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.TH FAKE-HWCLOCK 8 "14 July 2024" Debian
.SH NAME
fake-hwclock \- Control fake hardware clock
.SH SYNOPSIS
\fBfake-hwclock\fP [ \fIcommand\fP ] [ \fBforce\fP ]
.SH BACKGROUND
Many embedded Linux systems do not have a functional hardware clock. Either
they simply don't have a hardware clock at all or they have a hardware clock
but it is not usable (e.g. because Linux doesn't know how to use it or because
no battery is present).
This can lead to time moving backwards to some default value (often 1970) when
the system is rebooted. Since lots of software assumes that time only moves
forward this is a bad thing. NTP can (and should where practical) be used to
sync with an external timeserver but it is not available early in the boot
process and may be unavailable for other reasons.
The design expectation of \fBfake-hwclock\fP is that it will be run
very late at shutdown and very early at boot. This will ensure that
fsck has a vaguely sensible idea of system time at boot and won't
complain that the last-modified time in the filesystem is not hugely
in the past or future. Some users may not worry about this too use
case, in which case it is possible to modify the init system
configuration to move things earlier/later as appropriate.
.SH DESCRIPTION
\fBfake-hwclock\fP sets and queries a fake "hardware clock" which stores the
time in a file. This program may be run by the system administrator
directly but is typically run by init (to load the time on startup and
save it on shutdown) and cron (to save the time hourly).
If no command is given then fake-hwclock acts as if the save command was used.
.SH COMMANDS
.SS
.TP
\fBsave\fP
Save the time to the file. As a sanity check, fake-hwclock will not
move the saved clock backwards to a time/date earlier than its own
release date. Use "force" to over-ride this check.
.TP
\fBload\fP
Load the time from the file. If force is specified fake-hwclock will move the
clock either backwards or forwards. Otherwise it will only move it forwards.
.SH FILES
.SS
.TP
\fB/etc/fake-hwclock.data\fR
The file used to store the time
.TP
\fB/etc/init.d/fake-hwclock\fR
The init script used to run fake-hwclock on startup and shutdown
.TP
\fB/usr/lib/systemd/system/fake-hwclock-load.service\fR
systemd service used to run fake-hwclock on startup
.TP
\fB/usr/lib/systemd/system/fake-hwclock-save.service\fR
systemd service used to run fake-hwclock on shutdown
.TP
\fB/usr/lib/systemd/system/fake-hwclock-save.timer\fR
systemd timer used to save the time hourly
.TP
\fB/etc/default/fake-hwclock\fR
Settings file for the init script.
.TP
\fB/etc/cron.hourly/fake-hwclock\fR
Cron job used to save the time hourly
.SH ENVIRONMENT VARIABLES
.SS
.TP
\fBFILE\fR
set the file used by fake-hwclock
.SH RETURN VALUES
1 is returned for invalid commands. 0 is returned in all other cases.
.SH BUGS
This approach can only provide a crude approximation of what a real hardware
clock provides. Use of NTP or another method to keep the time in sync is
strongly advised.
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