1. User Account

• UNIX intrinsically support multiple users. Each user has its unique identification, login name (or UID number in system sense).
• The purpose of user identifications in multiple user environment is for management of resources such as storage space, memory, CPU time and I/O.
• Not all users are equal, the root user (UID = 0) is the system administrator and has complete acess to every resources in the system.
• Other than root user, all users are equal. They can use the resource what they are assigned to. Such as accessing their own files.
• User are also divided into sets, called groups. A user may belong to several groups. Each group is defined by a list of users that are part of that set.

• Command line tools: useradd, userdel

2. Managing Filesystems

• denotes the entire file tree and the algorithms to use to manage it.
• a subsection of the file tree where the files are stored in a separate disp partition.

• mount: is a process to attach a filesystem on a disk partition to a directory (mount point) in the root file tree.
• umount: to detach a filesystem from the root file tree. All unsaved contents of this filesystem will be written.
• to check the filesystems mounted, use the command mount or usermount.

$ mount     /dev/hda3 on / type ext2 (rw)     none on /proc type proc (rw)     /dev/hda1/ on /dos type vfat (rw)     none on /dev/pts type devpts (rw,gid=5,mode=620)

• to check the size of the filesystems mounted, use the command df.

$ df     Filesystem           1k-blocks      Used Available Use% Mounted on     /dev/hda7               996116    744416    201100  79% /     /dev/hda1              2096160   1642528    453632  78% /dos

• filesystem configuration are stored in the following files:

/etc/fstab: file system table where filesystems will be mount on startup.
/etc/mtab: table of mounted file systems.

• Linux filesystem are organized in hierarchical manner. Every file is stored on the system with a unique name, in directories which can also hold other files and directories -- or, subdirectories.
• /: the root filesystem, top of the directory tree.
• /usr: contains all commands, libraries, man pages, and other unchanging files needed during normal operation.
• /var: contains files that changesm such as spool directories (for mail, news, printers, etc), log files, formatted manual pages, and temporary files.
• /home: contains the users' home directory.
• /bin: command needed during bootup that might be yuser by normal users.
• /sbin: commands for system administrator on bootup.
• /etc: system configuration files
• /root: the home directory for user root.
• /lib: shared libraries needed by the programs on the root filesystem.
• /lib/modules: loadable kernel modules.
• /dev: devuce files.
• /tmp: temporary files.
• /boot: files used by LILO (Linux Loader).
• /mnt: mount point for temporary mounts by the system administrator.
• /usr/bin: almost all user commands.
• /usr/sbin: system administration commands that are not needed on the root filesystem.
• /usr/lib: unchanging data files for programs and subsystems, including some site-wide configuration files.

• Linux is a multi-user system, file permissions are one way the users to protect their files.
• All files and directories are "owned" by the person who created them. Only root can change the ownership of files and directories.
• Users can change the permission of their files or directories.
• to check the ownership and permission of a file or directory, use the list command ls.

$ cd /tmp     $ cat > file1     from file1     ^D     $ cat file1     from file1     $ ls -l /tmp

• to change the ownership of a file or directory, becoming the root user and use change owner command chown or File Property dialog in the GNOME / KDE File Manager (gmc / kfm).

$ chown wsuser.users /tmp/file1     $ ls -l /tmp/file1

• to change the permission of a file or directory, use the change mode command chmod or File Property dialog in File Manager.
• Todo: use GNOME File Manager to change the owner and group of the file /tmp/file1 to nobody.nobody.
1. Start File Manager in GNOME menu->File Manager
2. Locate the file /tmp/file1
3. Select the file, right click on the file, select Properties...
4. In Properties dialog, select Permissions.
5. in File Ownership:

Owner: nobody
Group: nobody
6. In the command shell, check the permission

$ ls -l /tmp/file1

3. Who am I? Who is using this system?

• to find who you are, try

$ id     $ whoami

• to find who is using the system, try

$ w     $ finger

• the primary function of finger is use to lookup information for a specific user.

$ finger root

4. Add New Software

• One of the system administrator's task is to install new software to the Linux since only root can access all disk in the Linux Filesystem.
• You can use rpm,  to manager software (packages) installed in the system.

5. Managing Process

• Processes are command/program/shell-script that is being run in memory.
• Each process identified by a number called Process ID(PID).
• There are special kind of process called daemons which running in background and performs a system-related tasks. Most of daemons are started as long as the system is up and stopped when on system shutdown. Examples include init, cron, inetd.
• to show the processes running in the system, use the process status command ps, top

$ ps -aux     $ top

• to control a process (terminate, suspend, resume), use the command kill .

6. Practical Security Tips

It (Unix) was not designed from the start to be secure. It was designed with the necessary characteristics to make security serviceable.  -- Dennis Ritchie

• Physical Security: lock your computer physical in a secure place.
• Password Security:
• Do not assign easy-to-guess password.
• Do not share your account with other person.
• Check user account with null passwd (without passwd) in /etc/shadow.
• Network Security:
• Close the door first by denying access from network by default.

$ cat "ALL:ALL" >> /etc/hosts.deny

• Stop all unused services such as sendmail, NFS.

$ chkconfig --list     $ chkconfig --del sendmail     $ chkconfig --del nfslock     $ chkconfig --del rpc

• Check system logs in /var/log regularly especially /var/log/secure.
• Update your Linux system regularly.
• Checking the errata (bug fixes) in http://www.redhat.com/support/errata
• The update packages can be found in

ftp://updates.redhat.com
ftp://ftp.cse.cuhk.edu.hk/pub/redhat/updates
• To upgrade the system, refer to Working with Update Agent and Freshening of Package Management with RPM in The Official RedHat Linux Reference Guide.