SYSTEM ADMINISTRATION

• Adding and removing users
• Adding and removing hardware
• Performing backups
• Installing new software
• Monitoring the system
• Troubleshooting
• Maintaining local documentation
• Auditing security
• Helping users

INTRODUCTION TO UNIX

• UNIX is an operating system
• Allows the various hardware components of a computer to work together
  • provides a consistent interface to the hardware
  • e.g., your word processor can save your files to disk
  • manages program interaction so one program won't crash another
• Platform-independent, multi-user, and multi-process
  • platform-independent: available for virtually all types of computers (mainframes, minicomputers, PCs, etc.)
  • multi-user: simultaneous users
  • multi-process: simultaneous processes (running programs)

UNIX History

• Originally developed by Ken Thompson at Bell Labs (1969-76) to play SpaceWars
• Rewritten with Dennis Ritchie in the newly developed C language
  • portable: first OS written primarily in a high-level language
• Bell Labs gave UNIX away to universities
• UC Berkeley made huge improvements to memory management and networking
• Started to market its own version known as BSD (Berkeley Standard Distribution)
• Original Bell/AT&T Unix is now known as System V
• There has been a division ever since
• Differences are largely cosmetic (e.g., different flags on commands)
• For kernel hackers, there are significant differences
• For sysadmins, the differences can be annoying

Linux

• In 1991 Linus Torvalds starts to write his own version - Linux
• Writes everything from scratch => able to give away source code
• Combines both System V and BSD functionality
• Several companies now support distributions
  • RedHat, Slackware, SUSE, Debian, ...
• Basically, all distributions are the same
• Only differences are
  • the way Linux is installed
  • some minor distribution-specific utilities

UNIX BASICS

Files, Programs, and Processes

• A file is just some data structure on disk that holds data
• A program is a file that can be executed
• A process is a program that is currently being executed

The UNIX Shell

• A shell is a process that acts as a middleman between you and the raw UNIX system
• Don't confuse the shell with "UNIX" the shell is just a regular program/process
• Accepts user commands and executes them (similar to DOS's COMMAND.COM)
• three most popular shells are
  • bash (Bourne Again Shell)
  • csh (C shell)
  • sh ("plain" shell)
• Each shell also has a built-in programming language

Special Keys

• Some keystrokes have a special meaning
• For example:
  • Control-C (^C) : terminates a process
  • Control-S/Control-Q : pauses/restarts a process
  • Control-D : end of input

The UNIX Filesystem

• UNIX has a hierarchical filesystem rooted at /
• UNIX has directories that are similar to Windows folders
• Common first-level directories are bin, etc, tmp, usr, etc.
• One special directory is your home directory
• Your working directory is the directory you're currently in
• Your default working directory is your home directory

Pathnames

• A pathname indicates the location of a file
• Absolute pathnames start with /
• e.g., /home/cs/jmache
• Relative pathnames are relative to the start of your working directory
• . refers to the current directory
• .. refers to the parent directory
• e.g., ../../student/foo.txt
  
• NB: file and directory names are case sensitive
• NB: not limited to three-letter-extensions; "." is a regular letter in a filename and can be used repeatedly
• E.g., "myFile.current.text" is perfectly legal
• Hidden files begin with a "."

Common Top-Level Directories

• bin: system binaries (executables)
• etc: system configuration files
• tmp: temporary files
• lib: system libraries
• mnt: multipurpose mount point
• sbin: system admin. binaries
• dev: devices files
• var: "variable"/changeable files
• usr: historical holdover
  • bin: more system binaries
  • sbin: more system admin utilities
• home: user home directories

Useful Commands

• man: displays information on commands
• pwd : print current working directory
• cd : change directory
• ls : displays a list of file in the current directory
• chmod : change the mode/permissions on a file
• cat : displays the contents of a file
• less : displays files one page at a time
• passwd: change your password

Output of ls -l

• Sample output:

  -rwxr-xr-x 1  jmache   faculty   44580  Jan 31 00:21    gcc
  

1. File type and permissions (user, group, other) [rwx]
2. Number of hard links
3. Owner of the file
   • every file has an owner (usually the person who creates it)
4. Group owner of the file
5. File size, in bytes
6. Date and time of last modification
7. Name of the file (up to 255 characters)

File Permissions

• First character is the file type "-" = regular file; "d" = directory; "l" = symbolic link
  • r = read permission
  • w = write permission
  • x = execute permission / s = set user or group id
  • - = permission denied
• Directory: "r" read permission, "w" add/removes files, "x" access subdirectories
• Permissions on new files is governed by a user's umask
• umask restricts the permissions: 0 = allow the permission bit to be set, 1 = don't allow the bit to be set
• Default umask is 022 (octal) this means don't allow write access for the group or world (000 010 010 = 022)
• Change permissions with the chmod(1) command

Other Useful Utilities

• head : displays the first n lines of a file
• tail : displays the last n lines of a file
• mv : renames/moves a file/directory
• mkdir : creates a new directory
• rmdir : removes a directory (must be empty)
• cp : makes a copy of a file (or directory with -r)
• rm : removes files and/or directories (with -r)
• lpr : prints files
• wc : displays number of lines (-l), words (-w), and/or characters (-c) in a file
• script : captures everything that appears on the screen to a file until the user types "exit"

Filename Wildcards

• Allows you to select multiple files based on their name
• Also called filename globbing
  • * matches any string, including the empty string
  • ? matches any single character
  • [...] matches any character in the bracket
• Examples:
  • ls -l *.c
  • ls ??.c
  • ls *.[ho]
  • more [a-d]*
• Quotes inhibit wildcard expansion
• E.g., ls *.c versus ls "*.c"
• Wildcards are expanded by the shell => every program supports wildcards!

I/O Redirection

• Each process has three I/O channels: stdin, stdout, and stderr
• stdin is where the process gets its input - by default from the keyboard
• stdout is where the process sends its output - by default to the screen
• stderr is where the process sends its errors/diagnostics - by default to the screen

Output Redirection

• Sends output to a file
• Use the redirection characters > and >>
  • command > file overwrites file
  • command >> file appends output to file
• Can redirect both stdout and stderr with this crazy-looking method:
  • command > file 2>&1

Input Redirection

• Reads input from a file
• Use the redirection character <
  • command < filename

Command Sequences

• Allow you to run one command after another
• Two flavors: unconditional and conditional

Unconditional

• Execute each command, regardless of the outcome
• Separate commands with semicolons (;)
  • command1 ; command2 ; ...

Conditional

• Two flavors: success sequences and failure sequences
• Success sequences only continue to execute commands if the previous command succeeds
• Separate commands with two ampersands (&&)
  • command1 && command2 && ...
• Failure sequences only continue to execute commands if the previous command fails
• Separate commands with two bars (||)
  • command1 || command2 || ...

Pipes

• Attaches the output of one command to the input of another
  • command1 | command2
• tee sends its output to a file, as well as sends it on to the next command in the pipeline
  • who | tee users | sort
• captures the output of who to the file users, then sends the list of users to the sort command which prints a sorted list of users to the screen

Command Substitution

• Substitute the output of a command into another command
• Example:

     $ echo Today is `date`
     Today is Tue Feb  6 08:47:45 PST 2001
  

• More complex example:

     $ echo There are `who | wc -l` people logged on.
     There are 2 people logged on.
  

Multiprocessing

• Processes can be run in either the foreground or the background
• Foreground processes
  • the user can interact with it (keystrokes go to the process)
  • the shell waits for it to finish
• Background processes
  • the user cannot directly interact with it
  • shell returns immediately and asks for the next command
• To run in the background: command &

Controlling Processes

• ps gives information about your processes
• pid (process id) is a unique number identifying the process
• jobs will list the processes you have running in the shell
• kill will kill a running process
• ^Z is used to suspend a foreground process
• bg will move a suspended process into the background
• fg will move a suspended process into the foreground

grep/egrep/fgrep

• grep finds lines in files that match a certain pattern
• grep/egrep/fgrep [-ilv] pattern files
  • -i: ignore case
  • -l: list filenames rather than display each line
  • -v: display lines that don't match the pattern
• fgrep is a "fast" grep that only searches for fixed strings
• grep finds basic regular expressions
• egrep finds extended regular expressions

Basic Regular Expressions

• Any simple character matches itself, e.g., a
• Catenation of REs, e.g., abc
• [...] matches any character in the list, e.g., [abc]
• [^...] matches any character NOT in the list, e.g., [^abc]
• . matches any character (including newline)
• ^ matches the beginning of the line
• $ matches the end of the line
• Suffix
  • *: 0 or more times
• Example:
  • ab*[cd][^0-9].*

Extended Regular Expressions

• expr1|expr2 matches either expr1 or expr2
• (expr): used for grouping
• Two new suffixes:
  • ?: 0 or 1 time
  • +: 1 or more time
• Example:
  • (ab|cd)+e?

find

• Locates files based on given criteria
• find path expression action
• path is where you want find to start looking
• expression describes the files you're looking for
• Common expressions:
  • -atime: was the file last accessed some time ago
  • -group: does the file belong to a given group
  • -name: does the filename match a given pattern
  • -perm: does the file's permissions match a pattern
  • -type: is the file of a certain type (directory, socket, etc.)
  • -user: is the file owned by a given user
• Common actions:
  • -print: print the full pathname
  • -exec: execute some command

find Example

• find . -name "*.c" -print
• find . -name "core" -exec rm -i {} \;