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- Unix / Linux for Beginners
- Unix / Linux - Home
- Unix / Linux - Getting Started
- Unix / Linux - File Management
- Unix / Linux - Directories
- Unix / Linux - File Permission
- Unix / Linux - Environment
- Unix / Linux - Basic Utilities
- Unix / Linux - Pipes & Filters
- Unix / Linux - Processes
- Unix / Linux - Communication
- Unix / Linux - The vi Editor
- Unix / Linux Shell Programming
- Unix / Linux - Shell Scripting
- Unix / Linux - What is Shell?
- Unix / Linux - Using Variables
- Unix / Linux - Special Variables
- Unix / Linux - Using Arrays
- Unix / Linux - Basic Operators
- Unix / Linux - Decision Making
- Unix / Linux - Shell Loops
- Unix / Linux - Loop Control
- Unix / Linux - Shell Substitutions
- Unix / Linux - Quoting Mechanisms
- Unix / Linux - IO Redirections
- Unix / Linux - Shell Functions
- Unix / Linux - Manpage Help
- Advanced Unix / Linux
- Unix / Linux - Regular Expressions
- Unix / Linux - File System Basics
- Unix / Linux - User Administration
- Unix / Linux - System Performance
- Unix / Linux - System Logging
- Unix / Linux - Signals and Traps
UNIX / LINUX Tutorial
Unix is a computer Operating System which is capable of handling activities from multiple users at the same time. The development of Unix started around 1969 at AT&T Bell Labs by Ken Thompson and Dennis Ritchie. This tutorial gives a very good understanding on Unix.
Audience
This tutorial has been prepared for the beginners to help them understand the basics to advanced concepts covering Unix commands, Unix shell scripting and various utilities.
Prerequisites
We assume you have adequate exposure to Operating Systems and their functionalities. A basic understanding on various computer concepts will also help you in understanding the various exercises given in this tutorial.
Execute Unix Shell Programs
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Unix / Linux - Getting Started
What is Unix?
The Unix operating system is a set of programs that act as a link between the computer and the user. The computer programs that allocate the system resources and coordinate all the details of the computer's internals is called the operating system or the kernel. Users communicate with the kernel through a program known as the shell. The shell is a command line interpreter; it translates commands entered by the user and converts them into a language that is understood by the kernel.
- Unix was originally developed in 1969 by a group of AT&T employees Ken Thompson, Dennis Ritchie, Douglas McIlroy, and Joe Ossanna at Bell Labs.
- There are various Unix variants available in the market. Solaris Unix, AIX, HP Unix and BSD are a few examples. Linux is also a flavor of Unix which is freely available.
- Several people can use a Unix computer at the same time; hence Unix is called a multiuser system.
- A user can also run multiple programs at the same time; hence Unix is a multitasking environment.
Unix Architecture
Here is a basic block diagram of a Unix system −
Login Unix When you first connect to a Unix system, you usually see a prompt such as the following − login: To log in
- Have your userid (user identification) and password ready. Contact your system administrator if you don't have these yet.
- Type your userid at the login prompt, then press ENTER. Your userid is case- sensitive , so be sure you type it exactly as your system administrator has instructed.
- Type your password at the password prompt, then press ENTER. Your password is also case-sensitive.
- If you provide the correct userid and password, then you will be allowed to enter into the system. Read the information and messages that comes up on the screen, which is as follows. login : amrood amrood's password: Last login: Sun Jun 14 09:32:32 2009 from 62.61.164. $ You will be provided with a command prompt (sometime called the $ prompt ) where you type all your commands. For example, to check calendar, you need to type the cal command as follows − $ cal June 2009 Su Mo Tu We Th Fr Sa 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 $ Change Password All Unix systems require passwords to help ensure that your files and data remain your own and that the system itself is secure from hackers and crackers. Following are the steps to change your password − Step 1 − To start, type password at the command prompt as shown below. Step 2 − Enter your old password, the one you're currently using. Step 3 − Type in your new password. Always keep your password complex enough so that nobody can guess it. But make sure, you remember it. Step 4 − You must verify the password by typing it again. $ passwd
Changing password for amrood (current) Unix password:****** New UNIX password:******* Retype new UNIX password:******* passwd: all authentication tokens updated successfully $ Note − We have added asterisk (*) here just to show the location where you need to enter the current and new passwords otherwise at your system. It does not show you any character when you type. Listing Directories and Files All data in Unix is organized into files. All files are organized into directories. These directories are organized into a tree-like structure called the filesystem. You can use the ls command to list out all the files or directories available in a directory. Following is the example of using ls command with - l option. $ ls - l total 19621 drwxrwxr-x 2 amrood amrood 4096 Dec 25 09:59 uml
- rw-rw-r-- 1 amrood amrood 5341 Dec 25 08:38 uml.jpg drwxr-xr-x 2 amrood amrood 4096 Feb 15 2006 univ drwxr-xr-x 2 root root 4096 Dec 9 2007 urlspedia
- rw-r--r-- 1 root root 276480 Dec 9 2007 urlspedia.tar drwxr-xr-x 8 root root 4096 Nov 25 2007 usr
- rwxr-xr-x 1 root root 3192 Nov 25 2007 webthumb.php
- rw-rw-r-- 1 amrood amrood 20480 Nov 25 2007 webthumb.tar
- rw-rw-r-- 1 amrood amrood 5654 Aug 9 2007 yourfile.mid
- rw-rw-r-- 1 amrood amrood 166255 Aug 9 2007 yourfile.swf $ Here entries starting with d..... represent directories. For example, uml, univ and urlspedia are directories and rest of the entries are files. Who Are You? While you're logged into the system, you might be willing to know : Who am I? The easiest way to find out "who you are" is to enter the whoami command − $ whoami amrood $ Try it on your system. This command lists the account name associated with the current login. You can try who am i command as well to get information about yourself. Who is Logged in? Sometime you might be interested to know who is logged in to the computer at the same time.
5 reboot Reboots the system 6 shutdown Shuts down the system You typically need to be the super user or root (the most privileged account on a Unix system) to shut down the system. However, on some standalone or personally-owned Unix boxes, an administrative user and sometimes regular users can do so.
Unix / Linux - File Management
In this chapter, we will discuss in detail about file management in Unix. All data in Unix is organized into files. All files are organized into directories. These directories are organized into a tree-like structure called the filesystem. When you work with Unix, one way or another, you spend most of your time working with files. This tutorial will help you understand how to create and remove files, copy and rename them, create links to them, etc. In Unix, there are three basic types of files −
- Ordinary Files − An ordinary file is a file on the system that contains data, text, or program instructions. In this tutorial, you look at working with ordinary files.
- Directories − Directories store both special and ordinary files. For users familiar with Windows or Mac OS, Unix directories are equivalent to folders.
- Special Files − Some special files provide access to hardware such as hard drives, CD-ROM drives, modems, and Ethernet adapters. Other special files are similar to aliases or shortcuts and enable you to access a single file using different names.
Listing Files
To list the files and directories stored in the current directory, use the following command − $ls Here is the sample output of the above command − $ls bin hosts lib res. ch07 hw1 pub test_results ch07.bak hw2 res.01 users docs hw3 res.02 work The command ls supports the - l option which would help you to get more information about the listed files −
$ls - l total 1962188 drwxrwxr-x 2 amrood amrood 4096 Dec 25 09:59 uml
- rw-rw-r-- 1 amrood amrood 5341 Dec 25 08:38 uml.jpg drwxr-xr-x 2 amrood amrood 4096 Feb 15 2006 univ drwxr-xr-x 2 root root 4096 Dec 9 200 7 urlspedia
- rw-r--r-- 1 root root 276480 Dec 9 2007 urlspedia.tar drwxr-xr-x 8 root root 4096 Nov 25 2007 usr drwxr-xr-x 2 200 300 4096 Nov 25 2007 webthumb-1.
- rwxr-xr-x 1 root root 3192 Nov 25 2007 webthumb.php
- rw-rw-r-- 1 amrood amrood 20480 Nov 25 2007 webthumb.tar
- rw-rw-r-- 1 amrood amrood 5654 Aug 9 2007 yourfile.mid
- rw-rw-r-- 1 amrood amrood 166255 Aug 9 2007 yourfile.swf drwxr-xr-x 11 amrood amrood 4096 May 29 2007 zlib-1.2. $ Here is the information about all the listed columns − - First Column − Represents the file type and the permission given on the file. Below is the description of all type of files. - Second Column − Represents the number of memory blocks taken by the file or directory. - Third Column − Represents the owner of the file. This is the Unix user who created this file. - Fourth Column − Represents the group of the owner. Every Unix user will have an associated group. - Fifth Column − Represents the file size in bytes. - Sixth Column − Represents the date and the time when this file was created or modified for the last time. - Seventh Column − Represents the file or the directory name. In the ls - l listing example, every file line begins with a d , - , or l. These characters indicate the type of the file that's listed. Sr.No. Prefix & Description 1 - Regular file, such as an ASCII text file, binary executable, or hard link. 2 b Block special file. Block input/output device file such as a physical hard drive. 3 c
To list the invisible files, specify the - a option to ls − $ ls - a
. .profile docs lib test_results .. .rhosts hosts pub users .emacs bin hw1 res.01 work .exrc ch07 hw2 res. .kshrc ch07.bak hw3 res. $ - Single dot (.) − This represents the current directory. - Double dot (..) − This represents the parent directory.
Creating Files
You can use the vi editor to create ordinary files on any Unix system. You simply need to give the following command − $ vi filename The above command will open a file with the given filename. Now, press the key i to come into the edit mode. Once you are in the edit mode, you can start writing your content in the file as in the following program − This is unix file....I created it for the first time..... I'm going to save this content in this file. Once you are done with the program, follow these steps −
- Press the key esc to come out of the edit mode.
- Press two keys Shift + ZZ together to come out of the file completely. You will now have a file created with filename in the current directory. $ vi filename $
Editing Files
You can edit an existing file using the vi editor. We will discuss in short how to open an existing file − $ vi filename Once the file is opened, you can come in the edit mode by pressing the key i and then you can proceed by editing the file. If you want to move here and there inside a file, then first you need to come out of the edit mode by pressing the key Esc. After this, you can use the following keys to move inside a file −
- l key to move to the right side.
- h key to move to the left side.
- k key to move upside in the file.
- j key to move downside in the file.
So using the above keys, you can position your cursor wherever you want to edit. Once you are positioned, then you can use the i key to come in the edit mode. Once you are done with the editing in your file, press Esc and finally two keys Shift + ZZ together to come out of the file completely.
Display Content of a File
You can use the cat command to see the content of a file. Following is a simple example to see the content of the above created file − $ cat filename This is unix file....I created it for the first time..... I'm going to save this content in this file. $ You can display the line numbers by using the - b option along with the cat command as follows − $ cat - b filename 1 This is unix file....I created it for the first time..... 2 I'm going to save this content in this file. $
Counting Words in a File
You can use the wc command to get a count of the total number of lines, words, and characters contained in a file. Following is a simple example to see the information about the file created above − $ wc filename 2 19 103 filename $ Here is the detail of all the four columns −
- First Column − Represents the total number of lines in the file.
- Second Column − Represents the total number of words in the file.
- Third Column − Represents the total number of bytes in the file. This is the actual size of the file.
- Fourth Column − Represents the file name. You can give multiple files and get information about those files at a time. Following is simple syntax − $ wc filename1 filename2 filename
Copying Files
To make a copy of a file use the cp command. The basic syntax of the command is − $ cp source_file destination_file Following is the example to create a copy of the existing file filename. $ cp filename copyfile
In this chapter, we will discuss in detail about directory management in Unix. A directory is a file the solo job of which is to store the file names and the related information. All the files, whether ordinary, special, or directory, are contained in directories. Unix uses a hierarchical structure for organizing files and directories. This structure is often referred to as a directory tree. The tree has a single root node, the slash character ( / ), and all other directories are contained below it.
Home Directory
The directory in which you find yourself when you first login is called your home directory. You will be doing much of your work in your home directory and subdirectories that you'll be creating to organize your files. You can go in your home directory anytime using the following command − $cd ~ $ Here ~ indicates the home directory. Suppose you have to go in any other user's home directory, use the following command − $cd ~username $ To go in your last directory, you can use the following command − $cd - $
Absolute/Relative Pathnames
Directories are arranged in a hierarchy with root (/) at the top. The position of any file within the hierarchy is described by its pathname. Elements of a pathname are separated by a /. A pathname is absolute, if it is described in relation to root, thus absolute pathnames always begin with a /. Following are some examples of absolute filenames. /etc/passwd /users/sjones/chem/notes /dev/rdsk/Os A pathname can also be relative to your current working directory. Relative pathnames never begin with /. Relative to user amrood's home directory, some pathnames might look like this − chem/notes personal/res To determine where you are within the filesystem hierarchy at any time, enter the command pwd to print the current working directory −
$pwd /user0/home/amrood $
Listing Directories
To list the files in a directory, you can use the following syntax − $ls dirname Following is the example to list all the files contained in /usr/local directory − $ls /usr/local X11 bin gimp jikes sbin ace doc include lib share atalk etc info man ami
Creating Directories
We will now understand how to create directories. Directories are created by the following command − $mkdir dirname Here, directory is the absolute or relative pathname of the directory you want to create. For example, the command − $mkdir mydir $ Creates the directory mydir in the current directory. Here is another example − $mkdir /tmp/test-dir $ This command creates the directory test-dir in the /tmp directory. The mkdir command produces no output if it successfully creates the requested directory. If you give more than one directory on the command line, mkdir creates each of the directories. For example, − $mkdir docs pub $ Creates the directories docs and pub under the current directory.
Creating Parent Directories
We will now understand how to create parent directories. Sometimes when you want to create a directory, its parent directory or directories might not exist. In this case, mkdir issues an error message as follows − $mkdir /tmp/amrood/test mkdir: Failed to make directory "/tmp/amrood/test"; No such file or directory
$mv mydir yourdir $
The directories. (dot) and .. (dot dot)
The filename. (dot) represents the current working directory; and the filename .. (dot dot) represents the directory one level above the current working directory, often referred to as the parent directory. If we enter the command to show a listing of the current working directories/files and use the - a option to list all the files and the - l option to provide the long listing, we will receive the following result. $ls - la drwxrwxr-x 4 teacher class 2048 Jul 16 17.. drwxr-xr-x 60 root 1536 Jul 13 14: .. ---------- 1 teacher class 4210 May 1 08:27 .profile
- rwxr-xr-x 1 teacher class 1948 May 12 13:42 memo $
Unix / Linux - File Permission / Access Modes
In this chapter, we will discuss in detail about file permission and access modes in Unix. File ownership is an important component of Unix that provides a secure method for storing files. Every file in Unix has the following attributes −
- Owner permissions − The owner's permissions determine what actions the owner of the file can perform on the file.
- Group permissions − The group's permissions determine what actions a user, who is a member of the group that a file belongs to, can perform on the file.
- Other (world) permissions − The permissions for others indicate what action all other users can perform on the file.
The Permission Indicators
While using ls - l command, it displays various information related to file permission as follows − $ls - l /home/amrood
- rwxr-xr-- 1 amrood users 1024 Nov 2 00:10 myfile drwxr-xr--- 1 amrood users 1024 Nov 2 00:10 mydir Here, the first column represents different access modes, i.e., the permission associated with a file or a directory. The permissions are broken into groups of threes, and each position in the group denotes a specific permission, in this order: read (r), write (w), execute (x) − - The first three characters (2-4) represent the permissions for the file's owner. For example, - rwxr-xr-- represents that the owner has read (r), write (w) and execute (x) permission.
- The second group of three characters (5-7) consists of the permissions for the group to which the file belongs. For example, - rwxr-xr-- represents that the group has read (r) and execute (x) permission, but no write permission.
- The last group of three characters (8-10) represents the permissions for everyone else. For example, - rwxr-xr-- represents that there is read (r) only permission.
File Access Modes
The permissions of a file are the first line of defense in the security of a Unix system. The basic building blocks of Unix permissions are the read , write , and execute permissions, which have been described below − Read Grants the capability to read, i.e., view the contents of the file. Write Grants the capability to modify, or remove the content of the file. Execute User with execute permissions can run a file as a program.
Directory Access Modes
Directory access modes are listed and organized in the same manner as any other file. There are a few differences that need to be mentioned − Read Access to a directory means that the user can read the contents. The user can look at the filenames inside the directory. Write Access means that the user can add or delete files from the directory. Execute Executing a directory doesn't really make sense, so think of this as a traverse permission. A user must have execute access to the bin directory in order to execute the ls or the cd command.
Changing Permissions
To change the file or the directory permissions, you use the chmod (change mode) command. There are two ways to use chmod — the symbolic mode and the absolute mode. Using chmod in Symbolic Mode The easiest way for a beginner to modify file or directory permissions is to use the symbolic mode. With symbolic permissions you can add, delete, or specify the permission set you want by using the operators in the following table.
3 Execute and write permission: 1 (execute) + 2 (write) = 3 - wx 4 Read permission r-- 5 Read and execute permission: 4 (read) + 1 (execute) = 5 r-x 6 Read and write permission: 4 (read) + 2 (write) = 6 rw- 7 All permissions: 4 (read) + 2 (write) + 1 (execute) = 7 rwx Here's an example using the testfile. Running ls - 1 on the testfile shows that the file's permissions are as follows − $ls - l testfile
- rwxrwxr-- 1 amrood users 1024 Nov 2 00:10 testfile Then each example chmod command from the preceding table is run on the testfile, followed by ls – l , so you can see the permission changes − $ chmod 755 testfile $ls - l testfile
- rwxr-xr-x 1 amrood users 1024 Nov 2 00:10 testfile $chmod 743 testfile $ls - l testfile
- rwxr---wx 1 amrood users 1024 Nov 2 00:10 testfile $chmod 043 testfile $ls - l testfile ----r---wx 1 amrood users 1024 Nov 2 00:10 testfile
Changing Owners and Groups
While creating an account on Unix, it assigns a owner ID and a group ID to each user. All the permissions mentioned above are also assigned based on the Owner and the Groups. Two commands are available to change the owner and the group of files −
- chown − The chown command stands for "change owner" and is used to change the owner of a file.
- chgrp − The chgrp command stands for "change group" and is used to change the group of a file.
Changing Ownership
The chown command changes the ownership of a file. The basic syntax is as follows − $ chown user filelist The value of the user can be either the name of a user on the system or the user id (uid) of a user on the system. The following example will help you understand the concept −
$ chown amrood testfile $ Changes the owner of the given file to the user amrood. NOTE − The super user, root, has the unrestricted capability to change the ownership of any file but normal users can change the ownership of only those files that they own.
Changing Group Ownership
The chgrp command changes the group ownership of a file. The basic syntax is as follows − $ chgrp group filelist The value of group can be the name of a group on the system or the group ID (GID) of a group on the system. Following example helps you understand the concept − $ chgrp special testfile $ Changes the group of the given file to special group.
SUID and SGID File Permission
Often when a command is executed, it will have to be executed with special privileges in order to accomplish its task. As an example, when you change your password with the passwd command, your new password is stored in the file /etc/shadow. As a regular user, you do not have read or write access to this file for security reasons, but when you change your password, you need to have the write permission to this file. This means that the passwd program has to give you additional permissions so that you can write to the file /etc/shadow. Additional permissions are given to programs via a mechanism known as the Set User ID (SUID) and Set Group ID (SGID) bits. When you execute a program that has the SUID bit enabled, you inherit the permissions of that program's owner. Programs that do not have the SUID bit set are run with the permissions of the user who started the program. This is the case with SGID as well. Normally, programs execute with your group permissions, but instead your group will be changed just for this program to the group owner of the program. The SUID and SGID bits will appear as the letter "s" if the permission is available. The SUID "s" bit will be located in the permission bits where the owners’ execute permission normally resides. For example, the command − $ ls - l /usr/bin/passwd
- r-sr-xr-x 1 root bin 19031 Feb 7 13:47 /usr/bin/passwd*
