Introduction to Unix

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Introduction to Unix

• Bent Thomsen
• Institut for Datalogi
• Aalborg Universitet

2
Unix Philosophy

• Designed by programmers for programmers
• Toolbox approach
• Flexibility and freedom
• Networked designed for server use
• Multi-user / Multitasking
• Conciseness
• Everything is a file or a process
• File system has places
• Processes have life

3
Unix Structure
4
Interacting with Unix

• Sometimes through a GUI interface

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OpenLook on Sun
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Common Desktop Environment
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MacOS X
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Interacting with Unix

• But most likely through a shell
• Xterm, telnet, Secure Shell
• A shell is the command line interpreter
  (like the DOS or command prompt in
  Windows)
• A shell is just another program
• There are several shells (sh, csh, tcsh, bash )
• A program or command
• Interacts with the kernel
• May be any of
• Built-in shell command
• Interpreted script
• Compiled object code file

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Telnet
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SSH Secure Shell
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Getting started - login

• The login is the users unique name
• Password is changeable
• Only known to user, not to system staff
• Except initial issued password
• Unix is case sensitive
• Login and password prompt
• System messages you have new mail
• The command prompt machinegt

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Example of login
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The command prompt

• Commands are the way to do things in Unix
• Commands are typed at the prompt
• Commands, as everything else, are case sensitive
  in Unix
• A command consists of a name, options (or flags)
  and sometimes arguments

promptgt ltcommandgt ltflagsgt ltargsgt
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Two Basic Commands

• The most useful commands youll ever learn
• man (short for manual)
• help
• They help you find information about other
  commands
• man ltcmdgt retrieves detailed information about
  ltcmdgt
• help lists useful commands

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Who am I?

• Commands that tell you who you are
• whoami displays your username
• id displays your username and groups
• Commands that tell you who others are
• finger ltnamegt displays info for ltnamegt
• id ltusernamegt displays info for ltusernamegt
• Commands that change who you are
• su ltusernamegt switch user to ltusernamegt
• login login as a different user

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Files and Directories

• In Unix, files are grouped together in other
  files called directories, which are analogous to
  folders in Windows
• Directory paths are separated by a forward slash
  /
• Example /home/bt/FIT/docs
• The hierarchical structure of directories (the
  directory tree) begins at a special directory
  called the root, or /
• Absolute paths start at /
• Example /home/bt/FIT/docs
• Relative paths start in the current directory
• Example FIT/docs (if youre currently in
  /home/bt)
• Your home directory is where your personal files
  are located, and where you start when you log in.
• Example /home/bt

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The File System
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Directories (contd)

• Handy directories to know
• Your home directory
• .. The parent directory
• . The current directory
• Other important directories
• /bin
• /tmp

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Simple commands

• ls
• LiSts the contents of specified files or
  directories (or the current directory if no files
  are specified)
• Syntax ls ltfilegt
• Example ls backups
• pwd
• Print Working Directory

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More commands

• cd
• Change Directory (or your home directory if
  unspecified)
• Syntax cd ltdirectorygt
• Examples
• cd backups/unix-tutorial
• cd ../class-notes
• mkdir
• MaKe DIRectory
• Syntax mkdir ltdirectoriesgt
• Example mkdir backups class-notes

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More commands

• rm
• ReMove
• Syntax rm ltoptionsgt ltfilesgt
• Example rm class-notes.txt
• Example rm ir backups
• rmdir
• ReMove DIRectory, which must be empty
• Syntax rmdir ltdirectoriesgt
• Example rmdir backups class-notes

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Files (contd)

• cp
• CoPies a file, preserving the original
• Syntax cp ltsourcesgt ltdestinationgt
• Example cp tutorial.txt tutorial.txt.bak
• mv
• MoVes or renames a file, destroying the original
• Syntax mv ltsourcesgt ltdestinationgt
• Examples
• mv tutorial.txt tutorial.txt.bak
• mv tutorial.txt tutorial-slides.ppt backups/

Note Both of these commands will over-write
existing files without warning you!
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File Permissions

• Every file has three access levels
• user (the user owner of the file)
• group (the group owner of the file)
• other (everyone else)
• At each level, there are three access types
• read (looking at the contents)
• write (altering the contents)
• execute (executing the contents)

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What You Can Do With Permissions
Permission File Directory
r (read) Read a file List files in
w (write) Write a file Create a file in Rename a file in Delete a file in
x (execute) Execute a file (eg shell script) Read a file in Write to a file in Execute a file/shell script in
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Changing Permissions

• The change mode command
• chmod ltlevelgtltopgtltpermissionsgt, ltfilenamegt
• ltlevelgt string of u, g, o, a (user, group,
  other, all)
• ltopgt one of , -, (gets, loses, equals)
• ltpermissionsgt string of r, w, x, s, t, u, g,
  o (read, write, execute, set-id, text, same
  as user, same as group, same as other),
• Examples
• chmod urwx,go-w foobar
• chmod gu,t temp/
• chmod urwx,grwxs,o shared/

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Process Management

• What can you do with it?
• Start programs in the background
• Run more than one program per terminal
• Kill bad and/or crashing programs
• Suspend programs mid-execution
• List all jobs running in a shell
• Move foreground jobs to the background
• More

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Three States of a Process

• Foreground
• Attached to keyboard
• Outputs to the screen
• Shell waits until the process ends
• Background, running
• Not attached to keyboard
• Might output to the screen
• Shell immediately gives you another prompt
• Background, suspended
• Paused mid-execution
• Can be resumed in background or foreground

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Background Processes

• Listing jobs
• jobs lists background jobs and job s
• ps lists processes and their process id (pid)
• ltjobgt expands to the process id of the job
• Stopping foreground jobs
• Press Z (Ctrl-Z) in the terminal window
• Starting a process in the background
• Append a character to the command line
• Examples ls lR gt ls-lR.out
• Resuming a stopped job
• In the foreground fg ltpidgt
• In the background bg ltpidgt

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Killing Processes

• The kill command
• kill -ltsignalgt ltpidgt
• Send ltsignalgt to process ltpidgt
• The killall command
• killall -ltsignalgt ltcommandgt
• Send ltsignalgt to all processes that start with
  ltcommandgt
• Useful signals (kill l for the complete list)
• TERM the default, terminate, kills things
  nicely
• KILL will kill anything, but not nicely
• HUP hangup, used to reload configurations
• STOP stops (suspends) a running process

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Redirecting input and output

• Simple!ltprogramgt lt ltFILEgt
• ltprogramgt gt ltFILEgt
• Examplesort lt my_grades.txtls gt dirlist

Note a file called dirlist will be created if it
doesnt exist Dirlist will be overwritten. gtgt
appends
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Piping

• Piping is connecting programs together by using
  the output of one program as the input to the
  next.
• Syntaxltprogram1gt ltprogram2gt ltprogramNgt
• A simple example (view a sorted file-listing a
  page at a time)ls sort less
• By combining Unix utilities in a pipeline, you
  can build tools on-the-fly as you need them.

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Shell Shortcuts

• Tab completion
• Type part of a file/directory name, hit lttabgt,
  and the shell will finish as much of the name as
  it can
• Works if youre running tcsh or bash
• Command history
• Dont re-type previous commands use the
  up-arrow to access them
• Wildcards
• Special character(s) which can be expanded to
  match other file/directory names
• Zero or more characters
• ? Zero or one character
• Examples
• ls .txt
• rm may-?-notes.txt

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Editing Text

• Which text editor is the best is a holy war.
  Pick one and get comfortable with it.
• Three text editors you should be aware of
• vi A lighter editor, used in programming
• emacs A heavily-featured editor commonly used
  in programming
• pico Comes with pine (Dantes email program)

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Printing

• Printing
• Use lpr to print
• Check the print queue with lpq
• lprm to remove print jobs
• For the above commands, youll need to specify
  the printer with Pltprinter namegt

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Exiting

• Logout leave the system
• Exit leave the shell
• C interrupt
• D can log user off often disabled

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Remember

• In Unix, youre expected to know what youre
  doing.
• Many commands will print a message only if
  something went wrong.
• Most often there is no undo button
• Make a backup copy if you are unsure
• Some commands have interactive options
• E.g. rm i
• Unix can be hard to learn, but it is loads of fun
  to use when you know what you are doing!