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-bash/zsh: hwclock command not found #Debian apt-get install util-linux #Ubuntu apt-get install util-linux #Alpine apk add util-linux #Arch Linux pacman -S util-linux #Kali Linux apt-get install util-linux #CentOS yum install util-linux #Fedora dnf install util-linux #OS X brew install util-linux #Raspbian apt-get install util-linux #Docker docker run cmd.cat/hwclock hwclock
Linux 的时间分为 System Clock（系统时间，就是 Linux Kernel 中的时间）和 Real Time Clock（硬件时间，简称 RTC，即主板上有电池供电的时间）
hwclock 是时钟的管理工具。它可以：
hwclock [function] [option...]
The following functions are mutually exclusive, only one can be given at a time. If none is given, the default is --show.
-a, --adjust
Add or subtract time from the Hardware Clock to account for
systematic drift since the last time the clock was set or
adjusted. See the discussion below, under The Adjust
Function.
--getepoch; --setepoch
These functions are for Alpha machines only, and are only
available through the Linux kernel RTC driver.
They are used to read and set the kernel’s Hardware Clock
epoch value. Epoch is the number of years into AD to which a
zero year value in the Hardware Clock refers. For example, if
the machine’s BIOS sets the year counter in the Hardware
Clock to contain the number of full years since 1952, then
the kernel’s Hardware Clock epoch value must be 1952.
The --setepoch function requires using the --epoch option to
specify the year. For example:
hwclock --setepoch --epoch=1952
The RTC driver attempts to guess the correct epoch value, so
setting it may not be required.
This epoch value is used whenever hwclock reads or sets the
Hardware Clock on an Alpha machine. For ISA machines the
kernel uses the fixed Hardware Clock epoch of 1900.
--predict
Predict what the Hardware Clock will read in the future based
upon the time given by the --date option and the information
in /etc/adjtime. This is useful, for example, to account for
drift when setting a Hardware Clock wakeup (aka alarm). See
rtcwake(8).
Do not use this function if the Hardware Clock is being
modified by anything other than the current operating
system’s hwclock command, such as '11 minute mode' or from
dual-booting another OS.
-r, --show; --get
Read the Hardware Clock and print its time to standard output
in the ISO 8601 format. The time shown is always in local
time, even if you keep your Hardware Clock in UTC. See the
--localtime option.
Showing the Hardware Clock time is the default when no
function is specified.
The --get function also applies drift correction to the time
read, based upon the information in /etc/adjtime. Do not use
this function if the Hardware Clock is being modified by
anything other than the current operating system’s hwclock
command, such as '11 minute mode' or from dual-booting
another OS.
-s, --hctosys
Set the System Clock from the Hardware Clock. The time read
from the Hardware Clock is compensated to account for
systematic drift before using it to set the System Clock. See
the discussion below, under The Adjust Function.
The System Clock must be kept in the UTC timescale for
date-time applications to work correctly in conjunction with
the timezone configured for the system. If the Hardware Clock
is kept in local time then the time read from it must be
shifted to the UTC timescale before using it to set the
System Clock. The --hctosys function does this based upon the
information in the /etc/adjtime file or the command line
arguments --localtime and --utc. Note: no daylight saving
adjustment is made. See the discussion below, under LOCAL vs
UTC.
The kernel also keeps a timezone value, the --hctosys
function sets it to the timezone configured for the system.
The system timezone is configured by the TZ environment
variable or the /etc/localtime file, as tzset(3) would
interpret them. The obsolete tz_dsttime field of the kernel’s
timezone value is set to zero. (For details on what this
field used to mean, see settimeofday(2).)
When used in a startup script, making the --hctosys function
the first caller of settimeofday(2) from boot, it will set
the NTP '11 minute mode' timescale via the
persistent_clock_is_local kernel variable. If the Hardware
Clock’s timescale configuration is changed then a reboot is
required to inform the kernel. See the discussion below,
under Automatic Hardware Clock Synchronization by the Kernel.
This is a good function to use in one of the system startup
scripts before the file systems are mounted read/write.
This function should never be used on a running system.
Jumping system time will cause problems, such as corrupted
filesystem timestamps. Also, if something has changed the
Hardware Clock, like NTP’s '11 minute mode', then --hctosys
will set the time incorrectly by including drift
compensation.
Drift compensation can be inhibited by setting the drift
factor in /etc/adjtime to zero. This setting will be
persistent as long as the --update-drift option is not used
with --systohc at shutdown (or anywhere else). Another way to
inhibit this is by using the --noadjfile option when calling
the --hctosys function. A third method is to delete the
/etc/adjtime file. Hwclock will then default to using the UTC
timescale for the Hardware Clock. If the Hardware Clock is
ticking local time it will need to be defined in the file.
This can be done by calling hwclock --localtime --adjust;
when the file is not present this command will not actually
adjust the Clock, but it will create the file with local time
configured, and a drift factor of zero.
A condition under which inhibiting hwclock’s drift correction
may be desired is when dual-booting multiple operating
systems. If while this instance of Linux is stopped, another
OS changes the Hardware Clock’s value, then when this
instance is started again the drift correction applied will
be incorrect.
For hwclock’s drift correction to work properly it is
imperative that nothing changes the Hardware Clock while its
Linux instance is not running.
--set
Set the Hardware Clock to the time given by the --date
option, and update the timestamps in /etc/adjtime. With the
--update-drift option also (re)calculate the drift factor.
Try it without the option if --set fails. See --update-drift
below.
--systz
This is an alternate to the --hctosys function that does not
read the Hardware Clock nor set the System Clock;
consequently there is not any drift correction. It is
intended to be used in a startup script on systems with
kernels above version 2.6 where you know the System Clock has
been set from the Hardware Clock by the kernel during boot.
It does the following things that are detailed above in the
--hctosys function:
• Corrects the System Clock timescale to UTC as needed.
Only instead of accomplishing this by setting the System
Clock, hwclock simply informs the kernel and it handles
the change.
• Sets the kernel’s NTP '11 minute mode' timescale.
• Sets the kernel’s timezone.
The first two are only available on the first call of
settimeofday(2) after boot. Consequently this option only makes
sense when used in a startup script. If the Hardware Clocks
timescale configuration is changed then a reboot would be
required to inform the kernel.
-w, --systohc
Set the Hardware Clock from the System Clock, and update the
timestamps in /etc/adjtime. With the --update-drift option
also (re)calculate the drift factor. Try it without the
option if --systohc fails. See --update-drift below.
-V, --version
Display version information and exit.
-h, --help
Display help text and exit.
--adjfile=filename
Override the default /etc/adjtime file path.
--date=date_string
This option must be used with the --set or --predict
functions, otherwise it is ignored.
hwclock --set --date='16:45'
hwclock --predict --date='2525-08-14 07:11:05'
The argument must be in local time, even if you keep your
Hardware Clock in UTC. See the --localtime option. Therefore,
the argument should not include any timezone information. It
also should not be a relative time like "+5 minutes", because
hwclock’s precision depends upon correlation between the
argument’s value and when the enter key is pressed.
Fractional seconds are silently dropped. This option is
capable of understanding many time and date formats, but the
previous parameters should be observed.
--delay=seconds
This option can be used to overwrite the internally used
delay when setting the clock time. The default is 0.5 (500ms)
for rtc_cmos, for another RTC types the delay is 0. If RTC
type is impossible to determine (from sysfs) then it defaults
also to 0.5 to be backwardly compatible.
The 500ms default is based on commonly used
MC146818A-compatible (x86) hardware clock. This Hardware
Clock can only be set to any integer time plus one half
second. The integer time is required because there is no
interface to set or get a fractional second. The additional
half second delay is because the Hardware Clock updates to
the following second precisely 500 ms after setting the new
time. Unfortunately, this behavior is hardware specific and
in same cases another delay is required.
-D, --debug
Use --verbose. The --debug option has been deprecated and may
be repurposed or removed in a future release.
--directisa
This option is meaningful for ISA compatible machines in the
x86 and x86_64 family. For other machines, it has no effect.
This option tells hwclock to use explicit I/O instructions to
access the Hardware Clock. Without this option, hwclock will
use the rtc device file, which it assumes to be driven by the
Linux RTC device driver. As of v2.26 it will no longer
automatically use directisa when the rtc driver is
unavailable; this was causing an unsafe condition that could
allow two processes to access the Hardware Clock at the same
time. Direct hardware access from userspace should only be
used for testing, troubleshooting, and as a last resort when
all other methods fail. See the --rtc option.
--epoch=year
This option is required when using the --setepoch function.
The minimum year value is 1900. The maximum is system
dependent (ULONG_MAX - 1).
-f, --rtc=filename
Override hwclock’s default rtc device file name. Otherwise it
will use the first one found in this order: /dev/rtc0,
/dev/rtc, /dev/misc/rtc. For IA-64: /dev/efirtc
/dev/misc/efirtc
-l, --localtime; -u, --utc
Indicate which timescale the Hardware Clock is set to.
The Hardware Clock may be configured to use either the UTC or
the local timescale, but nothing in the clock itself says
which alternative is being used. The --localtime or --utc
options give this information to the hwclock command. If you
specify the wrong one (or specify neither and take a wrong
default), both setting and reading the Hardware Clock will be
incorrect.
If you specify neither --utc nor --localtime then the one
last given with a set function (--set, --systohc, or
--adjust), as recorded in /etc/adjtime, will be used. If the
adjtime file doesn’t exist, the default is UTC.
Note: daylight saving time changes may be inconsistent when
the Hardware Clock is kept in local time. See the discussion
below, under LOCAL vs UTC.
--noadjfile
Disable the facilities provided by /etc/adjtime. hwclock will
not read nor write to that file with this option. Either
--utc or --localtime must be specified when using this
option.
--test
Do not actually change anything on the system, that is, the
Clocks or /etc/adjtime (--verbose is implicit with this
option).
--update-drift
Update the Hardware Clock’s drift factor in /etc/adjtime. It
can only be used with --set or --systohc.
A minimum four hour period between settings is required. This
is to avoid invalid calculations. The longer the period, the
more precise the resulting drift factor will be.
This option was added in v2.26, because it is typical for
systems to call hwclock --systohc at shutdown; with the old
behavior this would automatically (re)calculate the drift
factor which caused several problems:
• When using NTP with an '11 minute mode' kernel the drift
factor would be clobbered to near zero.
• It would not allow the use of 'cold' drift correction.
With most configurations using 'cold' drift will yield
favorable results. Cold, means when the machine is turned
off which can have a significant impact on the drift
factor.
• (Re)calculating drift factor on every shutdown delivers
suboptimal results. For example, if ephemeral conditions
cause the machine to be abnormally hot the drift factor
calculation would be out of range.
• Significantly increased system shutdown times (as of
v2.31 when not using --update-drift the RTC is not read).
Having hwclock calculate the drift factor is a good starting
point, but for optimal results it will likely need to be adjusted
by directly editing the /etc/adjtime file. For most
configurations once a machine’s optimal drift factor is crafted
it should not need to be changed. Therefore, the old behavior to
automatically (re)calculate drift was changed and now requires
this option to be used. See the discussion below, under The
Adjust Function.
This option requires reading the Hardware Clock before setting
it. If it cannot be read, then this option will cause the set
functions to fail. This can happen, for example, if the Hardware
Clock is corrupted by a power failure. In that case, the clock
must first be set without this option. Despite it not working,
the resulting drift correction factor would be invalid anyway.
-v, --verbose
Display more details about what hwclock is doing internally.
hwclock 显示硬件时钟报告的当前时间：
hwclock Fri 15 Oct 2021 03:35:07 PM CST -0.534618 seconds
hwclock 将当前软件时钟时间写入硬件时钟（有时在系统设置期间使用）：
hwclock --systohc
hwclock 将当前硬件时钟时间写入软件时钟：
hwclock --hctosys 或者 clock --hctosys --------------- 上面命令中，--hctosys 表示 Hardware Clock to System clock