To schedule a once off command to run sometime in the future:
  echo "wget url" | at 01:00
%
Bash arrays:

array=(zero one two)
array[3]=three
echo "The fourth element of array is ${array[3]}"
echo "There are ${#array} elements in array"

for example, the following 2 are equivalent:

VER=`cut -d' ' -f4 /etc/fedora-release`
VER=($(< /etc/fedora-release)); VER=${VER[3]}

Note, consider using a more powerful language
like python if you need to use bash arrays
%
In bash one can expand the last parameter used with !$
i.e. if one did `mkdir mydir` then `cd !$` would move to it.
Search for "history expansion" in `info bash` for more info.
Note this is only for interactive use, it doesn't work in scripts.
See also http://www.pixelbeat.org/settings/.inputrc
%
To display a quick calendar on the console:
  cal -3
%
Data and audio CDs

save copy of data cdrom
    dd bs=1M if=/dev/cdrom | gzip > cdrom.iso.gz
create cdrom image from directory
    mkisofs -r dir | gzip > cdrom.iso.gz
burn cdrom image
    gzip -dc cdrom.iso.gz | cdrecord dev=0,0,0
rip auto tracks from CD to wav files in current dir
    cdparanoia -B
make audio CD from all wavs in current dir
    cdrecord dev=0,0,0 -audio *.wav
make ogg file from wav file
    oggenc --tracknum="track" track.cdda.wav -o "track.ogg"
%
If you want to turn off the "clear screen"
done by some terminal apps on exit:

infocmp $TERM |
sed 's/\([rs]mcup=\)[^,]*,/\1,/' > /tmp/$TERM.src
tic -o ~/.terminfo/ /tmp/$TERM.src
rm /tmp/$TERM.src

Remove the resultant file in ~/.terminfo to undo
%
To setup terminal application foreground colours
appropriate for a dark background:

ls
    cp /etc/DIR_COLORS ~/.dircolors
vim
    echo "set background=dark" >> ~/.vimrc

Note to change colours for linux virtual terminals
    setterm -background white -foreground black -store
%
#what day does xmas fall on, this year
date --date='25 Dec' +%A

#convert number of seconds since the epoch to a date
date --date '1970-01-01 UTC 130204800 seconds'

#What time is it on West coast of US
#(use tzselect to get TZ to use)
TZ="America/Los_Angeles" date
%
Quickly search word list for prefix
  look prefix
Highlight occurances of word in word list
  look '' | grep -Fi --color word
Lookup word in online dictionaries/info
  gnome-dictionary ireland
%
Directory navigation

cd -             go to previous directory
cd               go to home directory
pushd .          put current dir on stack so you can popd back to it
(cd dir && cmd)  go to dir, execute cmd and return automatically
%
Handy disk space commands

ls -lSr      #show files, biggest last
df -h        #free disk space
df -i        #free inodes
du -hs ...   #disk usage of specified files/dirs
fdisk -l     #show disks partitions sizes (run as root)

See also: http://www.pixelbeat.org/scripts/dutop
%
Handy find commands to check permissions in a web tree

find -type f ! -perm -444 #files not readable by all
find -type d ! -perm -111 #dirs not accessible by all
%
Wondering what the optimum flags for your
compiler and CPU combination?
http://www.pixelbeat.org/scripts/gcccpuopt
%
Transforming documentation. The following changes the tar docs
from man, interactive help and info documentation,
to HTML, PDF and HTML respectively.

gzip -dc `man -aW tar` | groff -Thtml -man /dev/stdin > tar.html
card tar -o- | ps2pdf - > tar.pdf
texi2html tar.texi #Note texi is the source format of info docs
%
A common cause of networking problems on linux
is the integrated firewall which is enabled by default.
You can easily control it with the following 3 commands:

/etc/init.d/iptables st{atus,op,art}

Editing of the rules is system dependent, but
many use a thirdparty rule editor like shorewall for e.g.
%
Common Email/IRC acronyms

YMMV = Your Mileage May Vary   IMHO = In My Humble Opinion
AFAIK = As Far As I Know       IIRC = If I Remember Correctly
BTW = By The Way               FYI  = For Your Information
%
For a single user box, adding the following to /etc/fstab,
will allow you to `mount /mnt/floppy` etc. as a normal user
/dev/fd0    /mnt/floppy  auto  nosuid,nodev,noauto,user           0 0
/dev/cdrom  /mnt/cdrom   auto  nosuid,nodev,noauto,user,ro,unhide 0 0
%
To scroll the console (linux or xterm) use Shift+PgUp & Shift+PgDn
%
Handy aliases (to add to ~/.bashrc)

#Quick dir listing
alias l='ls -l --color=auto'
#Show the directories in the current dir
alais lld='ls -lUd */'
#A handy hexdump
alias hd='od -Ax -tx1z -v'
%
Multimedia conversion

Use `sox` to convert sound file formats and
`convert` for graphics file formats.
For e.g. this adds a comment to a jpg file.
convert infile.jpg -comment "a comment" outfile.jpg
%
Have a look at the following for common command line operations
http://www.pixelbeat.org/cmdline.html
%
To view the contents of an archive (tar.gz, .rpm, .deb, ...)
use Midnight Commander (mc) (Just hit enter on the higlighted file).
This even works remotely (over ftp, fish, ...)
Non obvious keys for mc are documented here:
http://www.pixelbeat.org/lkdb/mc/html
%
For useful non obvious keyboard bindings to various linux apps, see:
http://www.pixelbeat.org/lkdb/
%
If you need to leave a program running after you log out, try screen:
http://www.pixelbeat.org/lkdb/screen.html
%
When you install a package on linux, generally
detailed documentation is in /usr/share/doc/$PACKAGE/
and binaries are in /usr/bin/ or /bin.
You can get specifics on rpm distributions as follows:
rpm -ql -p package.rpm #show where files in RPM would be installed
rpm -ql    package     #show where files in package were installed
rpm -ql -f /path/file  #show files for package /path/file belongs to
%
To convert a DOS text file to Unix so that the extra ASCII 13
does not show up and vice versa, the unix2dos and dos2unix
commands are very handy. You can do the same thing with
sed, tr, recode, ...
%
Multiple X sessions

It is possible to open more than one "X" session  on  a single box.
As User #1 (from console) : $startx -- :0
As User #2 (from console) : $startx -- :1  ... and so on
Toggle between sessions with Ctrl+Alt+F(n) and Ctrl+Alt+F(n+1)
and so on ... where n is the first free tty (usually F7)
vnc or xnest can be handy for the same task also.
%
To show the dependent libraries for a binary use:
`ldd /path/to/binary`
%
Linux Documentation

looking for a command related to a word? Try apropos
Already know the command name? Try `man command` or `info command`
Keys for man/info are here: http://www.pixelbeat.org/lkdb/info.html
Additional documentation for a package is usually stored under
/usr/share/doc/$PACKAGE/
%
Process managment

ps -Af --forest  #show all process commands and hierarchy

You can send signals using the command `kill -num pid`.
The default num is 15 (SIGTERM). `kill -l` will give a list.
Note kill -9 should never be used in normal operation.
%
Multi boot : Installation order

For multiboot machine, the OS install order is "dumbest first"
dos -> Win9x -> Win NT/W2k -> Linux
%
Shell auto completion

Autocompletion is a feature provided by shells to complete any
part command from the console. For different shells these are:

Shell                          csh        ksh       bash
Single option completion       Esc-Esc    Esc-Esc   Tab
Unresolved reference menu      Ctrl-D     Esc =     Tab-Tab

Note bash has programmable auto completion which can be very handy.
%
User management on redhat 9 at least has the
following tools, going from high level to low level:

redhat-config-users
  user{add,mod,del}
    vipw

redhat-config-users
  group{add,mod,del}
    vigr
%
login messages

The file /etc/motd displays a message at every login. To have
it change, point MOTD_FILE in /etc/login.defs to another file.
Users can suppress this message by doing `touch ~/.hushlogin`
%
Type [Ctrl+R] in the bash prompt and you will get a
(reverse-i-search)`': TYPE THE SEARCH WORD HERE
This will show the matching commands executed before.
%
Console hotkeys

  Add entries like the following to ~/.inputrc
  "\e[[A":"lynx\C-M" # F2

Gnome 2 hotkeys

  Using gconf-editor edit the following keys as appropriate:
  apps->metacity->keybinding_commands->command_{1,12}
  apps->metacity->global_keybindings->run_command_{1,12}
%
File encryption

gpg -c file  #encrypts file
gpg file.gpg #decrypts file

Vim has builtin encryption (simple like pkzip).
Use ':X' to encrypt your file. Enter a blank
password to save a decrypted copy again.
%
Environment variables

VAR=value command #for command only
VAR=value         #for this shell only
export VAR=value  #for this shell and child processes

Environment variables can never be passed to parent processes
It's useful to set environment variables like above in
startup files. To get details for bash do:
info bash "Bash Features" "Bash Startup Files"
%
Turn off annoying beeps

readline:       add "set bell-style none" to ~/.inputrc
X:              xset b off
gnome-terminal: uncheck "Terminal bell" in profile's settings
vim:            add "set visualbell" to ~/.vimrc
%
Command execution timing

To see how long it takes a command to run, type the word
"time" before the command name.
%
nslookup is obsolete. Use host for DNS lookup instead

host www.pixelbeat.org    # A record for domain name
host -t MX draigBrady.com #MX record for domain name
host 192.0.34.16          #reverse dns on ip address
host `hostname`           #ip address of current sys
hostname -i               #same as above without DNS
%
Downloading directly from web

To download html directly as text:
  lynx -dump -width=1000 http://www.pixelbeat.org/vim.tips.html > vim.txt
To download any file directly:
  wget http://www.pixelbeat.org/cmdline.html
%
Math in shell

You can use bc, python, expr etc. to do math in the shell.
Note expr can only parse and output integers.

precision=2 num1=1.23; num2=3.21
answer=`echo "scale=${precision}; $num2/$num1" | bc`
answer=`echo "print '%.${precision}f' % ($num2/$num1)" | python`
answer=`expr 1 / 2`
%
Backing up a directory

Backing up a directory preserving ownerships, permissions and
links, to another location:

  cp -a /dir/to/copy /where/to/
or
  ( tar cf - /dir/to/copy ) | ( cd /where/to/; tar xf - )

Note the previous 2 create the copy dir under /where/to/.
If instead you would like just all the contents of copy, do:
  ( cd /dir/to/copy && tar cf - . ) | ( cd /where/to/ && tar xf - )
%
Finding installed PCI devices

Want to know which all PCI devices are  installed on your box
and are picked up by the kernel ? Depending upon verbosity of
reply needed, do: #lspci, 'lspci -v' or 'lspci -v -v'.
%
Synchronising time with net time servers

To synchronise the time on your system with Time Servers on
the net (e.g. time.nuri.net, time.hmt.org):
  rdate -s some.time.server;/usr/sbin/setclock
%
X hotkeys

Ctrl+Alt+Backspace      kill X server
Shift+Ctrl+Numlock      Toggle numpad mouse move keys
Ctrl+Alt+KeypadPlus     next X mode (resolution usually)
Ctrl+Alt+KeypadMinus    previous X mode (resolution usually)

See http://www.pixelbeat.org/lkdb/
%
Changing boot parameters using GRUB

At the grub boot menu press "e" and edit the boot parameters
For example append "single" to boot in single user mode.
Then boot the entry using "b" [boot]
%
Pushing process into background

To temporarily stop a running process, press Ctrl-z. And then
to push it in the background (so that it keeps running) type
'bg'. To get it back to foreground later, type 'fg'.
You can test with this command `cat </dev/zero >/dev/null`
%
Wondering how to make a safe password ? Use mkpasswd.
The following will produce a unique 8 letter password
with minimum 2 digits and 3 letters in upper case:

mkpasswd -l 8 -d 2 -C 3
%
Tracking your reboots

To know the history of your last few re-boot sequences:
  last reboot
last looks back in the wtmp file for all logins. The pseudo
user "reboot" also logs in at all reboots of the system
%
Redraw/Clear text console

If your console application is messed up
(with wall messages for e.g.) a Ctrl-L will
cause a redraw.
%
Changing priority of a process

The nice value (-20 to 19) represents the priority of a process.
The lower the value, the higher the priority (not as nice to others).
For non interactive processes (especially on multi-user systems) do:
  nice low_priority_command
You can retroactively give a process less priority like:
  renice 19 -p low_priority_pid
For e.g. this is useful at the start of long running shell scripts:
  renice 19 -p $$ > /dev/null
Only root can give processes more priority.
%
To take screen shots of a linux virtual console, do:
setterm -dump -file screen.dump
%
USB storage devices

USB mass storage devices are mounted as virtual scsi devices.
Usually it can be mounted like:
  mount /dev/sda1 /mount/point
If you have other scsi devices look in /proc/scsi to
see which scsi device to use.
You may find devlabel useful for helping with device consistency.
Have a look at http://www.laks.com/ for a nice USB storage watch.
%
video encoder in mplayer

A little known feature of mplayer (www.mplayerhq.hu) is that
it contains a movie-file encoder, 'mencoder'. Try divx enco-
ding with 'libavcodec' that comes bundled with it; to shrink
larger 'mpeg' files to half-sized 'avi' files.
%
Testing/ stepping through your shell script

$ sh -x script.sh 2> script.log

Your script runs a bit slower but, every line that is
executed (after variable expansion) gets logged.
View the log later with any pager or editor.
%
Genesis of (weird) unix names

awk
  This pattern scanning and text processing language was named
  after its authors: Al Aho, Peter Weinberger, Brian Kernighan.
grep
  "Global Regular Expression Print"
  g/re/p is the ed command to print all lines matching a
  certain pattern (where "re" is a "regular expression").
rc (as in ".bashrc" ...)
  "rc" derives from "runcom", from the MIT CTSS system,ca. 1965.
  There was a facility that would  execute a bunch of commands
  stored in a file; it was called "runcom" for "run commands",
  and the file began to be called "a runcom"
%
Free opensource books online

http://www.pixelbeat.org/links.html
%
Benchmarking your HTTP server

The standard Apache distribution has a built-in benchmark tool
for webservers called ApacheBench (/usr/bin/ab) ...  It  can
simulate 100s of simultaneous requests to same page; Try:
  ab -n 10000 -c 100 http://localhost:[port]/index.html
%
Text-to-pdf

To convert any text file or *nix command output into pdf use a
tiny utility called mpage (http://www.mesa.nl/pub/mpage) which
converts any text file  to postscript. The 'ps2pdf' script of
ghostscript can be used for doing the pdf conversion. Try:
$ man -t ls | mpage -X"ls man page" -m50l -1 | ps2pdf - ls.pdf
$ cat fname.txt | mpage -X"Title" -1 | ps2pdf - fname.pdf
%
Sending winpopup message from linux

To send a winpopup message to windows machines through SAMBA:
  echo "message" | smbclient -M netbiosname
Note M$ are disabling the messenger service by default as of
windows XP service pack 2
%
Mounting an ISO image directly

With a single command, you can mount an ISO file directly off
the hard drive, as if it were a CD. Try :
 mount -o loop filename.iso /mnt/cdrom
%
Midnight Commander (mc) remote filesystems

access remote files using ssh
  cd /#sh:user@machine/

access remote files using ftp
  cd /#ftp:user:pass@isp/public_html
%
Useful mozilla/firefox commands (type in URL bar):
  about:config    low level config editor
  about:about     show about items available
  view-source:    view page source
Note the view-source: command can be a link from a web page
but it requires an absolute URL as an argument. Therefore
relative links need to be transformed using javascript for e.g.
%
If you are transitioning from windows and would like vim or gvim
to behave more like windows editors then the following is useful

:source $VIMRUNTIME/mswin.vim

You can also get even more like windows editors by using evim
which starts in insert mode (use Ctrl+L to get to Normal Mode).
%
#list internet services on a system
netstat -lp --inet

#list active connections to/from system
netstat --inet
%
New Linux networking commands

ifconfig, route, mii-tool etc. are out of date
The new kids on the block are ip and ethtool.

ip link show                          #list interfaces
ethtool interface                     #list interface status
ip link set dev eth0 name home        #rename eth0 to home
ip addr add 1.2.3.4/24 brd + dev eth0 #add ip and mask(255.255.255.0)
ip link set dev interface up          #bring interface up (or down)
ip route add default via 1.2.3.254    #set default route to 1.2.3.254
%
Exporting filesystems over NFS

To export filesystem in NFS with read-only access and no root
write access, edit the /etc/exports file and add the following line:
/dir/to/export (ro,insecure,all_squash).

'man 5 exports' for other details

The `exportfs -r` command makes the /etc/exports command take affect
%
A common requirement in shell scripts is to print
numbers with thousands seperators. You can use
glibc functionality to do this like: (notice the ')

printf "%'d\n" 1234
%
On an rpm distribution, you can list packages by size like:

rpm -qa  --queryformat "%10{SIZE}\t%{NAME}\n" | sort -k1,1n

The equivalent for debian is:

dpkg-query -W --showformat='${Installed-Size;10}\t${Package}\n' |
sort -k1,1n
%
Redirection

cmd >/dev/null 2>&1     #redirect stdout & stderr to /dev/null
cmd 2>&1 >/dev/null     #redirect stdout to /dev/null & stderr out
cmd >file2 <file1       #stdin from file1, stdout to file2
exec 1>file 2>&1        #all stdout & stderr in shell (script) to file
%
rsync really is worth knowing. Here are two common uses.

To only get differences between file on the remote and
local server. For e.g. if you're having problems downloading,
you can run this several times to get the file.

rsync -P rsync://rsync.server.com/path/to/file file

This would copy file1 to file2 at around 1 MByte/s
Think of it as nice for I/O.

rsync --bwlimit=1000 file1 file2
%
In gnome at least, one can get a screenshot of the whole
screen and current window with the PrtSc and Alt+PrtSc
keys respectively. However quite often it's necessary
to get screenshots in situations where these keys don't
work. For these situations I find a delayed screenshot
initiated from a terminal on a different virtual desktop
very useful. I usually do this like:

sleep 5; import -window root -quality 90 screenshot.png
%
Shell conditionals

if [ "$str" = "1" ] || [ "$int" -eq "1" ]; then
    echo "str or int is 1"
elif [ "$int" -lt "10" ] && [ -e "file" ]; then
    echo "int less than 10 and file exists"
fi

#man test gives details of what you can put between the [ ]
%
Shell loops

for item in 1 2 3 4; do          while read line; do
    echo "item=$item"                echo "line=$line"
done                             done

Note you can pipe into and out of loops also
but be wary as they're run in a subshell so
any change in environment variables are local
to the loop.
%
Piping Vs Redirection:

> and < etc. are used for connecting files to commands
| is used for connecting commands

You can't just use "command | file" as this puts
commands and files in the same namespace. In other
words if one had a file with the same name as a
command then there would be ambiguity about what to do.
%
Trying distributions without rebooting
    http://umlbuilder.sourceforge.net/walkthrough.shtml
%
Users

su            #change to root user
su -l         #same but get correct root $PATH etc. (login shell)
su -l user    #change to user
su -c "cmd"   #just run cmd as root and exit
sudo cmd      #just run cmd as root without password and exit
visudo        #setup sudo (as root). `man sudoers` for more info
%
If you don't select "System clock uses UTC" then the time
will not be automatically adjusted for daylight savings etc.
%
Having difficulty reading the output from the diff command?
Consider using tkdiff instead, for a graphical side-by-side
view. You can use it like diff, i.e. tkdiff file1 file2
Also you can use vim like: vimdiff file1 file2
%
There are a few handy shortcuts common to all linux window managers:

lower window     Middle mouse click on titlebar
resize window    Alt + Middle mouse drag in appropriate bit of window
move window      Alt + Left mouse drag anywhere in window
%
Note when you copy something to the clipboard or selection buffer
in an X application, it's thrown away when the app is closed.
(unless you use an app to manage the clipboard (like xclipboard)).
%
There are some handy mouse shortcuts for selecting text in X.
As normal a left mouse button drag selects text but most apps
also allow the right mouse button to extend the selection.
I.E. you can click or drag the right button to define the
extent of the selection. Also in general double clicking
selects a word, while triple clicking selects a line.
See http://www.pixelbeat.org/docs/xclipboard.html for more.