UNIX_Module 1.pdf

1. Module 1

2. Module 1 Introduction Unix Components/Architecture. Features of Unix. The UNIX Environment and UNIX Structure, Posix and Single Unix specification. General features of Unix commands/ command structure. Command arguments and options. Understanding of some basic commands such as echo, printf, ls, who, date, passwd, cal, Combining commands. Meaning of Internal and external commands. The type command: knowing the type of a command and locating it. The man command knowing more about Unix commands and using Unix online manual pages. The man with keyword option and whatis. The more command and using it with other commands. Knowing the user terminal, displaying its characteristics and setting characteristics. Managing the non uniform behaviour of terminals and keyboards. The root login. Becoming the super user: su command. The /etc/passwd and /etc/shadow files. Commands to add, modify and delete users.

3. Introduction • Why Unix? – Portable – Multiuser – Multitasking – Networking – Organized file system – Device Independence – Utilities – Services

4. Introduction • Computer system • Hardware • Input Device • Processing Unit • Storage devices • Software • System software • Application software • Operating System (OS) • Hardware / Software ? • Why OS • OS load

5. Module 1 IntroductionUnix Components/Architecture. Features of Unix. The UNIX Environment and UNIX Structure, Posix and Single Unix specification. General features of Unix commands/ command structure. Command arguments and options. Understanding of some basic commands such as echo, printf, ls, who, date, passwd, cal, Combining commands. Meaning of Internal and external commands. The type command: knowing the type of a command and locating it. The man command knowing more about Unix commands and using Unix online manual pages. The man with keyword option and whatis. The more command and using it with other commands. Knowing the user terminal, displaying its characteristics and setting characteristics. Managing the non uniform behaviour of terminals and keyboards. The root login. Becoming the super user: su command. The /etc/passwd and /etc/shadow files. Commands to add, modify and delete users.

6. The UNIX Architecture • BSD (originally: Berkeley Software Distribution) refers to the particular version of the UNIX operating System that was developed at and distributed from the University of California at Berkeley • Berkeley Software Distribution (BSD) is a Unix operating system derivative developed and distributed by the Computer Systems Research Group (CSRG) of the University of California, Berkeley, from 1977 to 1995.

7. The UNIX Architecture 1. Division of labor • Kernel : communicates directly with hardware • Shell : command interpreter, translates commands in to action –Sh : Bourne Shell –Csh: C shell –Ksh : Korn shell 2.The file and Process • File • Process

8. The UNIX Architecture 3. System calls • Kernel, shell, apllications --- C • All UNIX flavors use the same system calls --- • POSIX (Portable Operating System Interface) • Examples : Open, write

9. The UNIX Architecture

10. The Architecture of Unix Operating System Unix has a graphical user interface similar to the Windows operating system that makes it easy for navigation and a good supportive environment. The internal design view of this operating system can be known from its architecture. The he architecture of this operating system is four layered. It consists of Hardware, Kernel, System Call interface(shell) and application libraries/tools, utilities, etc…The kernel controls the hardware of the computer and resides at the core of the architecture. System calls acts as the interface between the kernel and other libraries. These libraries include general functions and built on top of the system calls. Shell is a special application that provides an interface to the other applications of the architecture.

11. Kernel For this operating system, Kernel is the central core that interacts directly with the hardware of the system. The main functions of Kernal are- ● Computer hardware such as memory, disc, printers, etc.. are controlled by the kernel. ● The kernel schedules the processes, control and executes various user- defined tasks. ● Manages the data storage and control the computer accesses by several users. ● The kernel is composed of several sub-components such as configurations including boot code, device drivers to control hardware, header files.

12. Shell It is the interface between the user and the kernel. Users can interact with the shell using shell commands. Shell has two main responsibilities which include interpreting the commands given by the users and execute them using the kernel, providing programming ability to the users to write shell commands for a shell script to perform specific tasks.

13. Commands Some of the major categories of commands used by the Unix operating system are – ‘sh’ – shell commands providing a primary user interface, ‘utilities’ forming the core toolkit of Unix commands includes sub-categories such as system utilities supporting administrative tools and User utilities for environment management tools.

14. Properties From the time of its development, Unix has been adopted by many companies. It still plays a big role in many data centers and research labs. Some of the properties of Unix that make it so preferable over other systems are- ● Multiuser accessing – Multiple users can work on the system simultaneously by connecting through a point known as terminal. ● Multi-tasking-It provides the facility to run multiple programs or processes by multiple users on one system.

15. ● Portability – It provides the flexibility to be used on multiple hardware architectures. As it uses a high-level language, it is easy to comprehend and modify the Unix code according to the hardware architecture of the computer. Thus to work on new hardware architecture, the user has to simply modify the Unix code and run on the system. ● Processes– Files are the collection of data created by the user. It includes documents, programming instructions, etc…Processes are the executions of the programs or files. Unix follows the hierarchical file structure that starts with a root directory followed by the subdirectories ending with the file name. ● Communication – Requests and commands given by the user are carried out by the kernel and shell together. The user communicates with the system using Shell. It also provides inter-system communication through UUCP. This operating system also obeys the TCP/IP protocol.

16. ● It also provides easy maintenance of the files ● This operating system provides Pipes and Filters to the user to design complex programs from simple programs. ● It provides a wide variety of tools for software development and software maintenance. ● It is very popularly used on Desktop computers, laptops, mainframes, and other computing hardware.

17. Types of Unix Operating Systems Unix gives the user the flexibility to modify the Unix code according to their requirements. Thus it is easy to port the Unix code on different types of hardware. The freely available source code of this operating system and its portability feature lead to various Unix like operating systems. Some of the popular Unix like operating systems are Solaris, Darwin, AIX, HP-UX, FreeBSD, NetBSD, Xenix, IRIX, Tru64, macOS, etc…

18. Module 1 Introduction, Brief history. Unix Components/Architecture. Features of Unix. The UNIX Environment and UNIX Structure, Posix and Single Unix specification. General features of Unix commands/ command structure. Command arguments and options. Understanding of some basic commands such as echo, printf, ls, who, date, passwd, cal, Combining commands. Meaning of Internal and external commands. The type command: knowing the type of a command and locating it. The man command knowing more about Unix commands and using Unix online manual pages. The man with keyword option and whatis. The more command and using it with other commands. Knowing the user terminal, displaying its characteristics and setting characteristics. Managing the non uniform behaviour of terminals and keyboards. The root login. Becoming the super user: su command. The /etc/passwd and /etc/shadow files. Commands to add, modify and delete users.

19. Features of UNIX 1. UNIX: A Multiuser system 2. UNIX: A Multitasking system 3. The Building Block Approach 4. The UNIX toolkit 5. Pattern Matching 6. Programming Facility 7. Documentation

20. The UNIX toolkit The Unix operating system introduced the idea of a toolkit. That is, rather than having a few generalized and complex utilities programs, the system contains a toolkit of small and simple programs which the user combines to perform sophisticated tasks and to create custom programs. Because Unix treats files, commands, and devices all pretty much as files, and since most of these tools will take their input either from files or from other commands (or standard input if no file name is given), this juxtaposition of tools is general and powerful.

21. Grep Command in Unix with Simple Examples Grep command in Unix/Linux is the short form of ‘global search for the regular expression’. • The grep command is a filter that is used to search for lines matching a specified pattern and print the matching lines to standard output. example grep [options] [pattern] [file] The pattern is specified as a regular expression. A regular expression is a string of characters that is used to specify a pattern matching rule. Special characters are used to define the matching rules and positions.

22. $ grep “^hello” file1 $ grep “done$” file1 $ grep “[a-e]” file1 $ grep “[^aeiou]” file1 $ grep “ *[0-9]” file1 $ grep -i “hello”

23. UNIX o/s provides shell. Shell works like a programming language. It provides commands and keywords.

24. Module 1 Introduction Unix Components/Architecture. Features of Unix. The UNIX Environment and UNIX Structure, Posix and Single Unix specification General features of Unix commands/ command structure. Command arguments and options. Understanding of some basic commands such as echo, printf, ls, who, date, passwd, cal, Combining commands. Meaning of Internal and external commands. The type command: knowing the type of a command and locating it. The man command knowing more about Unix commands and using Unix online manual pages. The man with keyword option and whatis. The more command and using it with other commands. Knowing the user terminal, displaying its characteristics and setting characteristics. Managing the non uniform behaviour of terminals and keyboards. The root login. Becoming the super user: su command. The /etc/passwd and /etc/shadow files. Commands to add, modify and delete users.

25. The UNIX Environment • Stand alone personal environment • Time sharing systems • Client server system

26. Module 1 Introduction. Unix Components/Architecture. Features of Unix. The UNIX Environment and UNIX Structure, Posix and Single Unix specification. General features of Unix commands/ command structure. Command arguments and options. Understanding of some basic commands such as echo, printf, ls, who, date, passwd, cal, Combining commands. Meaning of Internal and external commands. The type command: knowing the type of a command and locating it. The man command knowing more about Unix commands and using Unix online manual pages. The man with keyword option and whatis. The more command and using it with other commands. Knowing the user terminal, displaying its characteristics and setting characteristics. Managing the non uniform behaviour of terminals and keyboards. The root login. Becoming the super user: su command. The /etc/passwd and /etc/shadow files. Commands to add, modify and delete users.

27. UNIX structure • The kernel: Process control, Resource management • The shell: interpreter, programming capability • Utilities: Text editors, search programs, sort , • Application

29. POSIX and the single UNIX specification Portable Operating System Interface for Computer Environment. IEEE • POSIX.1: C application programming interface, system calls. • POSIX.1a: • POSIX.1b • POSIX.2: Shell and utilities • X/Open & IEEE: Single Unix Specification Version 3 (SUSV3).

31. Command structure • Since the introduction of UNIX System V, Release 3, any new commands must obey a particular syntax governed by the following rules: • Command names must be between 2 and 9 characters in length. • Command names must be comprised of lowercase characters and digits. • Option names must be one character in length.

32. • command [options] [arguments] • where an argument indicates on what the command is to perform its action, usually a file or series of files. ... • Commands are case sensitive. ... • Options are generally preceded by a hyphen (-), and for most commands, more than one option can be strung together, in the form: ls -alR will perform a long list on all files in the current directory and recursively perform the list through all sub-directories. • For most commands you can separate the options, preceding each with a hyphen, e.g.: • command -option1 -option2 -option3 as in: ls -a -l -R

33. • Whitespace • Cat README • Options ls -l ls -l filename ls –l –a –t ls –lat

35. Command structure 2. Filename Arguments ls –lat chap01 chap02 chap03 cp chap01 chap02 rm chap01 chap02 3. Exceptions who ls pwd etc….

36. Module 1 Introduction Unix Components/Architecture. Features of Unix. The UNIX Environment and UNIX Structure, Posix and Single Unix specification.eneral features of Unix commands/ command structure. Command arguments and options. Understanding of some basic commands such as echo, printf, ls, who, date, passwd, cal, Combining commands. Meaning of Internal and external commands. The type command: knowing the type of a command and locating it. The man command knowing more about Unix commands and using Unix online manual pages. The man with keyword option and whatis. The more command and using it with other commands. Knowing the user terminal, displaying its characteristics and setting characteristics. Managing the non uniform behaviour of terminals and keyboards. The root login. Becoming the super user: su command. The /etc/passwd and /etc/shadow files. Commands to add, modify and delete users.

37. Module 1 Introduction, Brief history. Unix Components/Architecture. Features of Unix. The UNIX Environment and UNIX Structure, Posix and Single Unix specification. The login prompt. General features of Unix commands/ command structure. Command arguments and options. Understanding of some basic commands such as echo, printf, ls, who, date, passwd, cal, Combining commands. Meaning of Internal and external commands. The type command: knowing the type of a command and locating it. The man command knowing more about Unix commands and using Unix online manual pages. The man with keyword option and whatis. The more command and using it with other commands. Knowing the user terminal, displaying its characteristics and setting characteristics. Managing the non uniform behaviour of terminals and keyboards. The root login. Becoming the super user: su command. The /etc/passwd and /etc/shadow files. Commands to add, modify and delete users.

38. echo • $ echo UNIX • $ echo $SHELL • $ echo $x • $ x=3 • $ echo $x

40. printf • Similar to echo • Differ in newline

42. ls • ls • LS • ls chap* • ls –l • ls –l chap*

43. ls options ls -x output in multiple columns ls –F Identifying directories and executable ls -a list all files including hidden file starting with . ls –x dir _name Listing directory contents ls –xR Recursive listing

44. ls • ls –d current directory • ls –l list with long format - show permissions • ls –la list long format including hidden file • ls –s list file size • ls –S sort by file size • ls –t sort by time & date

46. who • $Who • $Who –Hu

48. date • $date • $date +%m • $date +%h • $date +”%H %M” • $date +%d • $date +%y • $date +%S • $date +%D • $date +%T

50. passwd • $passwd To change the password.

52. cal • $cal • $cal 03 2018 • $cal 2018

53. Commands • Examples: 1.ps (process status):In UNIX, everything you do, every command you type, is considered a "process". This command will list all the jobs you have running in your account, which includes the login shell. 2.Cat filename 3.Wc filename

55. Flexibility of command usage 1. Combining commands wc chap01 ; ls –l chap01 ( wc chap01; ls –l chap01)>newlist

56. Flexibility of command usage 2. A command line can overflow or be split into multiple lines echo “5th sem 2 section is a very good class. I love this class”

57. Flexibility of command usage 3. Entering a command before previous command has finished 4. man: Browsing the manual pages on line man argument 5. Navigation and search f or spacebar b

59. Meaning of Internal and external commands Internal commands are commands that are already loaded in the system. They can be executed any time and are independent.Example: cd, source,XCOPY, CHKDSK, LABEL, TREE etc External commands are loaded when the user requests for them.Example:ls, cat etc

61. Locating commands The type command is used to describe how its argument would be translated if used as commands. It is also used to find out whether it is built-in or external binary file. • type cd • type -a This option is used to find out whether it is an alias, keyword or a function and it also displays the path of an executable, if available. -t : This option will display a single word as an output. • alias – if command is a shell alias • keyword – if command is a shell reserved word • builtin – if command is a shell builtin • function – if command is a shell function • file – if command is a disk file

63. man man argument

65. whatis explanation of command in a line

66. Understanding the man documentation 1. Understanding a man page Name, Synopsis and description [] … pipeline

67. Understanding the man documentation 2. Using man to understand man man man more chap01

69. more more example

71. Knowing your terminal • The who command will give us the information we are looking for. • The one-letter command w requires less typing and provides more information. USER: The user name. TTY: The type of terminal they are logged in at. This will usually be a pts (a pseudo-teletype). :0 means the physical keyboard and screen connected to this computer. FROM: The name of the remote host if this is a remote connection. LOGIN@: The time at which the user logged in. IDLE: Idle time. This shows ?xdm? in the screenshot because we’re running under an X- windows Display Manager, which does not provide that information.

72. JCPU: Joint CPU time, this is the CPU time used by all processes that have been attached to this tty. In other words, the total CPU time of this user in this logged in session. PCPU: Process CPU time, this is the CPU time used by the current process. The current process is named in the WHAT column. WHAT: The command line of this user’s current process. • The whoami command provides a very pithy answer • teletype The tty command of terminal basically prints the file name of the terminal connected to standard input. tty is short of teletype, but popularly known as a terminal it allows you to interact with the system by passing on the data (you input) to the system, and displaying the output produced by the system

74. Displaying its characteristics and setting characteristics $stty –a • On Unix operating systems, the stty command changes and prints terminal line settings. stty -echo stty echo stty -a

75. Displaying its characteristics and setting characteristics • Backspace should erase characters $stty –echoe • Enter password through a shell script $stty –echo • Changing the interrupt key $stty intr ^c • Changing the end of file $stty eof ^a • When everything else fails $stty sane

77. Managing the non uniform behavior of terminals and keyboards Keystroke Function [Ctrl-h] Erase text [Ctrl-c] or [delete] Interrupts a command [Ctrl-d] Terminates login session or a program that expects its input from the key board [Ctrl-s] Stops scrolling of screen output and locks keyboard [Ctrl-q] Resumes scrolling of screen output and unlocks keyboard [Ctrl-u] Kills command line without executing it

78. Managing the non uniform behavior of terminals and keyboards Keystroke Function [Ctrl-] Kills running command but creates a core file containing the memory image of the program [Ctrl-z] Suspends process and returns shell prompt [Ctrl-j] Alternative to [enter] [Ctrl-m] As above Stty sane Restores terminal to normal status

80. The root login root login: root Password: ****

81. • A critical means for preventing users from directly damaging Unix-like systems or increasing the vulnerability of such systems to damage by others is the avoidance of using the root account except when absolutely necessary • rather than routinely logging into the system as root, administrators should log in with their admin account and then use the su command. This will provide you with root privileges only as needed and without requiring a new login.

82. Example: To list the contents of a directory owned by root you would type: su root ls /root You will be prompted for your root password. The account of the admin user can be returned to by typing exit.

84. Becoming a super user su password: **** sudo su On UNIX this special user is known as superuser or root (not to be confused with the root directory). Superuser can override file security and do almost anything she wants on the system (she cannot see your password, since it is encrypted, but she can change it). In fact, any user with a userid of 0 is a superuser. Naturally, such users should always have a password.

85. Becoming a super user • creating user environment su –henry [ctrl-d]/ exit • The administrator's privileges – Change the content or attribute of any file like its permission and ownership. He can delete any file even if the directory is write protected. – Initiate or kill any process

86. • The administrator's privileges – Change any user’s password without knowing the existing one – Set the system clock with date – Address all users concurrently with wall – Limit the maximum size of files that users are permitted to create with ulimit – Control users access to the scheduling services like at and cron – Control users access to many networking services like FTP, SSH etc. Becoming a super user

87. • date: setting the system date date MMDDhhm • wall: Communicating with users wall • ulimit: Setting limits on file size ulimit 20971510 Becoming a super user

88. • Changing any password passwd Becoming a super user

90. /etc/passwd is the key file in any Linux Unix system. • The password file is a human-readable file that contains information about users on the system including their encrypted passwords • Some systems don’t have encrypted passwords in this file if /etc/shadow file is generated. • cat /etc/passwd

91. • Username:Its user name being used by the user to login • Password:Its password in an encrypted format.you see x instead of encrypted password since /etc/shadow file is generated on the system. The encrypted password is found in /etc/shadow file in such case. • UID:Its user id. Its unique number assigned to every account on the system. • GID:Its group id. Its unique number of groups of which account is member of. • Comment:This field is introduced to have some descriptions against the account. This is purely for humans to identify/understand what related account is or to whom it belongs to • Home directory:Its a directory where normally user lands into when he/she login. • Login shell:This is a shell that will be spawn when the user successfully logs in. root:x:0:0:root:/root:/bin/bash root:$6$h./k6HFM$EmRq6TQl80Lq7BF5k0v9A8MvCZx5P ddhczyUViwtfj.FWkl6xsE0TtYJ91V30FZw6nd5maIQXSkbN RpPG6o8v.:18527:0:99999:7::: /etc/passwd and /etc/shadow

93. • adduser username : add user • Command to add a new user to the system. This command can be as short as just one argument of userid. When running with just userid as an argument then it takes all default values for creating that user as defined in /etc/default/useradd file. Or else a number of options can be specified which defines parameters of this new user while creation. for more command usage refer: • https://kerneltalks.com/linux/linux-user-management-useradd-userdel- usermod/ # cat /etc/default/useradd # useradd defaults file GROUP=100 HOME=/home INACTIVE=-1 EXPIRE= SHELL=/bin/bash SKEL=/etc/skel CREATE_MAIL_SPOOL=yes add, modify and delete users

94. • Command: userdel As the name suggests its a command to delete users. It has only two options – ● -r Remove user’s home_dir & mail spool ● -f Removes user even if he/she logged in. Removes home_dir, mail spool & group of the same name even these are being shared by another user. Dangerous!

95. Command: usermod This command used to modify user parameters which we saw in useradd command. All parameter options with useradd command compatible with this command. Apart from those options, it supports below ones – ● -l <new_login> Change login name to different. You have to manually rename home_dir ● -L Lock account. Basically it puts ! in front of encrypted password in passwd or shadow file. ● -U Unlock account. It removes! ● -m <new_home> Moves home_dir to new_dir. -d is mandatory to use with it.

96. Module 1 Unix files. Naming files. Basic file types/categories. Organization of files. Hidden files. Standard directories. Parent child relationship. The home directory and the HOME variable. Reaching required files- the PATH variable, manipulating the PATH, Relative and absolute pathnames. Directory commands – pwd, cd, mkdir, rmdir commands. The dot (.) and double dots (..) notations to represent present and parent directories and their usage in relative path names. File related commands – cat, mv, rm, cp, wc and od commands. File attributes and permissions and knowing them. The ls command with options. Changing file permissions: the relative and absolute permissions changing methods. Recursively changing file permissions. Directory permissions.

97. Unix files • Unix uses a hierarchical file system structure, much like an upside-down tree, with root (/) at the base of the file system and all other directories spreading from there.

98. Unix files • It has a root directory (/) that contains other files and directories. • Each file or directory is uniquely identified by its name, the directory in which it resides, and a unique identifier, typically called an inode. • File inode numbers can be seen by specifying the - i option to ls command. • It is self-contained. There are no dependencies between one file system and another.

99. Naming files • Alphabetic characters and numerical • The period (.), hyphen (-) and underscore (_). • . Last_time • list. • ^V^B^D-++bcd • -{}[] • @#$%*abcd • A.b.c.d.e • Abcd.txt abcd.txt ABCD.txt aBCD.txt

100. Basic file types/categories 1. Ordinary file 1. Text file 2. Binary file 2. Directory file 3. Device file/Special files 4. Pipes 5. Sockets 6. Symbolic link

101. 1. Ordinary files – An ordinary file is a file on the system that contains data, text, or program instructions. ● Used to store your information, such as some text you have written or an image you have drawn. This is the type of file that you usually work with. ● Always located within/under a directory file. ● Do not contain other files. ● In long-format output of ls -l, this type of file is specified by the “-” symbol. 2. Directories – Directories store both special and ordinary files. For users familiar with Windows or Mac OS, UNIX directories are equivalent to folders. A directory file contains an entry for every file and subdirectory that it houses. If you have 10 files in a directory, there will be 10 entries in the directory. Each entry has two components. (1) The Filename (2) A unique identification number for the file or directory (called the inode number)

102. Special Files – Used to represent a real physical device such as a printer, tape drive or terminal, used for Input/Output (I/O) operations. Device or special files are used for device Input/Output(I/O) on UNIX and Linux systems. They appear in a file system just like an ordinary file or a directory. Pipes – UNIX allows you to link commands together using a pipe. The pipe acts a temporary file which only exists to hold data from one command until it is read by another.A Unix pipe provides a one-way flow of data.The output or result of the first command sequence is used as the input to the second command sequence. To make a pipe, put a vertical bar (|) on the command line between two commands.For example: who | wc -l

103. Sockets – A Unix socket (or Inter-process communication socket) is a special file which allows for advanced inter-process communication. A Unix Socket is used in a client-server application framework. In essence, it is a stream of data, very similar to network stream (and network sockets), but all the transactions are local to the filesystem. In long-format output of ls -l, Unix sockets are marked by “s” symbol. Symbolic Link – Symbolic link is used for referencing some other file of the file system.Symbolic link is also known as Soft link. It contains a text form of the path to the file it references. To an end user, symbolic link will appear to have its own name, but when you try reading or writing data to this file, it will instead reference these operations to the file it points to. If we delete the soft link itself , the data file would still be there.If we delete the source file or move it to a different location, symbolic file will not function properly.

104. Organization of files

105. ● / : The slash / character alone denotes the root of the filesystem tree. ● /bin : Stands for “binaries” and contains certain fundamental utilities, such as ls or cp, which are generally needed by all users. ● /boot : Contains all the files that are required for successful booting process. ● /dev : Stands for “devices”. Contains file representations of peripheral devices and pseudo-devices. ● /etc : Contains system-wide configuration files and system databases. Originally also contained “dangerous maintenance utilities” such as init,but these have typically been moved to /sbin or elsewhere.

106. ● /home : Contains the home directories for the users. ● /lib : Contains system libraries, and some critical files such as kernel modules or device drivers. ● /media : Default mount point for removable devices, such as USB sticks, media players, etc. ● /mnt : Stands for “mount”. Contains filesystem mount points. These are used, for example, if the system uses multiple hard disks or hard disk partitions. It is also often used for remote (network) filesystems, CD-ROM/DVD drives, and so on. ● /proc : procfs virtual filesystem showing information about processes as files. ● /

107. ● root : The home directory for the superuser “root” – that is, the system administrator. This account’s home directory is usually on the initial filesystem, and hence not in /home (which may be a mount point for another filesystem) in case specific maintenance needs to be performed, during which other filesystems are not available. Such a case could occur, for example, if a hard disk drive suffers physical failures and cannot be properly mounted. ● /tmp : A place for temporary files. Many systems clear this directory upon startup; it might have tmpfs mounted atop it, in which case its contents do not survive a reboot, or it might be explicitly cleared by a startup script at boot time. ● /usr : Originally the directory holding user home directories,its use has changed. It now holds executables, libraries, and shared resources that are not system critical, like the X Window System, KDE, Perl, etc. However, on some Unix systems, some user accounts may still have a home directory that is a direct subdirectory of /usr, such as the default as in Minix. (on modern systems, these user accounts are often related to server or system use, and not directly used by a person).

108. ● /usr/bin : This directory stores all binary programs distributed with the operating system not residing in /bin, /sbin or (rarely) /etc. ● /usr/include : Stores the development headers used throughout the system. Header files are mostly used by the #include directive in C/C++ programming language. ● /usr/lib : Stores the required libraries and data files for programs stored within /usr or elsewhere. ● /var : A short for “variable.” A place for files that may change often – especially in size, for example e-mail sent to users on the system, or process-ID lock files. ● /var/log : Contains system log files. ● /var/mail : The place where all the incoming mails are stored. Users (other than root) can access their own mail only. Often, this directory is a symbolic link to /var/spool/mail. ● /var/spool : Spool directory. Contains print jobs, mail spools and other queued tasks. ● /var/tmp : A place for temporary files which should be preserved between system reboots.

109. Hidden files ls -a

110. Standard directories / This is the root directory which should contain only the directories needed at the top level of the file structure /lib Contains shared library files and sometimes other kernel-related files /bin This is where the executable files are located. These files are available to all users /boot Contains files for booting the system /dev These are device drivers /home Contains the home directory for users and other accounts /etc Supervisor directory commands, configuration files, disk configuration files, valid user lists, groups, Ethernet, hosts, where to send critical messages /mnt Used to mount other temporary file systems, such as cdrom and floppy for the CD-ROM drive and floppy diskette drive, respectively

111. Standard directories /proc Contains all processes marked as a file by process number or other information that is dynamic to the system /sbin Contains binary (executable) files, usually for system administration. For example, fdisk and ifconfigutlities /tmp Holds temporary files used between system boots /kernel Contains kernel files /usr Used for miscellaneous purposes, and can be used by many users. Includes administrative commands, shared files, library files, and others /var Typically contains variable-length files such as log and print files and any other type of file that may contain a variable amount of data

112. Organization of files

113. The home directory and the HOME variable $echo $HOME

114. When you log in to the system, the shell undergoes a phase called initialization to set up the environment. This is usually a two-step process that involves the shell reading the following files − ● /etc/profile ● profile The process is as follows − ● The shell checks to see whether the file /etc/profile exists. ● If it exists, the shell reads it. Otherwise, this file is skipped. No error message is displayed. ● The shell checks to see whether the file .profile exists in your home directory. Your home directory is the directory that you start out in after you log in. ● If it exists, the shell reads it; otherwise, the shell skips it. No error message is displayed. ● As soon as both of these files have been read, the shell displays a prompt

115. The PATH variable, manipulating the PATH • echo $PATH • $PATH=“set your own path” • echo $PATH

116. When you type any command on the command prompt, the shell has to locate the command before it can be executed. The PATH variable specifies the locations in which the shell should look for commands. Usually the Path variable is set as follows − $PATH=/bin:/usr/bin Here, each of the individual entries separated by the colon character (:) are directories. If you request the shell to execute a command and it cannot find it in any of the directories given in the PATH variable, a message similar to the following appears $hello hello: not found

117. DISPLAY: Contains the identifier for the display that X11 programs should use by default. HOME:Indicates the home directory of the current user: the default argument for the cd built-in command. IFS:Indicates the Internal Field Separator that is used by the parser for word splitting after expansion. LANG:LANG expands to the default system locale; LC_ALL can be used to override this. For example, if its value is pt_BR, then the language is set to (Brazilian) Portuguese and the locale to Brazil. LD_LIBRARY_PATH:A Unix system with a dynamic linker, contains a colonseparated list of directories that the dynamic linker should search for shared objects when building a process image after exec, before searching in any other directories. PATH:Indicates the search path for commands. It is a colon-separated list of directories in which the shell looks for commands.

118. PWD:Indicates the current working directory as set by the cd command. RANDOM:Generates a random integer between 0 and 32,767 each time it is referenced. SHLVL:Increments by one each time an instance of bash is started. This variable is useful for determining whether the built-in exit command ends the current session. TERM:Refers to the display type. TZ:Refers to Time zone. It can take values like GMT, AST, etc UID:Expands to the numeric user ID of the current user, initialized at the shell startup.

119. Relative and absolute pathnames • Absolute pathnames $cat /home/ankitha/login.txt • Relative pathname cd progs cat login.txt cd progs/scripts cd ../.. cp ../vasu/ex.txt . cd ./pis

120. cd cd pathname 1. cd dir1 2. cd / 3. cd .. 4. cd ../.. 5. cd

121. mkdir mkdir dir_name 1. mkdir dir1 2. mkdir dir1 dir2 dir3 3. mkdir sem3 sem3/usp sem3/ade

122. rmdir rmdir dir_name 1. rmdir dir1 2. rmdir dir1 dir2 dir3 3. rmdir sem3/usp sem3/ade sem3

123. dot (.) and double dots (..) notations • cd ../.. • cp ../ankitha/profile .

124. cat mv rm cp wc od

125. Cat cat filename 1. cat p1.c 2. cat p1.c p2.c 3. cat > filename 4. cat –v p1.c //non printing ASCII character 5. cat –n p1.c // line number

126. mv rename or move 1. mv oldfilename newfilename 2. mv file1 file2 file3 dir

127. rm delete a file 1. rm filename 2. rm –i file1 file2 3. rm –r dir1 4. rm –R dir1 5. rm –r * 6. rm –f * 7. rm –rf *

128. wc wc filename 1. wc file1 2. wc file1 file2 3. wc –l file1 4. wc –w file1 5. wc –c file1

129. cp cp source destination 1. cp file1 file2 2. cp file1 dir1/file2 3. cp dir1/file2 . 4. cp file1 file2 file3 dir1 5. cp *.pdf dir1 6. cp –i file1 file2 7. cp –R dir1 dir2

130. od octal form 1. od file //ASCII octal form 2. od –b file //octal value of each printable character 3. od –c file // print the character 4. od –bc file //displays character and corresponding octal value

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