Setting locale, timezone and reconfiguring packages non-interactively on Debian and Ubuntu

Goal

Set the system locale and timezone, and re-run a package's debconf configuration step, all from a script with no dialog box or terminal prompt.

Prerequisites

Root access; on a systemd system (default on Debian 12/13 and Ubuntu 22.04/24.04), localectl/timedatectl are available out of the box.

Steps

1. List available locales if unsure of the exact name: localectl list-locales.
2. Set the system-wide locale: localectl set-locale LANG=en_US.UTF-8. The command "Set[s] the system locale", accepting either one locale value or one or more VARIABLE=LOCALE assignments (e.g. LC_TIME=de_CH.UTF-8 alongside LANG) in the same call.
3. Equivalently, on a non-systemd system or when only the legacy mechanism is available, update-locale LANG=en_US.UTF-8 — a program that "can be called by maintainer scripts" to "Modify global locale settings" in /etc/default/locale, and takes the same VAR=value syntax.
4. Confirm the locale is actually generated (Debian/Ubuntu do not auto-generate every locale): check /etc/locale.gen for the entry uncommented, then run locale-gen if it was just added.
5. List available timezones if unsure of the exact name: timedatectl list-timezones.
6. Set the timezone: timedatectl set-timezone Europe/Zurich. The command sets "the system time zone to the specified value" and, per its own manual page, also updates the RTC time if the clock is configured to use local time, and alters the /etc/localtime symlink.
7. To re-run a package's own configuration questions non-interactively — for example after preseeding new debconf answers with debconf-set-selections — run DEBIAN_FRONTEND=noninteractive dpkg-reconfigure -f noninteractive <package>. dpkg-reconfigure's -f/--frontend option lets a caller "Select the frontend to use" for that one invocation, overriding the configured default (the manual page notes the reverse case explicitly: if debconf's default is already noninteractive, dpkg-reconfigure normally switches to the dialog frontend instead so a human actually sees it, so passing -f noninteractive explicitly is what keeps it silent).

Expected result

localectl status and timedatectl status show the new locale/timezone; dpkg-reconfigure -f noninteractive tzdata (for example) exits 0 with no output and updates /etc/timezone//etc/localtime to match.

Limits and test basis

Locale and timezone changes take effect for new shells/processes immediately; already-running services that cache the old locale or timezone (some daemons read it once at startup) need a restart, not a reboot, to pick up the change. To undo, re-run the same commands with the previous value.