Locales on Ubuntu and how to set it in a Docker container

by yaobin.wen

What is a locale?

As [1] says:

In computing, a locale is a set of parameters that defines the user’s language, region and any special variant preferences that the user wants to see in their user interface. Usually a locale identifier consists of at least a language code and a country/region code.

Locales on Ubuntu

Run locale -a to list all the available locales on the system:

$ locale -a
C
C.UTF-8
en_US.utf8
POSIX

Run locale to display the current settings:

$ locale
LANG=en_US.UTF-8
LANGUAGE=en_US
LC_CTYPE="en_US.UTF-8"
LC_NUMERIC="en_US.UTF-8"
LC_TIME="en_US.UTF-8"
LC_COLLATE="en_US.UTF-8"
LC_MONETARY="en_US.UTF-8"
LC_MESSAGES="en_US.UTF-8"
LC_PAPER="en_US.UTF-8"
LC_NAME="en_US.UTF-8"
LC_ADDRESS="en_US.UTF-8"
LC_TELEPHONE="en_US.UTF-8"
LC_MEASUREMENT="en_US.UTF-8"
LC_IDENTIFICATION="en_US.UTF-8"
LC_ALL=en_US.UTF-8

As [2] explains:

• LANG provides the “default value for LC_* variables that have not been explicitly set”.
• LC_ALL “overrides individual LC_* settings: if LC_ALL is set, none of the below have any effect.”

As [3] explains, LC_* are the environment variables “meant to override LANG and affecting a single locale category only”.

[2] doesn’t explain the purpose of LANGUAGE, but [3.1] explains:

Not all programs have translations for all languages. By default, an English message is shown in place of a nonexistent translation. If you understand other languages, you can set up a priority list of languages. This is done through a different environment variable, called LANGUAGE.

Or, as [4] explains:

The LANGUAGE environment variable can have one or more language values and is responsible for the order of the languages in which the messages will be displayed.

So if LANGUAGE is set to fr_FR:en_EN, French is preferred if the French translation is available, otherwise English is used. This is why we say LANGUAGE specifies a priority list of languages.

The package locales

The package locales provides the utility locale-gen. If the locale you want to use does not appear in the list of locale -a, you can run sudo locale-gen <locale-name> to generate it:

$ sudo locale-gen en_US.UTF-8
[sudo] password for ywen:
Generating locales (this might take a while)...
  en_US.UTF-8... done
Generation complete.

The file /usr/share/i18n/SUPPORTED lists all the locales that can be generated.

locales also installs the utility update-locale which modifies /etc/default/locale to store the locale settings permanently:

$ sudo update-locale LANG=en_US.UTF-8
$ cat /etc/default/locale
#  File generated by update-locale
LANG=en_US.UTF-8

Change locales for the current session

You can define/export the environment variables in the current session to change the locale settings for the current session. When you do this, note that LANG and LC_ALL may override the locale you are setting.

Change locales permanently

update-locale can modify /etc/default/locale which is sourced into the current environment after a fresh login. /etc/default/locale is a system-wide setting. $HOME/.pam_environment is user-specific settings.

However, note that $HOME/.pam_environment is only effective when UsePAM is yes in /etc/ssh/sshd_config.

Configure in a Docker container

You can include the following code in Dockerfile to configure the locales in a Docker container:

ARG LOCALE="en_US.UTF-8"
ARG LANGUAGE="en_US"

# Generate and set the locale to UTF-8.
RUN locale-gen ${LOCALE}
ENV LANG ${LOCALE}
ENV LC_ALL ${LOCALE}
ENV LANGUAGE ${LANGUAGE}

References

• [1] Locale (computer software)
• [2] Ubuntu Community Wiki: Locale
• [3] GNU gettext utilities
  • [3.1] 2.3.3 Specifying a Priority List of Languages
• [4] Locale Environment Variables in Linux