"Breaking" out from a locked GUI session:

Cntrl-Alt-F2,Cntrl-Alt-F3,Cntrl-Alt-F4, or Cntrl-Alt-F5 to get a "login:" prompt.
Next, login as root, find the process ID of the Xserver and kill it.

Creating users:

useradd {user name} w/ optional switches:

-m Creates a new home directory for a new user.
-M Tells useradd NOT to create a home directory for the new user.
-s Defines path to users default shell (such as /bin/bash).
-h Defines path to user's home directory.
-c Attaches comment text to user.
-e Defines account expiration date.
-u Sets user ID.
-D Changes the default values for useradd command.

* Don't forget to set a password for the new user! Do that with passwd {user name}

Deleting users:

userdel {user name}

Finding files by name:

find / -name {filename}

Kill X in an emergency:

If X has stalled (e.g. during re-configuration after an update etc.), or if a graphical application has crashed and left X unstable, use Ctrl+Alt+Backspace to kill X and return you to either the shell prompt or the login manager, depending on how X was started.

SysRq:

This key may be labelled Print Screen or Prt Scrn on some keyboards. If your system has crashed and become unstable, just cutting the power will at the very least cause an fsck when you eventually restart and it may well damage the system itself. A safer option is to attempt to prevent disaster in the hope that the kernel is still responding, even if everything else has died. (If your computer is on a network, try ssh or telnet first and see if you can get root access before giving up on the system.) Otherwise, use these in this order!
Alt+SysRq+S synchronises the discs.
Alt+SysRq+U unmounts ALL filesystems.
Alt+SysRq+B reboots.

Memory monitoring:

Find out which processes are taking the most memory: $ ps -aux | sort +5n | less

Find out who is listening to your box:

$ netstat -na | grep -i listen gives you a list of the daemons listening on TCP or UDP ports

$ netstat -na | grep -i established will let you see who is connected.

Logged on user listings:

finger
or users
or w

Finding library dependencies:

If an installed application is causing problems, or if you want to copy it to another machine, find out which libraries are required by the binary. e.g. for /usr/bin/procmail: $ ldd /usr/bin/procmail produces:
libm.so.6 => /lib/libm.so.6 (0x40025000)
libnsl.so.1 => /lib/libnsl.so.1 (0x40048000)
libdl.so.2 => /lib/libdl.so.2 (0x4005e000)
libc.so.6 => /lib/libc.so.6 (0x40062000)
/lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x40000000)

Tab completion:

Linux files names can be long but you don't always have to type the whole thing. In the shell, type the first few characters and press the TAB key. If there is more than one file with those letters in common, Linux will prompt you, showing the remaining choices. Type a few more characters and hit TAB again. If no files match, you may get a beep.

To fetch email without having to configure a full mail-transfer-agent like sendmail, use: mda "/usr/bin/procmail -d username" in your ~/.fetchmail.rc file.

Root password failure (Failure of your memory as opposed to failure of the operating system!):

At the LILO prompt (boot:), enter linux init=/bin/sh This will put you into a shell session and you can now use: $ passwd to enter a new root password. (Hence the need to protect your system with a LILO password. Anyone with physical access to your machine could change the root password.) The LILO password can be changed provided you can log in as root. If you forget the LILO password AND the root password then maybe you should invest in another form of security for the written passwords you will need for Linux. In all other cases, avoid writing down your root password.

Protect your system using LILO:

LILO can use password control to prevent other users changing the boot parameters of various boot images. Put the password password=mypassword at the top of /etc/lilo.conf and the keyword optional on a new line beneath. Now add the keyword restricted to any of the image sections to allow those to boot without a password only if no extra parameters are specified at the LILO prompt. Images without restricted added will require a password for every boot attempt. Finally, make sure that you run (as root): $ chmod 600 /etc/lilo.conf and $ lilo sometime before next rebooting the system! Check man lilo and man lilo.conf for more info.

LILO error messages:

If LILO fails to start Linux, it should give you one of a few possible error codes which may help pinpoint the problem (although not necessarily start the system!).
L means that boot.b cannot be loaded, possibly a disk error.
LI means that boot.b has been moved or a disk error occurred.
LIL means that LILO cannot allocate data for the map file.
LIL? means another possible disk error.
LIL- means the map file data is invalid.

Mounting drives and devices:

Floppy drives:

1) Create a mount point... mkdir /myfloppy
2) Mount... mount -t vfat /dev/fd0 /myfloppy
*(unmount /myfloppy to undo.)
CDROM drives:

1) mkdir /mycd
2) mount /dev/cdrom /mycd



Mounting other devices:


A:floppy = /dev/fd0 or /dev/floppy
B:floppy = /dev/fd1
Keyboard = /dev/tty
Master drive on primary IDE = /dev/hda
First partition master drive on primary IDE = /dev/hda1
Third partition on master drive on primary IDE = /dev/hda3
Slave drive on primary IDE = /dev/hdb
Master drive on secondary IDE = /dev/hdc
Slave drive on secondary IDE = /dev/hdd
SCSI target ID 0 = /dev/sd0
SCSI target ID 1 = /dev/sd1
Second partition on hard disk at SCSI target ID 0 = /dev/sd02
CDROM (various) = Either /dev/cdrom or IDE/SCSI designation (/dev/hdb, /dev/sd5, etc.)
Modem = /dev/modem
Sound system = /dev/sound

Adding hard drives, partitioning, and formatting:

Linux Planet
http://www.linuxplanet.com/linuxplanet/print/4232/

File system permissions and ownership:

Linux Powered
http://www.linuxpowered.com/html/editorials/file.html

Starting up Gnome session:

xinit
gnome-session&

Starting up a KDE session:

xinit
startkde&

Creating archive/tarball files:

1) tar -cvf newfilename.tar filetoadd1 filetoadd2 fil...
2) tar -cvf newfilename.tar /directorytobackup /directory2tobackup ...

1) tar -cvzf newfilename.tgz /directorytobackup
*Note: You could also call the new file newfilename.tar.gz, Linux does not differentiate.

1) tar -df /dev/st0
*Note: The d argument means "difference". It compares current directory contents to those on the tape device. The -v option shows every file name.

1) tar -tvf /dev/st0
*Note:The -v option will show owners and permissions.

1) tar -xvpf /dev/st0
*Note: This will restore EVERYTHING! The -p argument will preserve permissions on the files.
2) tar -xvpf /dev/st0 /directorytorestore
*Note: This will restore a particular directory

1) which programname
*Example - which smbd

1) ls -al (The -a argument is the key.)

1) chmod a+x filename

1) chkconfig --list (Lists all services at each run level.)
2) chkconfig --level2345 gpm off (Example - Turns off mouse service at 4 run levels.)
*NOTE: Service stays off after next reboot.

1) smbmount //server/share /mountpoint -o username=jdoe,password=whatever

OR try using...

mount -t smbfs -o username=jdoe,password=whatever //servername/share /mountpoint

(*NOTE: Password arguments are optional, you will be prompted for one if not included. You must be logged in as root to mount the remote share. Also, the user account used MUST be a member of the domain THAT THE TARGET MACHINE IS A MEMBER OF.)

1) ln -s file newlinklocation