Introduction to Linux
Martin Read
C#
• C# development driven more by Microsofts’ business need to reinvigorate Windows desktop application development against perceived competitive threats like Java
– designed to maximize application developer productivity
• Microsoftorigins,so
– usually targeted towards Windows – benefits from the .NET framework
• High-level programming language
– originally very much like Java, now very different
C#
• High-level programming language
– object-oriented, event-driven with large systems libraries
• Provides a managed memory model that adds a higher level of abstraction & protects programmers
– does a lot of what could be done in C for you e.g. memory management
– simpler syntax is less demanding (& error prone) & has a shallower learning curve for new programmers
– adds convenience and improves development times
– complicates access to lower level APIs
• Similarities between C & C# are few
– syntax is similar, but languages very different behind the scenes
C#
• Provides a number of features to make it easier to code
– dynamic typing, implicit variable declarations, & local functions
– error handling (type checking, bounds checking, uninitialized variable checking)
– exception handling & garbage collection
• Generally interpreted – most commonly compiled into byte-code (not machine code) & executes on a virtual machine that converts into machine code on the fly
C#
• Higher level languages are not efficient
– C# code is much slower & useless for high performance scientific computing
• C# remains a common choice for internal/enterprise applications & games development, but is less common for commercial software
• Rapid client application development & high performance Server development
What about C?
• Low-level programming language
– easier to understand and implement than object oriented programming
– usually compiles to assembly language
– performs almost as efficiently as assembly code
– provides base-level access to memory, & requires very little runtime support
– lets you do almost anything provided correct syntax
– can cause some real damage to the OS
What about C?
• Complete binary data transparency
• Consecutive data is placed consecutive in memory
– an int, followed by a double, followed by a char, is exactly 15 bytes long
• Data allocated in a function gets allocated on the stack exactly as you declare it (usually in the same order)
– memory layout of data is completely under your control
Lecture aims
• To introduce the Unix/Linux Operating System.
Lecture Objectives
• To examine the file & directory structures used.
• To look at a few basic Unix commands. • Practical in Linux command line
The UNIX Operating System
• Common Operating System – can be found on Sun, Vax computers, as well as being ported into Intel based computers.
• UNIX has been around since 1969. At one stage it was available free….
• The entire source code was once available…
• Most commonly found as a scientific/engineering Operating System, but…
Commonly found on ‘servers’ to, for example, the web
– because it handles concurrency (pseudo concurrency) easily
Bell labs (AT&T) 1969 Multics –
slow, limited interaction.
Originally written in assembler and ‘B’. Rewritten in C
1975 – University only
1979 – first sold commercially
1983 – improved commercial appeal 1989 – 1.2 million Unix installations
Linux 2.2 kernel released.
1972
Version 6 Version 7
System V: Release 1 Release 4
1999
–
History
There are 2 ‘main’ versions of UNIX, referred to as System V – Bell labs or AT&T UNIX
BSD – Berkley UNIX (mostly universities)
Why UNIX?
• It is a ‘Real’ Operating System – supporting virtual memory management, multitasking, multiuser, etc…
• Probably only OS available from several sources
• Almost completely written in a high level language. Thus it is portable to new processors (if they support a compiler and OS to run it on). Used on micros to supercomputers .
• Resource requirements are relatively small & don’t require very specialised hardware
BUT
• UNIX has been around for some time & much of it ‘evolved’.
• Is UNIX really a business OS? It was never intended to be.
• Although UNIX is available from many sources, each source is slightly different
• It is not a real-time OS
• Machines like ICL29000 hardware & the VME Operating System were designed around each other
– > better optimised system?
The ‘Onion’ Model
H/ware
Kernel
Shell (interface)
Tools & Application
Tools
• Tools and applications
There are hundreds of tools available to users. Email, word processing, business applications, or programming.
• Account and invoicing of system users
• Computer aided instruction
• On line manual (very comprehensive)
• Programme debugger
• Pattern scanning and processing language
• Languages – C, Basic, FORTRAN
• System status info
• Much file management, inc encryption & database
– and more – a vast software library
Interfaces
Most versions work with a windowing system (GUI)
• Every Unix/Linux system has a command-line interface
• Command-line more flexible for advanced use
– programmes used together in combinations impossible under windows
• Shell scripts automate jobs which would have to be done manually and repetitively using windows
• Text-based interfaces easier than GUIs for sight- & hearing-impaired users
Linux
• Linux is a free Unix-type OS
• It is a clone of Unix
• Linux supports most of the popular Unix software
• Linux is mostly System V, mostly BSD compatible & mostly POSIX compliant
Linux
It has all the features you would expect in a modern fully-fledged Unix:
• Multitasking
• Virtual memory
• Shared libraries
• Proper memory management
• TCP/IPnetworking
Linux
• Linux is easily portable to most general- purpose 32- or 64-bit architectures as long as they have:
• a paged memory management unit (PMMU)
• a GNU C-compiler
• GNU – recursive acronym for “GNU’s Not Unix”
Linux
• Much of Linux is really an aggregation of hundreds of independently developed utilities that make up the typical Unix system
• Linux offers a windows-based shell interface, commonly known as X-Windows or simply as X
Linux
• Unlike Windows 10, there’s no single version of Linux
• Linux distributions take the Linux kernel
• Combined with other software like the GNU core utilities, X.org graphical server, a desktop environment, web browser, etc
• Each distribution unites some combination of these elements into a single OS you can install
Which Linux am I running?
• The uname command lets you check some basic information about the copy of Linux you’re running:
> uname -a
• will display all the information, including the Linux release and CPU information
What is Debian?
• The Debian Project is an association to create a free OS using the Linux kernel
• Most of the basic OS tools come from the GNU project – usually called GNU/Linux
• GNU/Linux – a complete Unix-like operating system
• Debian comes with more than 50,000 packages
Ubuntu
• Ubuntu is a cross-platform, open-source OS based on Debian
• Ubuntu builds on the Debian architecture & infrastructure, but there are important differences
• Ubuntu is a good place to start for former Windows users
What do you need to know?
• Print your working directory
• Change directories & list all contents
• Create a file & a directory
• Redirect the output of a file
• View the full/partial contents of file
• Search for something
• Delete files/directories
• Run a programme in the background – Then end that process
root super user
• In a multi-user environment, you must be able to
protect files from hackers, etc
• There is a single super user-id called root which has the power to do anything
• Normally the root user prompt is different e.g. #
Linux naming convention
• A name may be up to 14 characters long
– recent versions have increased this to 28 characters
• Anamecannotcontainspaces
• Any other characters may be used
• However, because a number of characters have special uses, it is suggested that only the following are used:
– Lowercase & uppercase letters (a-z) – Numbers (0-9)
– Underscore ( _ ), period ( . ) and comma ( , )
• Linux is case sensitive – Readme, readme, ReadMe, readMe are all different files
Naming conventions
• .c
• .h
• .o
• .ps-Postcriptsourcecode
• .sh-Shellprogramme
– ‘C’ programme source file
– header file
– compiled and assembled code (object file)
• .z – compressed file using pack command
• .Z – compressed file using compress compile
Permissions
Files and directories
• Any file or directory has three sets of access permissions – r, w, x
• Any file has 3 classes of people who can access it:
– Owner – Group – Other
u (for user)
g (owners group) o sometimes rest,
world, etc
• The root user overrides all settings and can access any file
• Groups can be set up by the system administer – to find out which ones you belong to, type
> groups
• You can also determine which groups another
user is a member of
E.g. groups martin (or other username)
Access permissions
Ordinary file
r (read)
can look at contents of
a file w (write)
can change or delete contents
x (execute)
can execute the file
Directory
Allows listing of files
Create/move/delete files in the directory
Can search the directory
Access permissions
-rw-r—– 1 root system 3953 Jan 12 2018 martin4 (1) (2) (3) (4) (5) (6) (7)
(1) File/directory permission bits
(2) Link count
(3) User name of person who owns the entry (4) Group that owns the file/directory
(5) Size (in bytes)
(6) Date file last modified
(7) Name of file or directory
Access permissions
• You can change access permissions of files and directories you own using chmod
+ = add permissions
– = remove permissions
chmod go +rw martin4
can also use octal or absolute mode rwx
4+ 2 + 1
chmod nnn filename e.g. chmod 740 martin4
. ..
Directories
Directory structures are separated by / (forward slash)
refers to your working directory refers to the parent directory
(DOS copied
from Unix)
From other parts of the system
your home directory can be referred to as ‘~’ cp bsctemp ~/bsctemp
cd } mkdir
rmdir
Directories
• Working directory or current directory
• To find out where you are in the system
pwd (Print Working Directory)
cd should take you to your home directory
(but takes you to sec204)
• Everything in your home directory you own
File structure
• Hierarchical structure
• Parent directory/root directory written as “/”
• Subdirectories include bin, etc, tmp & usr – usually containing specific kinds of system files
• /bin programmes
• /etc configuration files
• /home or /user home directories of users
• /usr
• /var
• /dev
application programmes
variable files (admin, mail, spool)
devices e.g. terminal, printer, drives
• .login • .logout • .cshrc
• .plan
• .profile
Useful files
(executes when you log in) (executes when you log out) (executes when you invoke a ‘C’ shell )
(used by ‘finger’ to display a little message to yourself)
Full & Relative Pathname
• Full Pathname
List each directory, starting from /, down to the file itself. Each directory and the filename must be separated by a “/”
OR
• Relative Pathname
If the file is in a directory near the Working Directory, a relative path may be used
• If the Working Directory is /home/martin, the file myfile.c in that directory may be referred to by:
Full Pathname: /home/martin/myfile.c Partial Pathname: myfile.c
Full & Relative Pathname
• If the Working Directory is /home/paul
• myfile.c under the directory /home/martin may be referred to by:
Full Pathname: /home/martin/myfile.c Relative Pathname: ../martin/myfile.c
Basic Command Syntax
• Most commands follow a consistent syntax, of the form:
command [options] [arguments]
• [options] switches that modify the function. Options usually begin with a “-“, such as “-f”.
• [arguments] usually (but not always) the name of a file or directory to perform the operations on.
• ls
• ls –alg
• ls –d
• ls –t
• ls –r
list
• ls –l a*
• ls–ltmartin?
List command
(all files, long format + group id) (directory)
(time order)
(reverse order)
Metacharacters
? one character compare
* multiple character compare [] list of comparative values [-]range for comparison
[!] negative comparison
!! Execute the previous command
!nn Execute command nn from history
!-n Execute the command that was executed n commands ago
The on-line manual
• To get help on any part of the system – use man or info
man 1 ls section number and title
• When the manual entry displays more than one screen, press the space bar for more, or CTL/C to stop, or q to quit
Sections of the manual
1. Commonlyusedcommands&tools
2. System calls (to the Unix kernel). Assumed written in C
3. Higher level subroutines – again in C
4. Special device files. Typically characteristics of disc drives, etc
5. File formats – of system files
6. Games
7. Misc file system hierarchy. Text formatting, macro pages, character sets, etc
8. System admin guide
on-line manual
• Some commands are in several sections of the manual
sleep nn causes your process to sleep for nn seconds
• Can be used as a Console command or
• A call to the operating system from within a programming language
– Found in sections 1 and 3
• ‘Sleep’ uses the OS functions ‘gettimer’ & ‘sigpause’, which have entries in section 2 of the manual
Finding files
• Finding files using the find programme find directory -options [-print -exec -ok]
find . –type f –name “myfile*”
• Look in working directory (& all subdirectories) for files with names starting ‘myfile’
find . -type f –mtime -4
• Look for all files created or modified in the last 4 days
find / -name “[a-z]*”
Other commands
• Linux has the GNU locate programme
locate alpha
• date
• who
• whoami • passwd
who is on the system who are you logged in as change your password
• cat filename (list a file – no pauses) • more (pg) filename
(list a file – with pauses – use for large files)
• tail filename (list the last few lines)
Processes
• To display all the processes use ps ps –ef
• Displays the user name, PPID (parent process id), start time & full command line
• To stop a process use kill – need to know the process id
kill 1481 kill process 1481
• Some processes will not terminate with a normal kill & require a kill absolute to terminate them
kill -9 1481