Change a locale definition (as opposed to a locale setting)

Question

Preliminary remark: This question is not about setting a locale by changing LC_ALL, LC_TIME, LANG, etc., it's rather about changing a locale definition, e.g. in /usr/share/i18n/locales/de_DE, or respectively, to get suggestions about more appropriate options than changing those system settings.

The intent is to obtain ISO time representations %Y-%m-%d and %H:%M:%S without undesired side effects.

Suggestions that I found on the net were yet all inappropriate (e.g., setting the locale to "danish" will not maintain the native language's spelled weekday names).

Now the only choice I currently see to get the desired behaviour is to change the definition of my native locale in /usr/share/i18n/locales/de_DE. This is not a perfect solution since with system updates those changed files may be overwritten again.

My questions are:

1. Are there any better locale related options to get the desired function than changing the specific system locale file?
2. Are there any problems to expect when changing a specific locale definition file? (And how could it be prevented that this file gets overwritten in case of system updates?)
3. Would it be better to define a new specific variant of an existing locale file and use that in the LC_* settings? (And how would that new file/definition correctly be made known to the system?)
4. Any other suggestions?

Answer 1

In a recent question I asked what's the best practice for this kind of change. There was no answer yet. However, I can give you the recipe I am using, which has proven to work fine. All the programs pick the date up correctly.

My description is for Ubuntu, but will likely work on Debian and Mint.

Preparations

Copy the locale you want to customize from /usr/share/i18n/locales to a new file. E.g.

cp /usr/share/i18n/locales/de_DE /usr/share/i18n/locales/de_DE@isodate

Adjust all with the exception of the LC_TIME sections to:

copy "de_DE"

Adjust the LC_TIME section to match your desired outcome. You can use the settings from my above linked question as a template. It looks like this is pretty much exactly what you want.

If your locale doesn't make use of "AM/PM" notation, set those to empty:

t_fmt_ampm ""
am_pm   "";""

Making it known to the system

Edit the file /var/lib/locales/supported.d/local and add the info about your locale definition there. I.e. add a line like this if your file name above was isodate, adjust otherwise:

de_DE.UTF-8@isodate UTF-8

If /var/lib/locales/supported.d/local doesn't exist, create a file of that name. Don't put your changes into the respective en or de file in that folder, as they may get overwritten as soon as the language-pack-*-base and language-pack-* packages on your system receive an update.

Now run dpkg-reconfigure locales:

# dpkg-reconfigure locales
Generating locales...
  de_DE.UTF-8@isodate... done
  de_DE.UTF-8... up-to-date
Generation complete.

In your case this will indicate that the de_DE.UTF-8@isodate locale has been generated (assuming you have no syntax issues).

Last but not least add the following to /etc/default/locale:

LC_TIME="de_DE.UTF-8@isodate"

This will ensure that only LC_TIME overrides the default locale defined using LANG.

Log in anew and you should be able to see the new ISO date/time when using date or other tools using the respective libc runtime function.

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The intent here is to make the least intrusive change while also not working against the system (e.g. against package manager and friends). Of course you can also simply create a copy of your locale, install it in a similar fashion as outlined above and then adjust LANG. The point is, as long as you don't want to run the risk of your changes being overwritten by a package update, you have to use a customized copy (whatever customization you go for; i.e. copy or simply keep the sections as they were in the original). And whether you change LANG to point to the full customized locale definition - or whether you add LC_TIME to point only to the relevant customized section of the same name of a customized locale definition file - you won't get around adjusting one of the global settings.