xref: /freebsd/usr.bin/fortune/datfiles/freebsd-tips (revision 278e370066734a06f6b35e585568ac34969b1e84)

Any user that is a member of the wheel group can use "su -" to simulate
a root login. You can add a user to the wheel group by editing /etc/group.
		-- Konstantinos Konstantinidis <kkonstan@duth.gr>
%
By pressing "Scroll Lock" you can use the arrow keys to scroll backward
through the console output.  Press "Scroll Lock" again to turn it off.
Don't have a "Scroll Lock" key? The "Pause / Break" key acts alike.
%
Can't remember if you've installed a certain port or not? Try "pkg info
-x port_name".
%
Ever wonder what those numbers after command names were, as in cat(1)?  It's
the section of the manual the man page is in.  "man man" will tell you more.
		-- David Scheidt <dscheidt@tumbolia.com>
%
Forget how to spell a word or a variation of a word? Use

	look portion_of_word_you_know
		-- Dru <genesis@istar.ca>
%
Forget what directory you are in? Type "pwd".
		-- Dru <genesis@istar.ca>
%
Forget when Easter is? Try "ncal -e". If you need the date for Orthodox
Easter, use "ncal -o" instead.
		-- Dru <genesis@istar.ca>
%
FreeBSD is started up by the program 'init'.  The first thing init does when
starting multiuser mode (ie, starting the computer up for normal use) is to
run the shell script /etc/rc.  By reading /etc/rc and the /etc/rc.d/ scripts,
you can learn a lot about how the system is put together, which again will
make you more confident about what happens when you do something with it.
%
Handy bash(1) prompt:  PS1="\u@\h \w \!$ "
		-- David Scheidt <dscheidt@tumbolia.com>
%
Having trouble using fetch through a firewall? Try setting the environment
variable FTP_PASSIVE_MODE to yes, and see fetch(3) for more details.
%
If other operating systems have damaged your Master Boot Record, you can
reinstall it with gpart(8). See
"man gpart" for details.
%
If you accidentally end up inside vi, you can quit it by pressing Escape, colon
(:), q (q), bang (!) and pressing return.
%
If you do not want to get beeps in X11 (X Windows), you can turn them off with

	xset b off
%
If you have a CD-ROM drive in your machine, you can make the CD-ROM that is
presently inserted available by typing 'mount /cdrom' as root.  The CD-ROM
will be available under /cdrom/.  Remember to do 'umount /cdrom' before
removing the CD-ROM (it will usually not be possible to remove the CD-ROM
without doing this.)

Note: This tip may not work in all configurations.
%
If you need a reminder to leave your terminal, type "leave +hhmm" where
"hhmm" represents in how many hours and minutes you need to leave.
		-- Dru <genesis@istar.ca>
%
If you need to ask a question on the FreeBSD-questions mailing list then

	https://docs.freebsd.org/en/articles/freebsd-questions

contains lots of useful advice to help you get the best results.
%
If you write part of a filename in tcsh,
pressing TAB will show you the available choices when there
is more than one, or complete the filename if there's only one match.
%
If you `set watch = (0 any any)' in tcsh, you will be notified when
someone logs in or out of your system.
%
If you use the C shell, add the following line to the .cshrc file in your
home directory to prevent core files from being written to disk:

	limit coredumpsize 0
		-- Dru <genesis@istar.ca>
%
If you want df(1) and other commands to display disk sizes in
kilobytes instead of 512-byte blocks, set BLOCKSIZE in your
environment to 'K'.  You can also use 'M' for Megabytes or 'G' for
Gigabytes.  If you want df(1) to automatically select the best size
then use 'df -h'.
%
If you want to play CDs with FreeBSD, a utility for this is already included.
Type 'cdcontrol' then 'help' to learn more.  (You may need to set the CDROM
environment variable in order to make cdcontrol want to start.)
%
If you'd like to keep track of applications in the FreeBSD ports tree, take a
look at FreshPorts;

	https://www.freshports.org/
%
In order to make fetch (the FreeBSD downloading tool) ask for
username/password when it encounters a password-protected web page, you can set
the environment variable HTTP_AUTH to 'basic:*'.
%
In order to search for a string in some files, use 'grep' like this:

	 grep "string" filename1 [filename2 filename3 ...]

This will print out the lines in the files that contain the string.  grep can
also do a lot more advanced searches - type 'man grep' for details.
%
In order to support national characters for European languages in tools like
less without creating other nationalisation aspects, set the environment
variable LC_ALL to 'en_US.UTF-8'.
%
"man firewall" will give advice for building a FreeBSD firewall using ipfw(8).
		-- David Scheidt <dscheidt@tumbolia.com>
%
"man hier" will explain the way FreeBSD filesystems are normally laid out.
		-- David Scheidt <dscheidt@tumbolia.com>
%
Man pages are divided into section depending on topic.  There are 9 different
sections numbered from 1 (General Commands) to 9 (Kernel Developer's Manual).
You can get an introduction to each topic by typing

	man <number> intro

In other words, to get the intro to general commands, type

	man 1 intro
%
"man ports" gives many useful hints about installing FreeBSD ports.
%
"man security" gives very good advice on how to tune the security of your
FreeBSD system.
%
"man tuning" gives some tips how to tune performance of your FreeBSD system.
		-- David Scheidt <dscheidt@tumbolia.com>
%
Need to do a search in a manpage or in a file you've sent to a pager? Use
"/search_word". To repeat the same search, type "n" for next or "p" for
previous.
		-- Dru <genesis@istar.ca>
%
Need to find the location of a program? Use "locate program_name".
		-- Dru <genesis@istar.ca>
%
Need to leave your terminal for a few minutes and don't want to logout?
Use "lock -p". When you return, use your password as the key to unlock the
terminal.
		-- Dru <genesis@istar.ca>
%
Need to quickly empty a file? Use ": > filename".
		-- Dru <genesis@istar.ca>
%
Need to quickly return to your home directory? Type "cd".
		-- Dru <genesis@istar.ca>
%
Need to remove all those ^M characters from a DOS file? Try

	tr -d \\r < dosfile > newfile
		-- Originally by Dru <genesis@istar.ca>
%
Need to see the calendar for this month? Simply type "cal".  To see the
whole year, type "cal -y".
		-- Dru <genesis@istar.ca>
%
Need to see which daemons are listening for connection requests? Use
"sockstat -4l" for IPv4, and "sockstat -l" for IPv4 and IPv6.
		-- Dru <genesis@istar.ca>
%
Need to see your routing table? Type "netstat -rn". The entry with the G
flag is your gateway.
		-- Dru <genesis@istar.ca>
%
Nice bash prompt: PS1='(\[$(tput md)\]\t <\w>\[$(tput me)\]) $(echo $?) \$ '
		-- Mathieu <mathieu@hal.interactionvirtuelle.com>
%
Over quota?  "du -sh * | sort -h " will give you a sorted list of your
directory sizes.
		-- David Scheidt <dscheidt@tumbolia.com>
%
nc(1) (or netcat) is useful not only for redirecting input/output to
TCP or UDP connections, but also for proxying them with inetd(8).
%
sh (the default Bourne shell in FreeBSD) supports command-line editing.  Just
``set -o emacs'' or ``set -o vi'' to enable it. Use "<TAB>" key to complete
paths.
%
Simple tcsh prompt: set prompt = '%# '
%
The default editor in FreeBSD is vi, which is efficient to use when you have
learned it, but somewhat user-unfriendly.  To use ee (an easier but less
powerful editor) instead, set the environment variable EDITOR to /usr/bin/ee
%
Time to change your password? Type "passwd" and follow the prompts.
		-- Dru <genesis@istar.ca>
%
To change an environment variable in /bin/sh use:

	$ VARIABLE="value"
	$ export VARIABLE
%
To change an environment variable in tcsh you use: setenv NAME "value"
where NAME is the name of the variable and "value" its new value.
%
To clear the screen, use "clear". To re-display your screen buffer, press
the scroll lock key and use your page up button. When you're finished,
press the scroll lock key again to get your prompt back.
		-- Dru <genesis@istar.ca>
%
You can press Ctrl-L while in the shell to clear the screen.
%
To determine whether a file is a text file, executable, or some other type
of file, use

	file filename
		-- Dru <genesis@istar.ca>
%
To do a fast search for a file, try

	 locate filename

locate uses a database that is updated every Saturday (assuming your computer
is running FreeBSD at the time) to quickly find files based on name only.
%
To erase a line you've written at the command prompt, use "Ctrl-U".
		-- Dru <genesis@istar.ca>
%
To find out the hostname associated with an IP address, use

	drill -x IP_address
		-- Dru <genesis@istar.ca>
%
To obtain a neat PostScript rendering of a manual page, use ``-t'' switch
of the man(1) utility: ``man -t <topic>''.  For example:

	man -t grep > grep.ps	# Save the PostScript version to a file
or
	man -t printf | lp	# Send the PostScript directly to printer
%
To quickly create an empty file, use "touch filename".
		-- Dru <genesis@istar.ca>
%
To read a compressed file without having to first uncompress it, use
"zcat" or "zless" to view it. There is also "bzcat", "bzless", "xzcat"
and "xzless".
		-- Dru <genesis@istar.ca>
%
To save disk space in your home directory, compress files you rarely
use with "gzip filename".
		-- Dru <genesis@istar.ca>
%
To search for files that match a particular name, use find(1); for example

	find / -name "*GENERIC*" -ls

will search '/', and all subdirectories, for files with 'GENERIC' in the name.
      	--  Stephen Hilton <nospam@hiltonbsd.com>
%
To see all of the directories on your FreeBSD system, type

	find / -type d | less

All the files?

	find / -type f | less
%
To see how long it takes a command to run, type the word "time" before the
command name.
		-- Dru <genesis@istar.ca>
%
To see how much disk space is left on your UFS partitions, use

	df -h
		-- Dru <genesis@istar.ca>
%
To see the 10 largest files in a directory or on a UFS partition, use

	du -h /partition_or_directory_name | sort -rh | head
		-- Dru <genesis@istar.ca>
%
To see the IP addresses currently set on your active interfaces, type
"ifconfig -u".
		-- Dru <genesis@istar.ca>
%
To see the last 10 lines of a long file, use "tail filename". To see the
first 10 lines, use "head filename". To see new lines as they're appended
to a file, use "tail -f filename".
		-- Dru <genesis@istar.ca>
%
To see the last time that you logged in, use lastlogin(8).
		-- Dru <genesis@istar.ca>
%
To see the MAC addresses of the NICs on your system, type

	ifconfig -a
		-- Dru <genesis@istar.ca>
%
To see the output from when your computer started, run dmesg(8).  If it has
been replaced with other messages, look at /var/run/dmesg.boot.
		-- Francisco Reyes <lists@natserv.com>
%
Want colour in your directory listings?  Use "ls -G".  "ls -F" is also useful,
and they can be combined as "ls -FG".
%
Want to find a specific port? Just type the following under /usr/ports
or one of its subdirectories:

	make search name=<port-name>
    or
	make search key=<keyword>
%
Want to know how many words, lines, or bytes are contained in a file? Type
"wc filename".
		-- Dru <genesis@istar.ca>
%
Want to see how much virtual memory you're using? Just type "swapinfo" to
be shown information about the usage of your swap partitions.
%
Want to strip UTF-8 BOM(Byte Order Mark) from given files?

	sed -e '1s/^\xef\xbb\xbf//' < bomfile > newfile
%
Want to use sed(1) to edit a file in place?  Well, to replace every 'e' with
an 'o', in a file named 'foo', you can do:

	sed -i.bak s/e/o/g foo

And you'll get a backup of the original in a file named 'foo.bak', but if you
want no backup:

	sed -i '' s/e/o/g foo
%
When you've made modifications to a file in vi(1) and then find that
you can't write it, type ``<ESC>!rm -f %'' then ``:w!'' to force the
write

This won't work if you don't have write permissions to the directory
and probably won't be suitable if you're editing through a symbolic link.

If you have sudo(8) installed and permissions to use it, type
``<ESC>w ! sudo tee %'' to force a write.
%
You can adjust the volume of various parts of the sound system in your
computer by typing 'mixer <type> <volume>'.  To get a list of what you can
adjust, just type 'mixer'.
%
You can automatically download and install binary packages by doing

	pkg install <package>

This will also automatically install the packages that are dependencies
for the package you install (ie, the packages it needs in order to work.)
%
You can change the video mode on all consoles by adding something like
the following to /etc/rc.conf:

	allscreens="80x30"

You can use "vidcontrol -i mode | grep T" for a list of supported text
modes.
		-- Konstantinos Konstantinidis <kkonstan@duth.gr>
%
You can disable tcsh's terminal beep if you `set nobeep'.
%
You can install extra packages for FreeBSD by using the ports system.
If you have installed it, you can download, compile, and install software by
just typing

	# cd /usr/ports/<category>/<portname>
	# make install && make clean

as root.  The ports infrastructure will download the software, change it so
it works on FreeBSD, compile it, install it, register the installation so it
will be possible to automatically uninstall it, and clean out the temporary
working space it used.  You can remove an installed port you decide you do not
want after all by typing

	# cd /usr/ports/<category>/<portname>
	# make deinstall

as root.
%
You can look through a file in a nice text-based interface by typing

	less filename
%
You can make a log of your terminal session with script(1).
%
You can often get answers to your questions about FreeBSD by searching in the
FreeBSD mailing list archives at

	http://freebsd.markmail.org
%
You can open up a new split-screen window in (n)vi with :N or :E and then
use ^w to switch between the two.
%
You can permanently set environment variables for your shell by putting them
in a startup file for the shell.  The name of the startup file varies
depending on the shell - csh and tcsh uses .login, bash, sh, ksh and zsh use
.profile.  When using bash, sh, ksh or zsh, don't forget to export the
variable.
%
You can press Ctrl-D to quickly exit from a shell, or logout from a
login shell.
		-- Konstantinos Konstantinidis <kkonstan@duth.gr>
%
You can press up-arrow or down-arrow to walk through a list of
previous commands in tcsh.
%
You can search for documentation on a keyword by typing

	apropos keyword
%
You can `set autologout = 30' to have tcsh log you off automatically
if you leave the shell idle for more than 30 minutes.
%
You can use aliases to decrease the amount of typing you need to do to get
commands you commonly use.  Examples of fairly popular aliases include (in
Bourne shell style, as in /bin/sh, bash, ksh, and zsh):

	alias lf="ls -FA"
	alias ll="ls -lA"
	alias su="su -m"

In csh or tcsh, these would be

	alias lf ls -FA
	alias ll ls -lA
	alias su su -m

To remove an alias, you can usually use 'unalias aliasname'.  To list all
aliases, you can usually type just 'alias'.
%
You can use /etc/make.conf to control the options used to compile software
on this system.  Example entries are in
/usr/share/examples/etc/make.conf and in make.conf(5).
For options that are set for building FreeBSD's kernel and its world, see
src.conf(5).
%
You can use "pkg info" to see a list of packages you have installed.
%
You can use the 'fetch' command to retrieve files over ftp, http or https.

	 fetch https://www.FreeBSD.org/index.html

will download the front page of the FreeBSD web site.
%
You can use "whereis" to search standard binary, manual page and source
directories for the specified programs. This can be particularly handy
when you are trying to find where in the ports tree an application is.

Try "whereis firefox" and "whereis whereis".
		-- Konstantinos Konstantinidis <kkonstan@duth.gr>
%
Want to run the same command again?
In many shells (e.g., tcsh, zsh, bash) you can type "!!".
%
Want to go the directory you were just in?
Type "cd -"
%
Can't delete /usr/obj? Enter "chflags -R noschg /usr/obj" to remove the
system immutable flag for all files in /usr/obj.

		-- Lars Engels <lme@FreeBSD.org>
%
Want to list all files of an installed package? Enter
"pkg info -l packagename".

		-- Lars Engels <lme@FreeBSD.org>
%
Are you looking for a package? Search for it with
"pkg search part_of_package_name"

		-- Lars Engels <lme@FreeBSD.org>
%
If you want to recursively copy a directory preserving file and directory
attributes use
"cp -a source target"

		-- Lars Engels <lme@FreeBSD.org>
%
Do you wonder what a terminal program is doing at the moment? dd(1) does not
show any throughput? Hit "^T" (Control + t) to send SIGINFO to the process
and see what it is doing.

		-- Lars Engels <lme@FreeBSD.org>
%
Do you want to know which version of FreeBSD you are running? Enter
"freebsd-version -ku" to display kernel and userland version.

		-- Lars Engels <lme@FreeBSD.org>
%
If you want to end one or more processes at a time using a regular expression
enter "pkill regex".

		-- Lars Engels <lme@FreeBSD.org>
%
Do you want to run a program directly after some other process has ended? Use
"pwait pid && new_program"

		-- Lars Engels <lme@FreeBSD.org>
%
When you want your users to be able to reboot or shutdown FreeBSD, add them
to the group "operator" and they are allowed to use shutdown(8) and poweroff(8).

		-- Lars Engels <lme@FreeBSD.org>
%
If you need to create a FAT32 formatted USB thumb drive, find out its devicename
running dmesg(8) after inserting it. Then create an MBR schema, a single slice and
format it:

# gpart create -s MBR ${devicename}
# gpart add -t fat32 ${devicename}
# newfs_msdos -F 32 -L thumbdrive ${devicename}s1

		-- Lars Engels <lme@FreeBSD.org>
%
If you want to get a sorted list of all services that are started when FreeBSD boots,
enter "service -e".

		-- Lars Engels <lme@FreeBSD.org>
%
To easily configure your installed FreeBSD use bsdconfig(8).

		-- Lars Engels <lme@FreeBSD.org>
%
After you compiled and installed a new version of FreeBSD, use etcupdate(8) to merge
configuration updates.
Run "etcupdate extract" once when your sources match your running system, then run
"etcupdate" after every upgrade and "etcupdate resolve" to resolve any conflicts.

		-- Lars Engels <lme@FreeBSD.org>
%
Do you want to do a binary upgrade of your running FreeBSD installation? Use freebsd-update(8).

To install updates and patches for the running branch use
# freebsd-update fetch install

To upgrade to a newer release use
# freebsd-update upgrade -r ${name_of_release}

		-- Lars Engels <lme@FreeBSD.org>
%
To run rc scripts in /etc/rc.d and /usr/local/etc/rc.d use service(8).
Run "service ${name_of_rc_script} start" to start a daemon and
"service ${name_of_rc_script} stop" to stop it.

		-- Lars Engels <lme@FreeBSD.org>
%
If you don't want to edit /etc/rc.conf directly, use sysrc(8) to add and remove entries.
Use "sysrc name=value" to add an entry and "sysrc -x name" to delete an entry.

		-- Lars Engels <lme@FreeBSD.org>
%
You can upload the dmesg of your system to help developers get an overview of commonly
used hardware and peripherals for FreeBSD. Use the curl package to upload it like this:
curl -v -d "nickname=$USER" -d "description=FreeBSD/$(uname -m) on \
$(kenv smbios.system.maker) $(kenv smbios.system.product)" -d "do=addd" \
--data-urlencode 'dmesg@/var/run/dmesg.boot' http://dmesgd.nycbug.org/index.cgi
%
Want to know how much memory (in bytes) your machine has installed? Let
sysctl(8) tell you with the following command:

sysctl hw.realmem

The realmem value is memory before the kernel and modules are loaded, whereas
hw.physmem is what is left after they were loaded.

The number of active CPUs is displayed using this command:

sysctl hw.ncpu

		-- Benedict Reuschling <bcr@FreeBSD.org>
%
When using ZFS as the file system the "df" command is reporting the pool size
and not file system sizes. It also does not know about descendent ZFS
datasets, snapshots, quotas, and reservations with their individual space usage.
Use the built-in "zfs list" command to get a better overview of space usage:

zfs list -o space

		-- Benedict Reuschling <bcr@FreeBSD.org>
%
To learn more about what your system is doing, take a look at systat(1). For
example, to get various statistics related to virtual memory usage, process
scheduling, device interrupts, system name translation caching, and disk I/O,
enter the following:

systat -vmstat

Other values are icmp, icmp6, ifstat, iostat, ip, ip6, netstat, pigs, sctp,
swap, tcp, or zarc. You can switch between displays using :<display> and exit
back to your shell by typing

:quit

		-- Benedict Reuschling <bcr@FreeBSD.org>
%
To set a quota of 10 GB for the user named foo on a ZFS dataset, run the
following command:

# zfs set userquota@foo=10G pool/home/foo

The zfs userspace command can display the quota and current space usage:

# zfs userspace pool/home/foo

To unset a quota, assign "none" as the value.
		-- Benedict Reuschling <bcr@FreeBSD.org>
%
ZFS can display I/O statistics for a given pool using the iostat subcommand.
By default, it will display one line of current activity.  To display stats
every 5 seconds run the following command (cancel with CTRL+C):

zpool iostat 5

To view individual disk activities, specify the -v parameter:

zpool iostat -v

Of course, both can be combined. For more options, see zpool(8).
		-- Benedict Reuschling <bcr@FreeBSD.org>
%
FreeBSD's top(1) utility displays CPU statistics by default.
To display I/O activity for each process instead, run top like this:

top -m io

		-- Benedict Reuschling <bcr@FreeBSD.org>
%
ZFS keeps a history of commands run against a specific pool using the
history subcommand to zpool:

zpool history

More details are available using the -i and -l parameters. Note that ZFS
will not keep the complete pool history forever and will remove older
events in favor of never ones.
		-- Benedict Reuschling <bcr@FreeBSD.org>
%
To display the compression ratio for the ZFS dataset /var/log on the pool
mypool, run the following command:

zfs get refcompressratio mypool/var/log

The refcompressratio will only display the compression ratio for that specific
dataset, not the descendant datasets. To include the child datasets, the
command looks like this:

zfs get compressratio mypool/var

		-- Benedict Reuschling <bcr@FreeBSD.org>
%
You can limit the depth of the displayed datasets in the "zfs list" output
using the -d parameter. To display only the first level of datasets below
mypool/usr and not the ones deeper than those, run this command:

zfs list -d 1 mypool/usr

		-- Benedict Reuschling <bcr@FreeBSD.org>
%
The "zfs list" command can be filtered in multiple ways. To display just
the dataset name, use the -o parameter:

zfs list -o name mypool/usr

More columns and their order can be defined by separating them with commas:

zfs list -o mountpoint,name,avail

		-- Benedict Reuschling <bcr@FreeBSD.org>
%
The output of "zfs list" can be sorted by a specific column using -s.  To
sort the datasets by the "used" column in ascending order, run this command:

zfs list -s used

To sort in descending order instead, use -S:

zfs list -S used

		-- Benedict Reuschling <bcr@FreeBSD.org>
%
To make the "zfs list" output more script-friendly, you can suppress the
output of the headers for each column by passing the -H parameter:

zfs list -H

Another helpful option for script writers is -p, which displays the numbers
in non-rounded, exact values:

zfs list -p

		-- Benedict Reuschling <bcr@FreeBSD.org>
%
Before deleting a dataset or snapshot, perform a dry run using the -n
parameter. This is to make sure you really want to delete just that
dataset/snapshot and not any dependent ones. ZFS will display the resulting
action when -n is combined with the -v option without actually performing
it:

zfs destroy -nrv mypool@mysnap

Once you are sure this is exactly what you intend to do, remove the -n
parameter to execute the destroy operation.
		-- Benedict Reuschling <bcr@FreeBSD.org>
%
You can delete a range of ZFS snapshots (a-z) in multiple ways.
The following will delete d and all earlier snapshots:

zfs destroy mypool/data@%d

To delete d and all later snapshots:

zfs destroy mypool/data@d%

To delete all dataset snapshots:

zfs destroy mypool/data@%

Make sure to let ZFS perform a dry run (-n option) first and display (-v) what
it would do to confirm that the delete operation is removing exactly what you
intended.
		-- Benedict Reuschling <bcr@FreeBSD.org>
%
To set a custom ZFS property on the mypool pool, you need to provide it
using the "key1:key2=value" syntax, where the colon (:) is used as the
separator and identifier from the built-in ZFS properties:

# zfs set warranty:expires=2038-01-19 mypool

The custom property is applied to all datasets and can be queried like any
built-in properties using zfs get:

zfs get warranty:expires mypool

To reset the value of a custom property, use the inherit subcommand:

# zfs inherit warranty:expires mypool

Removing a custom property from a pool is done using the -r flag to the
"zfs inherit" command:

# zfs inherit -r warranty:expires mypool

		-- Benedict Reuschling <bcr@FreeBSD.org>
%
To delete a range of ZFS snapshots, use the % (percent) character after the
full path to the first snapshot that should be included. For example, to
simulate deleting snapshots a through (including) d, use this command:

# zfs destroy -rvn mypool/tmp@a%d

Once you are sure that this is what you want, remove the -n option:

# zfs destroy -rv mypool/tmp@a%d

		-- Benedict Reuschling <bcr@FreeBSD.org>
%
You can prevent the removal of a ZFS snapshot by using the hold subcommand.
For example, to prevent the snapshot called milestone from deletion, run the
following command:

# zfs hold milestone_hold mypool/projects@my_milestone

The "zfs holds" command will list all current snapshots that are protected
this way (-r for a recursive list):

# zfs holds -r mypool

The TIMESTAMP column in the output of the above command is from when the
hold was created, not the snapshot it holds. The "zfs destroy" command will
echo a "dataset is busy" message on the console when it encounters a hold.
Use "zfs release" to release the hold on the snapshot:

# zfs release milestone_hold mypool/projects@my_milestone

		-- Benedict Reuschling <bcr@FreeBSD.org>
%
A user "sender" needs the following permissions set to send a ZFS dataset:

# zfs allow -u sender send,snapshot txpool

On the receiving side, the user "receiver" requires these permissions:

# zfs allow -u receiver compression,mountpoint,mount,create,receive rxpool

		-- Benedict Reuschling <bcr@FreeBSD.org>
%
Don't let your zpool fill up completely by creating a dataset with
reservation.

# zfs create -o refreservation=<5% of total pool space> <poolname>/reserved

You can always shrink the reserve if you need the space, but your pool will
always have space left this way.

		-- Benedict Reuschling <bcr@FreeBSD.org>
%
Sometimes a single slow HDD can cripple the performance of your entire system.
You can spot one like this:

# gstat -I5s | sort -rn -k9 | head

		-- Alan Somers <asomers@FreeBSD.org>
%
FreeBSD's ps(1) can create a dependency tree based on parent/child
relationships between processes, like this:

$ ps -d

		-- Daniel Ebdrup Jensen <debdrup@FreeBSD.org>
%
It is possible to measure the resident memory set:

$ vmstat -o | awk 'NR>1 { t[$7] += $1 } \
END { for (i in t) printf "%s %d\n",i,t[i] }'

The rows have the following meaning:
df = default (not assigned a specific pager)
sw = swap
df = virtual
vn = vnode
ph = heap
md = memory device

This will be reported in number of pages, so it needs to be multiplied by the
page size of the architecture which can be found via:

$ sysctl -n hw.pagesize

		-- Daniel Ebdrup Jensen <debdrup@FreeBSD.org>
%
To establish a serial connection to anything including a USB device,
nothing more than cu(1) is needed:

$ cu -s 9600 -l /dev/ttyU0

		-- Daniel Ebdrup Jensen <debdrup@FreeBSD.org>
%
You can control kernel stack(9) traces on ^T (tty info) by setting
kern.tty_info_kstacks to 0 (off), 1 (on), or 2 (verbose), e.g.:

# sysctl kern.tty_info_kstacks=2

		-- Michael Gmelin <grembo@FreeBSD.org>
%