hwclock - read or set the hardware clock (RTC)
is a tool for accessing the Hardware Clock. It can: display the
Hardware Clock time; set the Hardware Clock to a specified time; set the
Hardware Clock from the System Clock; set the System Clock from the Hardware
Clock; compensate for Hardware Clock drift; correct the System Clock
timescale; set the kernel's timezone, NTP timescale, and epoch (Alpha only);
and predict future Hardware Clock values based on its drift rate.
Since v2.26 important changes were made to the --hctosys
function and the
option, and a new option --update-drift
See their respective descriptions below.
The following functions are mutually exclusive, only one can be given at a time.
If none is given, the default is --show
- 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.
- --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
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 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
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
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
- Set the Hardware Clock to the time given by the
--date option, and update the timestamps in /etc/adjtime.
With the --update-drift option (re)calculate the drift factor.
- 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
- 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'
- 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. When the --update-drift
option is given, then also (re)calculate the drift factor.
- -V, --version
- Display version information and exit.
- -h, --help
- Display help text and exit.
- Override the default /etc/adjtime file path.
- 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.
- -D, --debug
- Display a lot of information about what hwclock is
doing internally. Some of its functions are complex and this output can
help you understand how the program works.
- 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
- This option is required when using the
- -f, --rtc=filename
- Override hwclock's default rtc device file name.
Otherwise it will use the first one found in this order:
- -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
- 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
- Do not actually change anything on the system, i.e., the
Clocks or adjtime file. This is useful, especially in conjunction with
--debug, in learning about the internal operations of hwclock.
- Update the Hardware Clock's drift factor in
/etc/adjtime. It is used with --set or --systohc,
otherwise it is ignored.
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 behaviour this
would automatically (re)calculate the drift factor which caused several
- When using ntpd 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
calculate the drift factor is a good starting point, but
for optimal results it will likely need to be adjusted by directly editing the
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
There are two types of date-time clocks:
The Hardware Clock:
This clock is an independent hardware device, with
its own power domain (battery, capacitor, etc), that operates when the machine
is powered off, or even unplugged.
On an ISA compatible system, this clock is specified as part of the ISA
standard. A control program can read or set this clock only to a whole second,
but it can also detect the edges of the 1 second clock ticks, so the clock
actually has virtually infinite precision.
This clock is commonly called the hardware clock, the real time clock, the RTC,
the BIOS clock, and the CMOS clock. Hardware Clock, in its capitalized form,
was coined for use by hwclock
. The Linux kernel also refers to it as
the persistent clock.
Some non-ISA systems have a few real time clocks with only one of them having
its own power domain. A very low power external I2C or SPI clock chip might be
used with a backup battery as the hardware clock to initialize a more
functional integrated real-time clock which is used for most other purposes.
The System Clock:
This clock is part of the Linux kernel and is driven by
a timer interrupt. (On an ISA machine, the timer interrupt is part of the ISA
standard.) It has meaning only while Linux is running on the machine. The
System Time is the number of seconds since 00:00:00 January 1, 1970 UTC (or
more succinctly, the number of seconds since 1969 UTC). The System Time is not
an integer, though. It has virtually infinite precision.
The System Time is the time that matters. The Hardware Clock's basic purpose is
to keep time when Linux is not running so that the System Clock can be
initialized from it at boot. Note that in DOS, for which ISA was designed, the
Hardware Clock is the only real time clock.
It is important that the System Time not have any discontinuities such as would
happen if you used the date(1)
program to set it while the system is
running. You can, however, do whatever you want to the Hardware Clock while
the system is running, and the next time Linux starts up, it will do so with
the adjusted time from the Hardware Clock. Note: currently this is not
possible on most systems because hwclock --systohc
is called at
The Linux kernel's timezone is set by hwclock
. But don't be misled --
almost nobody cares what timezone the kernel thinks it is in. Instead,
programs that care about the timezone (perhaps because they want to display a
local time for you) almost always use a more traditional method of determining
the timezone: They use the TZ environment variable or the
file, as explained in the man page for tzset(3)
However, some programs and fringe parts of the Linux kernel such as
filesystems use the kernel's timezone value. An example is the vfat
filesystem. If the kernel timezone value is wrong, the vfat filesystem will
report and set the wrong timestamps on files. Another example is the kernel's
NTP '11 minute mode'. If the kernel's timezone value and/or the
variable are wrong, then the Hardware Clock
will be set incorrectly by '11 minute mode'. See the discussion
below, under Automatic Hardware Clock Synchronization by the Kernel
sets the kernel's timezone to the value indicated by TZ or
with the --hctosys
The kernel's timezone value actually consists of two parts: 1) a field
tz_minuteswest indicating how many minutes local time (not adjusted for DST)
lags behind UTC, and 2) a field tz_dsttime indicating the type of Daylight
Savings Time (DST) convention that is in effect in the locality at the present
time. This second field is not used under Linux and is always zero. See also
uses many different ways to get and set Hardware Clock values.
The most normal way is to do I/O to the rtc device special file, which is
presumed to be driven by the rtc device driver. Also, Linux systems using the
rtc framework with udev, are capable of supporting multiple Hardware Clocks.
This may bring about the need to override the default rtc device by specifying
one with the --rtc
However, this method is not always available as older systems do not have an rtc
driver. On these systems, the method of accessing the Hardware Clock depends
on the system hardware.
On an ISA compatible system, hwclock
can directly access the "CMOS
memory" registers that constitute the clock, by doing I/O to Ports 0x70
and 0x71. It does this with actual I/O instructions and consequently can only
do it if running with superuser effective userid. This method may be used by
specifying the --directisa
This is a really poor method of accessing the clock, for all the reasons that
userspace programs are generally not supposed to do direct I/O and disable
provides it for testing, troubleshooting, and
because it may be the only method available on ISA systems which do not have a
working rtc device driver.
On an m68k system, hwclock
can access the clock with the console driver,
via the device special file /dev/tty1
The Hardware Clock is usually not very accurate. However, much of its inaccuracy
is completely predictable - it gains or loses the same amount of time every
day. This is called systematic drift. hwclock
function lets you apply systematic drift corrections to the Hardware Clock.
It works like this: hwclock
keeps a file, /etc/adjtime
, that keeps
some historical information. This is called the adjtime file.
Suppose you start with no adjtime file. You issue a hwclock --set
command to set the Hardware Clock to the true current time. hwclock
creates the adjtime file and records in it the current time as the last time
the clock was calibrated. Five days later, the clock has gained 10 seconds, so
you issue a hwclock --set --update-drift
command to set
it back 10 seconds. hwclock
updates the adjtime file to show the
current time as the last time the clock was calibrated, and records 2 seconds
per day as the systematic drift rate. 24 hours go by, and then you issue a
consults the adjtime
file and sees that the clock gains 2 seconds per day when left alone and that
it has been left alone for exactly one day. So it subtracts 2 seconds from the
Hardware Clock. It then records the current time as the last time the clock
was adjusted. Another 24 hours go by and you issue another
does the same thing: subtracts
2 seconds and updates the adjtime file with the current time as the last time
the clock was adjusted.
When you use the --update-drift
option with --set
, the systematic drift rate is (re)calculated by comparing the
fully drift corrected current Hardware Clock time with the new set time, from
that it derives the 24 hour drift rate based on the last calibrated timestamp
from the adjtime file. This updated drift factor is then saved in
A small amount of error creeps in when the Hardware Clock is set, so
refrains from making any adjustment that is less than 1
second. Later on, when you request an adjustment again, the accumulated drift
will be more than 1 second and --adjust
will make the adjustment
including any fractional amount.
also uses the adjtime file data to compensate
the value read from the Hardware Clock before using it to set the System
Clock. It does not share the 1 second limitation of --adjust
, and will
correct sub-second drift values immediately. It does not change the Hardware
Clock time nor the adjtime file. This may eliminate the need to use
, unless something else on the system needs the Hardware Clock
to be compensated.
While named for its historical purpose of controlling adjustments only, it
actually contains other information used by hwclock
from one invocation
to the next.
The format of the adjtime file is, in ASCII:
Line 1: Three numbers, separated by blanks: 1) the systematic drift rate in
seconds per day, floating point decimal; 2) the resulting number of seconds
since 1969 UTC of most recent adjustment or calibration, decimal integer; 3)
zero (for compatibility with clock(8)
) as a decimal integer.
Line 2: One number: the resulting number of seconds since 1969 UTC of most
recent calibration. Zero if there has been no calibration yet or it is known
that any previous calibration is moot (for example, because the Hardware Clock
has been found, since that calibration, not to contain a valid time). This is
a decimal integer.
Line 3: "UTC" or "LOCAL". Tells whether the Hardware Clock
is set to Coordinated Universal Time or local time. You can always override
this value with options on the hwclock
You can use an adjtime file that was previously used with the clock(8)
program with hwclock
You should be aware of another way that the Hardware Clock is kept synchronized
in some systems. The Linux kernel has a mode wherein it copies the System Time
to the Hardware Clock every 11 minutes. This mode is a compile time option, so
not all kernels will have this capability. This is a good mode to use when you
are using something sophisticated like NTP to keep your System Clock
synchronized. (NTP is a way to keep your System Time synchronized either to a
time server somewhere on the network or to a radio clock hooked up to your
system. See RFC 1305.)
If the kernel is compiled with the '11 minute mode' option it will
be active when the kernel's clock discipline is in a synchronized state. When
in this state, bit 6 (the bit that is set in the mask 0x0040) of the kernel's
variable is unset. This value is output as the 'status'
line of the adjtimex --print
It takes an outside influence, like the NTP daemon ntpd(1)
, to put the
kernel's clock discipline into a synchronized state, and therefore turn on
'11 minute mode'. It can be turned off by running anything that
sets the System Clock the old fashioned way, including
. However, if the NTP daemon is still running,
it will turn '11 minute mode' back on again the next time it
synchronizes the System Clock.
If your system runs with '11 minute mode' on, it may need to use
in a startup script, especially if
the Hardware Clock is configured to use the local timescale. Unless the kernel
is informed of what timescale the Hardware Clock is using, it may clobber it
with the wrong one. The kernel uses UTC by default.
The first userspace command to set the System Clock informs the kernel what
timescale the Hardware Clock is using. This happens via the
kernel variable. If --hctosys
is the first, it will set this variable according to the
adjtime file or the appropriate command-line argument. Note that when using
this capability and the Hardware Clock timescale configuration is changed,
then a reboot is required to notify the kernel.
should not be used with NTP
'11 minute mode'.
There is some sort of standard that defines CMOS memory Byte 50 on an ISA
machine as an indicator of what century it is. hwclock
does not use or
set that byte because there are some machines that don't define the byte that
way, and it really isn't necessary anyway, since the year-of-century does a
good job of implying which century it is.
If you have a bona fide use for a CMOS century byte, contact the hwclock
maintainer; an option may be appropriate.
Note that this section is only relevant when you are using the "direct
ISA" method of accessing the Hardware Clock. ACPI provides a standard way
to access century values, when they are supported by the hardware.
This discussion is based on the following conditions:
- Nothing is running that alters the date-time clocks, such
as ntpd(1) or a cron job.
- The system timezone is configured for the correct local
time. See below, under POSIX vs 'RIGHT'.
- Early during startup the following are called, in this
adjtimex --tick value --frequency value
- During shutdown the following is called:
Systems without adjtimex
may use ntptime
Whether maintaining precision time with ntpd(1)
or not, it makes sense to
configure the system to keep reasonably good date-time on its own.
The first step in making that happen is having a clear understanding of the big
picture. There are two completely separate hardware devices running at their
own speed and drifting away from the 'correct' time at their own rates. The
methods and software for drift correction are different for each of them.
However, most systems are configured to exchange values between these two
clocks at startup and shutdown. Now the individual device's time keeping
errors are transferred back and forth between each other. Attempt to configure
drift correction for only one of them, and the other's drift will be overlaid
This problem can be avoided when configuring drift correction for the System
Clock by simply not shutting down the machine. This, plus the fact that all of
's precision (including calculating drift factors) depends upon
the System Clock's rate being correct, means that configuration of the System
Clock should be done first.
The System Clock drift is corrected with the adjtimex(8)
options. These two work together: tick is
the coarse adjustment and frequency is the fine adjustment. (For systems that
do not have an adjtimex
ntptime -f ppm
may be used instead.)
Some Linux distributions attempt to automatically calculate the System Clock
drift with adjtimex
's compare operation. Trying to correct one drifting
clock by using another drifting clock as a reference is akin to a dog trying
to catch its own tail. Success may happen eventually, but great effort and
frustration will likely precede it. This automation may yield an improvement
over no configuration, but expecting optimum results would be in error. A
better choice for manual configuration would be adjtimex
It may be more effective to simply track the System Clock drift with
, or date -Ins
and a precision timepiece, and then
calculate the correction manually.
After setting the tick and frequency values, continue to test and refine the
adjustments until the System Clock keeps good time. See adjtimex(8)
more information and the example demonstrating manual drift calculations.
Once the System Clock is ticking smoothly, move on to the Hardware Clock.
As a rule, cold drift will work best for most use cases. This should be true
even for 24/7 machines whose normal downtime consists of a reboot. In that
case the drift factor value makes little difference. But on the rare occasion
that the machine is shut down for an extended period, then cold drift should
yield better results.
Steps to calculate cold drift:
- Ensure that ntpd(1) will not be launched at
- The System Clock time must be correct at
- Shut down the system.
- Let an extended period pass without changing the Hardware
- Start the system.
- Immediately use hwclock to set the correct time,
adding the --update-drift option.
Note: if step 6 uses --systohc
, then the System Clock must be set
correctly (step 6a) just before doing so.
calculate the drift factor is a good starting point, but
for optimal results it will likely need to be adjusted by directly editing the
file. Continue to test and refine the drift factor until
the Hardware Clock is corrected properly at startup. To check this, first make
sure that the System Time is correct before shutdown and then use sntp
or date -Ins
and a precision timepiece, immediately after
Keeping the Hardware Clock in a local timescale causes inconsistent daylight
saving time results:
- If Linux is running during a daylight saving time change,
the time written to the Hardware Clock will be adjusted for the
- If Linux is NOT running during a daylight saving time
change, the time read from the Hardware Clock will NOT be adjusted for the
The Hardware Clock on an ISA compatible system keeps only a date and time, it
has no concept of timezone nor daylight saving. Therefore, when hwclock
is told that it is in local time, it assumes it is in the 'correct' local time
and makes no adjustments to the time read from it.
Linux handles daylight saving time changes transparently only when the Hardware
Clock is kept in the UTC timescale. Doing so is made easy for system
administrators as hwclock
uses local time for its output and as the
argument to the --date
POSIX systems, like Linux, are designed to have the System Clock operate in the
UTC timescale. The Hardware Clock's purpose is to initialize the System Clock,
so also keeping it in UTC makes sense.
Linux does, however, attempt to accommodate the Hardware Clock being in the
local timescale. This is primarily for dual-booting with older versions of MS
Windows. From Windows 7 on, the RealTimeIsUniversal registry key is supposed
to be working properly so that its Hardware Clock can be kept in UTC.
A discussion on date-time configuration would be incomplete without addressing
timezones, this is mostly well covered by tzset(3)
. One area that seems
to have no documentation is the 'right' directory of the Time Zone Database,
sometimes called tz or zoneinfo.
There are two separate databases in the zoneinfo system, posix and 'right'.
'Right' (now named zoneinfo-leaps) includes leap seconds and posix does not.
To use the 'right' database the System Clock must be set to
(UTC + leap seconds), which is equivalent to
(TAI - 10). This allows calculating the exact number of seconds
between two dates that cross a leap second epoch. The System Clock is then
converted to the correct civil time, including UTC, by using the 'right'
timezone files which subtract the leap seconds. Note: this configuration is
considered experimental and is known to have issues.
To configure a system to use a particular database all of the files located in
its directory must be copied to the root of /usr/share/zoneinfo
are never used directly from the posix or 'right' subdirectories, e.g.,
'. This habit was becoming so common that the
upstream zoneinfo project restructured the system's file tree by moving the
posix and 'right' subdirectories out of the zoneinfo directory and into
Unfortunately, some Linux distributions are changing it back to the old tree
structure in their packages. So the problem of system administrators reaching
into the 'right' subdirectory persists. This causes the system timezone to be
configured to include leap seconds while the zoneinfo database is still
configured to exclude them. Then when an application such as a World Clock
needs the South_Pole timezone file; or an email MTA, or hwclock
the UTC timezone file; they fetch it from the root of
, because that is what they are supposed to do.
Those files exclude leap seconds, but the System Clock now includes them,
causing an incorrect time conversion.
Attempting to mix and match files from these separate databases will not work,
because they each require the System Clock to use a different timescale. The
zoneinfo database must be configured to use either posix or 'right', as
described above, or by assigning a database path to the
- If this variable is set its value takes precedence over the
system configured timezone.
- If this variable is set its value takes precedence over the
system configured timezone database directory path.
- The configuration and state file for hwclock.
- The system timezone file.
- The system timezone database directory.
Device files hwclock
may try for Hardware Clock access:
Written by Bryan Henderson, September 1996 (firstname.lastname@example.org), based on
work done on the clock(8)
program by Charles Hedrick, Rob Hooft, and
Harald Koenig. See the source code for complete history and credits.
The hwclock command is part of the util-linux package and is available from