11.1.0/2"

Go into daylight saving the second Sunday in March at 2 am (M3.2.0/2) - "0" being "Sunday"
Go into standard time the first Sunday in November at 2 am (M11.1.0/2)

EST is +5 hours from GMT.

I don't care about any imprecision on the diff between GMT and UTC, leap seconds and other esoterica.

I expect my user experience at the console to change from "just okay" to "ZOOOMomyGod!" with this improvement.

[khayyam]

[mv]

[mv]

[cboldt]

My /etc/sudoers and /etc/rc.conf make no mention of TZ. In all of /etc here, the only mentions of TZ are in /etc/localtime (a copy of the EST5EDT file, graciously installed by the timezone-data package), /etc/exim/exim.conf.dist (which merely informs me that exim handles TZ separately, whatever that means), and also commented out as one of the forms of ENV_TZ in /etc/login.defs, with the other form being "ENV_TZ /etc/tzname", also effective only if uncommented.

Good point about the propagation of TZ. If set in the environment, it only goes where bash/csh does.

[cboldt]

Oh, the issue with the symlink is that "/usr on a separate partition" thing.

[mv]

[cboldt]

Ahhh, I mistook your "instead of clearing it [the TZ variable]" as sudo taking some affirmative action against that specific variable. cron also doesn't pass the environment, more on that below.

Good tip that openrc-run picks up the setting of "random" environment variables in rc.conf. The man page says as much, but with no example, I would not have thought to use that.

[steveL]

[Mr. T.]

I have musl as system library and I set the TZ environment variable in /etc/environment. I have installed timezone-data for the sake of convenience and
set TZ to "Europe/Paris" but "NFT-1DST,M3.5.0,M10.5.0" works too.

[R0b0t1]

At this point I've seen a number of these relatively undocumented tricks. Is there a list of common and not-so-common optimizations anywhere? The Wiki, perhaps?