Introduction to Linux

Instructor: Bennett M. Tanyag

UNIT - 2

UNIT 2: Using the Linux System

Learning ObjectiveTo learn how to log in and outTo understand the command formatTo learn about input and output redirectionTo learn to use some basic commandsTo understand the usage of pipes and filters

Logging in and out

Users identify themselves to the system through the login procedure.

Every Linux system has a system administrator who administers, monitors and manages the system.

The system administrator creates accounts and provide login names for the accounts.

The administrator also provides the initial password for every account

Each user of the system is identified with a unique account. The users are identified by their login name, and protection is

provided through user passwords.

A typical login prompt looks like this:mycomputer login:

mycomputer is the name of the computer Every computer is uniquely identified ba a name given to it. The user enters the login name Linus and immediately the

password prompt is shown as mycomputer login: Linuspassword:

The password entered is not echoed to the screen If the password is incorrectly entered, a message appears

login incorrect

Logging in and out

UNIX systems are case sensitive systems User is logged in to the system, when both the login name and

password match The space where the user is logged into is called as the home

directory When a user logs in, the command prompt appears as follows

/home/linus The above is the home directory of the user. The shell is now in command, waiting to take input from the user The default shell is the bash (Bourne Again Shell) Other shells are csh (C shell), ksh (Korn shell), sh (shell) and tcsh

(enchanced C shell).

Logging in and out

Before leaving the system, it is important that we log out of the system.

This can be accomplished by typing either Logout Exit Ctrl-d

Anyone of the above can be used to log out of the system.

Remember to log out always to prevent other users from messing around with your files.

Logging in and out

Command Format

A command is a program that does a specific job There are hundreds of commands and most of them have options

available, yielding thousands of possibilities. The command man, meaning manual pages, is used to find the

details about each command To run even the man command we need to know the structure of a

Linux command linecommand –opn1, opn2 …,arg1, arg2… argn

Where Command is the command name Opn1 to Opnn are the range of n options possible for that command Arg1 to argn are the range of n arguments that may be passed to the

command.

passwd command

The command to change the password is passwd./home/Linus$ passwdChanging password for Linus(current) UNIX password: TorvaldsNew Unix password: HelsinkiRetype new Unix password: Helsinki/home/Linus$ The password entered by the user cannot resemble the old

password Some of the error messages given by the system are:

BAD PASSWORD: is too similar to the old one. BAD PASSWORD: it is too short BAD PASSWORD: is too simple

BASIC UNIX COMMANDS

To execute a UNIX command, simply type the name of the command at the UNIX prompt (much like in DOS) followed by the Enter or Return key ().

Some basic UNIX commands:1. date

The date command gives the user the current date and time. Only the system administrator (or the superuser) can change the system date and time.

Example :$ date Mon Oct 25 19:24:50 PST 1993

clear

The clear command will simply clear the screen and move the $ prompt to the upper left-hand corner of the screen. This is similar to the cls command in DOS.

BASIC UNIX COMMANDS

who The who command gives a list of users currently logged in the system. The output will give the user name or account name together with the TTY or terminal number (the exact terminal being used by the user).

Example :

$ who root tty0user1 tty17user5 tty6mitch tty8

BASIC UNIX COMMANDS

whoamiThe whoami command gives the account name of the user currently logged in at a particular terminal. For example, if someone happens to see a terminal left on with someone logged in, typing whoami at the prompt will give the account name of that user.

Example :$ whoami mitch

BASIC UNIX COMMANDS

bcThe bc command will enable user to access the on-line calculator of UNIX.

Example:$ bc 13 + 18 312 * 8 16<^D>$ _

BASIC UNIX COMMANDS

cal or cal year or cal month year

The cal command gives the user the calendar for the current month and year. The cal year command will give the user the whole calendar for the year specified. The cal month year command will give the calendar for the specified month and year.

BASIC UNIX COMMANDS

Example :$ cal

JulySun Mon Tue Wed Thu Fri Sat 1 23 4 5 6 7 8 910 11 12 13 14 15 1617 18 19 20 21 22 2324 25 26 27 28 29 30 31

BASIC UNIX COMMANDS

banner

The banner command prints large letters on screen.Example:$ banner HI# # ## # ## # ## # # # # # # ## # ## # ## # #

BASIC UNIX COMMANDS

BASIC UNIX COMMANDS

ls commandThe ls (list) command lists the name of files in a given directory. Typing ls without any option and argument at the prompt gives the user a listing of the files in columns and in alphabetical order.

Example:$ ls arithmetic chapter1.doc eric.letfigures memo.to.henry report.1sample.c test.program.c

Some options and arguments of the ls command:The l option gives a long listing of the files that includes information such as the size of the file, the owner of the file, the date and time the file was last modified, etc.Example:

$ ls -l -r-xr-xr-x 1 linus staff 4222 Jan 13 11:10 arithmetic-rw-r----- 1 linus staff 512 Dec 1 8:00 chapter1.doc-rw-rw-r-- 1 linus staff 1020 Feb 9 10:15 eric.letdrwx------ 2 linus staff 2023 Jun 4 9:31 figures

BASIC UNIX COMMANDS

The a option gives a listing of all the files including hidden files (those which start with a ".")Example:

$ ls -a . .. .profile.sh_history arithmetic chapter1.doceric.let figures memo.to.henryreport.1 sample.c test.program.c

In some UNIX versions, the ls command will list the files one line per file. Use the C option to list the files by columns.

BASIC UNIX COMMANDS

The ls command can also have arguments.

Example:$ ls arithmetic arithmetic$ ls *.c sample.c test.program.c

BASIC UNIX COMMANDS

The ls command can also have a combination of arguments and options.

$ ls -l sample.c

-rw-r--r-- 2 linus staff 20432 Oct 25 10:30 sample.c

Multiple Options

The rule of separation can be applied in case of multiple options. A dash "-" symbol should precede an option and a space should separate each option.

Example:$ ls -a -l

This will give you a long listing of all the files.

A user may also combine multiple options in a single dash symbol.

Example:$ ls -al

Take note that there should be no space between the a and the l.Also, the ordering of the options is not important. Therefore, ls -l -a and ls -la will also work.

Multiple Argunments

The rule of separation is also applicable in case of multiple arguments. A space separates one argument from another.

Example:

$ ls -l arithmetic eric.let sample.c

Multiple Commands

A user may enter several commands at a single UNIX prompt. In such a case, a semi-colon (;) delimits each command unit. The commands will then be executed consecutively.

Example:

$ ls; ls -a; ls -l

UNIX FILENAMES

UNIX filenames can have fourteen (14) characters consisting of letters, numbers, periods, and/or dashes.

UNIX filenames do not follow the concept of file extensions as in DOS. In other words, the period is just like an ordinary character.

Example:letters.to.me and c.prog.linus are valid UNIX filenames

UNIX filenames are case sensitive unlike in DOSExample:

payroll.1994, PAYROLL.1994, and Payroll.1994 are all different filenames in UNIX.

WILD CARDS Like in DOS, UNIX also uses wild cards in

selecting certain files to be operated on. In UNIX systems have 3 wild cards, these are:

* asterisk ? Question mark [ ] brackets

1. The Asterisk "*"

The asterisk wild card will match any number of characters that occur at the designated position in the pattern.

Examples:1. ch* represents all filenames that start with ch followed by any

number of characters.$ ls ch* chapter1 change.doc cherry_text

2. *er represents all filenames that start with any number of characters and end with an er.

$ ls *er files.october system.ver

3. *itch* represents all filenames that start with any number of characters, followed by the string itch and then followed by any number of characters again.

$ ls *itch* ritchie.doc itchy.let mitch.let

In DOS, *.* represents all files (any name and any extension). This is not applicable in UNIX since there are no such things as file extensions. Instead, *.* will be interpreted by UNIX as any file with a period in its filename. To represent all files in UNIX, just simply type *.

2. The Question Mark "?"

Like in DOS, the question mark will match any single character. Examples:

1.?ark represents all filenames that start with any single character followed by the string ark.

$ ls ?ark mark lark 5ark ark

2. henry? represents all filenames that start with the string henry and ends with any single character. $ ls henry?

henry1 henry2 henryx henry3. letter?.doc represents all filenames that start with the

string letter, followed by any single character, and then followed by the string doc.$ ls letter?.doc

letter1.doc letters.doc letter.doc

The Square Brackets "[ ]"

The square brackets are similar to the question mark in the sense that it will match any single character. However, square brackets provide the user with options in choosing the character.

Examples:1. letter[246].doc represents all filenames that start with the string

letter, followed by either a 2, 4, or 6, and then followed by the string.doc.

$ ls letter[246].doc letter2.doc letter4.doc letter6.doc

2. memo_[a-c] represents all filenames that start with the string memo_ followed by any letter from a to c.$ ls memo_[a-c].doc

memo_a.doc memo_b.doc memo_c.doc3. letter[3-5] represents all filenames that start with the

string letter followed by any number from 3 to 5.$ ls letter[3-5]

letter3 letter4 letter5

4.message[4-6m-o] represents all filenames that start with the string message followed by any number from 4 to 6 or any letter from m to o.

$ ls message[4-6m-o] message4 message5 message6

messagem messagen messageo

5.notice[1-2][b-d] represents all filenames that start with the string notice followed by any number from 1 to 2 and then by any letter from b to d.

$ ls notice[1-2][b-d] notice1b notice1c notice1dnotice2b notice2c notice2d

6.element[!5-9] represents all filenames that start with the string element and NOT followed by any number from 5 to 9.

UNIX FILE SYSTEM The UNIX File System organizes all files

into a single hierarchy of directories just like in DOS. In fact, DOS copied its directory structure from UNIX. The highest directory level is called the root (/). Take note that DOS represents its own root directory by a backward slash (\).

The root directory contain numerous subdirectories, many of which are standard in UNIX systems. Some of UNIX’s important subdirectories are:1.bincontains the executable programs that make up most of the important UNIX system commands2.etccontains important system administration programs and data files3.libcontains some of the libraries that programming language compilers use

4. dev

contains UNIX device files5. tmp

contains temporary files to hold data for a short period of time

6. usr

contains files of general use to the users of a UNIX system

Path

A file could be in any directory and its location is called its path.

a chain of directory names that tell UNIX how to find a particular fileAn Absolute Path traces the location of the file from

the root directoryExamples:

/usr/linus/docs/mydoc/usr/linus/letters/letter1

A Relative Path traces the location of the file starting at the current working directory

Examples:Assuming that the current working directory is docs:

../letters/letter1

../letters/letter2

Files and Directories

What are files?Everything in UNIX is a fileA file is sequence of bytesFiles a re placeholders for dataData can be simple ASCII text, formatted

documents, source, programs, executable files, pictures etc.

What are Directories? A directory is a placeholder for files. Directories can hold any kind of files. UNIX directory is a special kind of file that maintains a list of all

the files in it. The organizational scheme is powerful, as a file in a directory

can be another directory. This would be called a subdirectory of the original directory. Directories are the branches of the tree and the files are the

leaves of the tree. As a tree has both internal and leaf nodes with a root at the top,

in UNIX too we have a root that is the basis of the structure of directories and files.

Typical representation of a directory

/

dir1 dir2 dir3

dir4 dir5 dir6

file1 file2 file3

UNIX FILE OWNERSHIP AND SECURITY In a multi-user environment like UNIX, each file has a designated

owner.Example:$ ls -l

-r-xr-xr-x 1 Linus staff 4222 Jan 13 11:10 arithmetic-rw-r----- 1 Linus staff 512 Dec 1 8:00 chapter1.doc-rw-rw-r-- 1 Linus staff 1020 Feb 9 10:15 eric.letdrwx------ 2 Linus staff 2023 Jun 4 9:31 figures

Displayed above is a sample output of the ls -l command. There are eight (8) columns shown.

Explanation of the ls-l Output -rw-rw-r-- 1 mitch staff 1020 Feb 9 10:15 eric.let

Column ExplanationFirst Security Access ModesSecond Number of LinksThird Owner of the File or

DirectoryFourth Group where the Owner

belongsFifth Size of the FileSixth Date the file was last

modifiedSeventh Time the file was last

modifiedEight Name of the File or Directory

What Security Access Mode means: - r w - r w - r - -

The security access mode consists of four components:

First Character A “d” means this is a directory, while a "-" means this is just a regular file (text or executable file).

Next Three The Security Access Mode for the owner of the file or directory.

Next Three The Security Access Mode for the group mates of the owner.

Last Three The Security Access Mode for the other users in the system who do not belong to the owner's group.

The security modes

"r" stands for the read permission. This is for printing files, copying files, and displaying the contents of the file. For directories, this allows the user to list the files (by using the ls command) that are in the directory. Octal code is 4.

"w" stands for write permission. This is needed in order for the user to modify or update a file. For directories, this allows the user to create and delete files in the directory. Octal code is 2.

The security modes

"x" stands for the execute permission. This is needed in order for a user to run or execute a program. It is not applicable for non-executable files such as ASCII text files. For directories, this allows the user to access the files in the directory. Without the execute permission, the write permission for a directory is useless. Octal code is 1.

The security modes

"-" stands for no access (neither r, w, nor x). Octal code is 0.

The security modes

chmod command

The chmod command allows the user to define or change the security access mode of files.

Syntax:

chmod XXX filename(s)

where:chmod command name

xxx stands for a three (3) - digit numerical code for the security access mode. The first

digit is for the owner, the second if for the group, and the third is for others.

filename(s) is the name of the file/directory whose security access mode is being changed

chmod command

Examples:

owner - read, write, execute (4 + 2 + 1 = 7)

group - read, write (4 + 2 = 6)

others - read (4)

then the security access code is 764

$ chmod 764 eric.let

Owner - read, write (4 + 2 = 6)

group - read (4)

others- no access (0)

then the security access code is 640

$ chmod 640 eric.let

Examples:

owner - read, write, execute (4 + 2 + 1 = 7)

group - read, write,execute (4 + 2 + 1 = 7)

others - read, write, execute (4 + 2 + 1 = 7)

then the security access code is 777

$ chmod 777 eric.let

Examples:

$ ls -l arithmetic--------- 1 mitch staff 4222 Jan 13 11:10 arithmeticTo give the read and execute access for the owner, execute for the group, and no access for others:

$ chmod 510 arithmetic$ ls -l arithmetic

-r-x--x--- 1 mitch staff 4222 Jan 13 11:10 arithmetic

Examples:

$ ls -l eric.let

-rw-rw---- 1 Linus staff 1020 Feb 9 10:15 eric.let

To give the read and write access for others:

$ chmod 666 eric.let

$ ls -l eric.let-rw-rw-rw- 1 Linus staff 1020 Feb 9 10:15 eric.let

Examples:

pwd (print working directory) command

Everytime a user logs-in into an UNIX system, he is automatically taken to his home directory (/usr/Linus). However, a user can move to any directory in the file system as long as he has access to it. In order for the user to determine which directory he is currently in, the command to use is pwd.

Example:$ pwd/usr/Linus

FILE / DIRECTORY COMMANDS

cd (change directory) command

If a user wants to move from one directory to another, he should use the cd command.

Format:

cd directory_name

Note: Typing cd alone will always take the user to his home directory

The mkdir command is for creating new directories or subdirectories within the user’s home directory (or in any directory to which he is authorized to do so) and for creating subdirectories within subdirectories.

Format:mkdir directory_name(s)

mkdir (make directory) command

rmdir (remove directory) command

The rmdir command is for removing or deleting directories. The command can remove a directory or subdirectory only if there are no files in it. However, this command is only possible if the user is authorized to do so.

Format:rmdir directory_name(s)

cp (copy) command

The cp command is for copying files within the same directory or to other directories. However, this command is only possible if the user is authorized to do so.

Format:

cp source_name destination_name

mv (move) command

The move command is for renaming files or moving files from one directory to another. However, this command is only possible if the user is authorized to do so.

Format:

mv source_name destination_name

UNIX TEXT FILES

DEFINITION OF A TEXT FILE A text file consists of almost exclusively of

printable characters (letters, digits, and punctuation). It contains lines of legible data, such as English text. Each line in a text ends with a special nonprintable (invisible) character called a newline, which UNIX uses to determine whether one line ends and the next begins.

A text file differs from an executable file in that the latter mostly consists of nonprintable characters and “strange” characters such as , , , and many others.

THE file COMMAND

The file command determines the type and organization of a particular file. It examines a few hundred bytes at the beginning of the file for hints about the kind of data the file contains.

The format of the file command is:

file filename

File Command reports

If the file command notes that the file consists almost entirely of printable characters, it may identify that file as ascii text.

2. If the file consists of printable characters, many of which happen form words that file finds in the dictionary, it may report the file as English text.

1. If the file command notes that the file consists almost entirely of printable characters, it may identify that file as ascii text.

2. If the file consists of printable characters, many of which happen form words that file finds in the dictionary, it may report the file as English text.

File Command reports

3. If the file consists almost entirely of nonprintable characters, it may report the file as executable.

4. If the file contains what seems to be programming constructs from C or FORTRAN, the file is identified as source code for those languages, reporting c program text or fortran program text.

File Command reports

VIEWING TEXT FILES

There are three different commands in viewing the contents of text files in UNIX:catmorepg

cat

cat commandFormat:

cat filename(s)The cat command is for displaying the contents of a text file on-screen. It is very much similar to the type command of DOS. However, cat will continuously display the contents of the file without regard to whether the user is finish reading the contents or not. In order for the user to catch up, he can freeze the screen by pressing <Ctrl-S>. To unfreeze the screen, simply type <Ctrl-Q>.

pg command

pg commandFormat:

pg filenameThe pg command is more appropriate for viewing large files since it displays the contents of text files one screen at a time. To view the next screen, the user simply presses the Enter key and it displays the next screen of data. pg repeats this process until it reaches the end of the file.

more command

more commandFormat:

more filenameThe more command is very much similar to the pg command in the sense that it also displays the contents of a text file one screenful at a time. However, in order to view the next screen, the user presses the spacebar key instead of Enter key . Furthermore, more updates the user on the percentage of the file that was viewed already. The user may also use the ENTER key if he wants to scroll-up the screen one line at a time.