UNIX basics

Chapter 1. The UNIX Operating System

Introduction

This chapter introduces you to the UNIX operating system. We first look at what is an operating system and then proceed to discuss the different features of UNIX that have made it a popular operating system.

Objectives                                                                                         

• What is an operating system (OS)?
• Features of UNIX OS
• A Brief History of UNIX OS, POSIX and Single Unix Specification (SUS)

1.     What is an operating system (OS)?

An operating system (OS) is a resource manager. It takes the form of a set of software routines that allow users and application programs to access system resources (e.g. the CPU, memory, disks, modems, printers, network cards etc.) in a safe, efficient and abstract way.

For example, an OS ensures safe access to a printer by allowing only one application program to send data directly to the printer at any one time. An OS encourages efficient use of the CPU by suspending programs that are waiting for I/O operations to complete to make way for programs that can use the CPU more productively. An OS also provides convenient abstractions (such as files rather than disk locations) which isolate application programmers and users from the details of the underlying hardware.

 

UNIX Operating system allows complex tasks to be performed with a few keystrokes. It doesn’t tell or warn the user about the consequences of the command.

Kernighan and Pike (The UNIX Programming Environment) lamented long ago that “as the UNIX system has spread, the fraction of its users who are skilled in its application has decreased.” However, the capabilities of UNIX are limited only by your imagination.

 2.     Features of UNIX OS

Several features of UNIX have made it popular. Some of them are:

 Portable

UNIX can be installed on many hardware platforms. Its widespread use can be traced to the decision to develop it using the C language.

Multiuser

The UNIX design allows multiple users to concurrently share hardware and software

Multitasking

UNIX allows a user to run more than one program at a time. In fact more than one program can be running in the background while a user is working foreground.

Networking

While UNIX was developed to be an interactive, multiuser, multitasking system, networking is also incorporated into the heart of the operating system. Access to another system uses a standard communications protocol known as Transmission Control Protocol/Internet Protocol (TCP/IP).

Organized File System

UNIX has a very organized file and directory system that allows users to organize and maintain files.

Device Independence

UNIX treats input/output devices like ordinary files. The source or destination for file input and output is easily controlled through a UNIX design feature called redirection.

Utilities

UNIX provides a rich library of utilities that can be use to increase user productivity.

3. A Brief History of UNIX

In the late 1960s, researchers from General Electric, MIT and Bell Labs launched a joint project to develop an ambitious multi-user, multi-tasking OS for mainframe computers known as MULTICS (Multiplexed Information and Computing System). MULTICS failed, but it did inspire Ken Thompson, who was a researcher at Bell Labs, to have a go at writing a simpler operating system himself. He wrote a simpler version of MULTICS on a PDP7 in assembler and called his attempt UNICS (Uniplexed Information and Computing System). Because memory and CPU power were at a premium in those days, UNICS (eventually shortened to UNIX) used short commands to minimize the space needed to store them and the time needed to decode them - hence the tradition of short UNIX commands we use today, e.g. ls, cp, rm, mv etc.

Ken Thompson then teamed up with Dennis Ritchie, the author of the first C compiler in 1973. They rewrote the UNIX kernel in C - this was a big step forwards in terms of the system's portability - and released the Fifth Edition of UNIX to universities in 1974. The Seventh Edition, released in 1978, marked a split in UNIX development into two main branches: SYSV (System 5) and BSD (Berkeley Software Distribution). BSD arose from the University of California at Berkeley where Ken Thompson spent a sabbatical year. Its development was continued by students at Berkeley and other research institutions. SYSV was developed by AT&T and other commercial companies. UNIX flavors based on SYSV have traditionally been more conservative, but better supported than BSD-based flavors.

Some of the commercial UNIX based on system V are:

• IBM's AIX
• Hewlett-Packard's HPUX
• SCO's Open Server Release 5
• Silicon Graphics' IRIS
• DEC's Digital UNIX
• Sun Microsystems' Solaris 2

Some of the commercial UNIX based on BSD are:

• SunOS 4.1.X (now Solaris)
• DEC's Ultris
• BSD/OS, 4.4BSD

Some Free UNIX are:

• Linux, written by Linus Torvalds at University of Helsinki in Finland.
• FreeBSD and NetBSD, a derivative of 4.4BSD

 Chapter 2. The UNIX Architecture and Command Usage

Introduction

In order to understand the subsequent chapters, we first need to understand the architecture of UNIX and the concept of division of labor between two agencies viz., the shell and the kernel. This chapter introduces the architecture of UNIX. Next we discuss the rich collection of UNIX command set, with a specific discussion of command structure and usage of UNIX commands. We also look at the man command, used for obtaining online help on any UNIX command. Sometimes the keyboard sequences don’t work, in which case, you need to know what to do to fix them. Final topic of this chapter is troubleshooting some terminal problems.

Objectives

• The UNIX Architecture
• Locating Commands
• Internal and External Commands
• Command Structure and usage
• Flexibility of Command Usage
• The man Pages, apropos and whatis
• Troubleshooting the terminal problems

1.    The UNIX Architecture

UNIX architecture comprises of two major components viz., the shell and the kernel. The kernel interacts with the machine’s hardware and the shell with the user.

The kernel is the core of the operating system. It is a collection of routines written in C. It is loaded into memory when the system is booted and communicates directly with the hardware. User programs that need to access the hardware use the services of the kernel via use of system calls and the kernel performs the job on behalf of the user. Kernel is also responsible for managing system’s memory, schedules processes, decides their priorities.

The shell performs the role of command interpreter. Even though there’s only one kernel running on the system, there could be several shells in action, one for each user who’s logged in. The shell is responsible for interpreting the meaning of metacharacters if any, found on the command line before dispatching the command to the kernel for execution.

The File and Proces

A file is an array of bytes that stores information. It is also related to another file in the sense that both belong to a single hierarchical directory structure.

A process is the second abstraction UNIX provides. It can be treated as a time image of an executable file. Like files, processes also belong to a hierarchical structure. We will be discussing the processes in detain in a subsequent chapter.

2. Locating Files

 All UNIX commands are single words like ls, cd, cat, etc. These names are in lowercase. These commands are essentially files containing programs, mainly written in C. Files are stored in directories, and so are the binaries associated with these commands. You can find the location of an executable program using type command:

            $ type ls

            ls is /bin/ls

This means that when you execute ls command, the shell locates this file in /bin directory and makes arrangements to execute it.

The Path

The sequence of directories that the shell searches to look for a command is specified in its own PATH variable. These directories are colon separated. When you issue a command, the shell searches this list in the sequence specified to locate and execute it.

3.     Internal and External Commands

Some commands are implemented as part of the shell itself rather than separate executable files. Such commands that are built-in are called internal commands. If a command exists both as an internal command of the shell as well as an external one (in /bin or /usr/bin), the shell will accord top priority to its own internal command with the same name. Some built-in commands are echo, pwd, etc.

 4.     Command Structure

UNIX commands take the following general form:

            verb [options] [arguments]

where verb is the command name that can take a set of optional options and one or more optional arguments.

Commands, options and arguments have to be separated by spaces or tabs to enable the shell to interpret them as words. A contiguous string of spaces and tabs together is called a whitespace. The shell compresses multiple occurrences of whitespace into a single whitespace.

Options

An option is preceded by a minus sign (-) to distinguish it from filenames.

Example: $ ls –l

There must not be any whitespaces between – and l.  Options are also arguments, but given a special name because they are predetermined. Options can be normally compined with only one – sign. i.e., instead of using

            $ ls –l –a –t

we can as well use,

            $ ls –lat

Because UNIX was developed by people who had their own ideas as to what options should look like, there will be variations in the options. Some commands use + as an option prefix instead of -.

Filename Arguments

Many UNIX commands use a filename as argument so that the command can take input from the file. If a command uses a filename as argument, it will usually be the last argument, after all options.

Example:         cp file1 file2 file3 dest_dir

                        rm file1 file2 file3

The command with its options and argumens is known as the command line, which is considered as complete after [Enter] key is pressed, so that the entire line is fed to the shell as its input for interpretation and execution.

Exceptions

Some commands in UNIX like pwd do not take any options and arguments. Some commands like who may or may not be specified with arguments. The ls command can run without arguments (ls), with only options (ls –l), with only filenames (ls f1 f2), or using a combination of both (ls –l f1 f2). Some commands compulsorily take options (cut). Some commands like grep, sed can take an expression as an argument, or a set of instructions as argument.

5.     Flexibility of Command Usage

UNIX provides flexibility in using the commands. The following discussion looks at how permissive the shell can be to the command usage.

Combining Commands

Instead of executing commands on separate lines, where each command is processed and executed before the next could be entered, UNIX allows you to specify more than one command in the single command line. Each command has to be separated from the other by a ; (semicolon).

            wc sample.txt ; ls –l sample.txt

You can even group several commands together so that their combined output is redirected to a file.

            (wc sample.txt ; ls –l sample.txt) > newfile

When a command line contains a semicolon, the shell understands that the command on each side of it needs to be processed separately. Here ; is known as a metacharacter.

Note: When a command overflows into the next line or needs to be split into multiple lines, just press enter, so that the secondary prompt (normally >) is displayed and you can enter the remaining part of the command on the next line.

Entering a Command before previous command has finished

You need not have to wait for the previous command to finish before you can enter the next command. Subsequent commands entered at the keyboard are stored in a buffer (a temporary storage in memory) that is maintained by the kernel for all keyboard input. The next command will be passed on to the shell for interpretation after the previous command has completed its execution.

6.     man: Browsing The Manual Pages Online

UNIX commands are rather cryptic. When you don’t remember what options are supported by a command or what its syntax is, you can always view man (short for manual) pages to get online help. The man command displays online documentation of a specified command.

A pager is a program that displays one screenful information and pauses for the user to view the contents. The user can make use of internal commands of the pager to scroll up and scroll down the information. The two popular pagers are more and less.  more is the Berkeley’s pager, which is a superior alternative to original pg command. less is the standard pager used on Linux systems. less if modeled after a popular editor called vi and is more powerful than more as it provides vi-like navigational and search facilities. We can use pagers with commands like ls | more. The man command is configured to work with a pager.

7.     Understanding The man Documentation

The man documentation is organized in eight (08) sections. Later enhancements have added subsections like 1C, 1M, 3N etc.) References to other sections are reflected as SEE ALSO section of a man page.

When you use man command, it starts searching the manuals starting from section 1. If it locates a keyword in one section, it won’t continue the search, even if the keyword occurs in another section. However, we can provide the section number additionally as argument for man command.

For example, passwd appears in section 1 and section 4. If we want to get documentation of passwd in section 4, we use,

            $ man 4 passwd          OR      $ man –s4 passwd (on Solaris)

Understanding a man Page

A typical man page for wc command is shown below:

 A man page is divided into a number of compulsory and optional sections. Every command doesn’t need all sections, but the first three (NAME, SYNOPSIS and DESCRIPTION) are generally seen in all man pages. NAME presents a one-line introduction of the command. SYNOPSIS shows the syntax used by the command and DESCRIPTION provides a detailed description.

The SYNOPSIS follows certain conventions and rules:

• If a command argument is enclosed in rectangular brackets, then it is optional; otherwise, the argument is required.
• The ellipsis (a set if three dots) implies that there can be more instances of the preceding word.
• The | means that only one of  the options shows on either side of the pipe can be used.

All the options used by the command are listed in OPTIONS section. There is a separate section named EXIT STATUS which lists possible error conditions and their numeric representation.

Note: You can use man command to view its own documentation ($ man man). You can also set the pager to use with man ($ PAGER=less ; export PAGER). To understand which pager is being used by man, use $ echo $PAGER.

The following table shows the organization of man documentation.

8.     Further Help with man –k, apropos and whatis

man –k: Searches a summary database and prints one-line description of the command.

Example:        

$ man –k awk

awk  awk(1)    -pattern scanning and processing language

nawk nawk(1)   -pattern scanning and processing language

apropos: lists the commands and files associated with a keyword.

Example:        

$ apropos FTP

ftp  ftp(1)        -file transfer program

ftpd in.ftpd(1m)   -file transfer protocol server

ftpusers  ftpusers(4)-file listing users to be disallowed

                                                ftp login privileges

whatis:  lists one-liners  for a command.

Example:        

$ whatis cp

cp   cp(1)         -copy files

9.     When Things Go Wrong

Terminals and keyboards have no uniform behavioral pattern. Terminal settings directly impact the keyboard operation. If you observe a different behavior from that expected, when you press certain keystrokes, it means that the terminal settings are different. In such cases, you should know which keys to press to get the required behavior. The following table lists keyboard commands to try when things go wrong.

 Chapter 3. The File System

Introduction

In this chapter we will look at the file system of UNIX. We also look at types of files their significance. We then look at two ways of specifying a file viz., with absolute pathnames and relative pathnames. A discussion on commands used with directory files viz., cd, pwd, mkdir, rmdir and ls will be made. Finally we look at some of the important directories contained under UNIX file system.

Objectives

• Types of files
• UNIX Filenames
• Directories and Files
• Absolute and Relative Pathnames
• pwd – print working directory
• cd – change directory
• mkdir – make a directory
• rmdir – remove directory
• The PATH environmental variable
• ls – list directory contents
• The UNIX File System

1. Types of files

A simple description of the UNIX system is this:

“On a UNIX system, everything is a file; if something is not a file, it is a process.”

A UNIX system makes no difference between a file and a directory, since a directory is just a file containing names of other files. Programs, services, texts, images, and so forth, are all files. Input and output devices, and generally all devices, are considered to be files, according to the system.

Most files are just files, called regular files; they contain normal data, for example text files, executable files or programs, input for or output from a program and so on.

While it is reasonably safe to suppose that everything you encounter on a UNIX system is a file, there are some exceptions.

Directories: files that are lists of other files.

Special files or Device Files: All devices and peripherals are represented by files. To read or write a device, you have to perform these operations on its associated file. Most special files are in /dev.

Links: a system to make a file or directory visible in multiple parts of the system's file tree.

(Domain) sockets: a special file type, similar to TCP/IP sockets, providing inter−process networking protected by the file system's access control.

Named pipes: act more or less like sockets and form a way for processes to communicate with each other, without using network socket semantics.

Ordinary (Regular) File

This is the most common file type. An ordinary file can be either a text file or a binary file.

A text file contains only printable characters and you can view and edit them. All C and Java program sources, shell scripts are text files. Every line of a text file is terminated with the newline character.

A binary file, on the other hand, contains both printable and nonprintable characters that cover the entire ASCII range. The object code and executables that you produce by compiling C programs are binary files. Sound and video files are also binary files.

Directory File

A directory contains no data, but keeps details of the files and subdirectories that it contains. A directory file contains one entry for every file and subdirectory that it houses. Each entry has two components namely, the filename and a unique identification number of the file or directory (called the inode number).

When you create or remove a file, the kernel automatically updates its corresponding directory by adding or removing the entry (filename and inode number) associated with the file.

Device File

All the operations on the devices are performed by reading or writing the file representing the device. It is advantageous to treat devices as files as some of the commands used to access an ordinary file can be used with device files as well.

Device filenames are found in a single directory structure, /dev. A device file is not really a stream of characters. It is the attributes of the file that entirely govern the operation of the device. The kernel identifies a device from its attributes and uses them to operate the device.

2. Filenames in UNIX

On a UNIX system, a filename can consist of up to 255 characters. Files may or may not have extensions and can consist of practically any ASCII character except the / and the Null character. You are permitted to use control characters or other nonprintable characters in a filename. However, you should avoid using these characters while naming a file. It is recommended that only the following characters be used in filenames:

            Alphabets and numerals.

            The period (.), hyphen (-) and underscore (_).

UNIX imposes no restrictions on the extension. In all cases, it is the application that imposes that restriction. Eg. A C Compiler expects C program filenames to end with .c, Oracle requires SQL scripts to have .sql extension.

A file can have as many dots embedded in its name. A filename can also begin with or end with a dot.

UNIX is case sensitive; cap01, Chap01 and CHAP01 are three different filenames that can coexist in the same directory.

3. Directories and Files

A file is a set of data that has a name. The information can be an ordinary text, a user-written computer program, results of a computation, a picture, and so on. The file name may consist of ordinary characters, digits and special tokens like the underscore, except the forward slash (/). It is permitted to use special tokens like the ampersand (&) or spaces in a filename.

Unix organizes files in a tree-like hierarchical structure, with the root directory, indicated

by a forward slash (/), at the top of the tree. See the Figure below, in which part of the hierarchy of files and directories on the computer is shown.

4. Absolute and relative paths

A path, which is the way you need to follow in the tree structure to reach a given file, can be described as starting from the trunk of the tree (the / or root directory). In that case, the path starts with a slash and is called an absolute path, since there can be no mistake: only one file on the system can comply.

Paths that don't start with a slash are always relative to the current directory. In relative paths we also use the . and .. indications for the current and the parent directory.

The HOME variable

When you log onto the system, UNIX automatically places you in a directory called the home directory. The shell variable HOME indicates the home directory of the user.

            E.g.,

                    $  echo $HOME

                        /home/kumar

What you see above is an absolute pathname, which is a sequence of directory names starting from root (/). The subsequent slashes are used to separate the directories.

5. pwd - print working directory

At any time you can determine where you are in the file system hierarchy with the pwd, print working directory, command,

E.g.,:

         $  pwd

/home/frank/src

6. cd - change directory

You can change to a new directory with the cd, change directory, command. cd will accept both absolute and relative path names.

Syntax

cd [directory]

Examples

cd        changes to user's home directory

cd /      changes directory to the system's root

cd ..     goes up one directory level

cd ../.. goes up two directory levels

cd /full/path/name/from/root changes directory to absolute path named

          (note the leading slash)

cd path/from/current/location changes directory to path relative to current 

location (no leading slash)

7. mkdir - make a directory

You extend your home hierarchy by making sub-directories underneath it. This is done with the mkdir, make directory, command. Again, you specify either the full or relative path of the directory.

Examples

            mkdir patch     Creates a directory patch under current directory

            mkdir patch dbs doc   Creates three directories under current directory

            mkdir pis pis/progs pis/data    Creates a directory tree with pis as a directory under

the current directory and progs and data as subdirectories under pis

Note the order of specifying arguments in example 3. The parent directory should be specified first, followed by the subdirectories to be created under it.

The system may refuse to create a directory due to the following reasons:

1. The directory already exists.

2. There may be an ordinary file by the same name in the current directory.

3. The permissions set for the current directory don’t permit the creation of files and directories by the user.

8. rmdir - remove directory

A directory needs to be empty before you can remove it. If it’s not, you need to remove the files first. Also, you can’t remove a directory if it is your present working directory; you must first change out of that directory. You cannot remove a subdirectory unless you are placed in a directory which is hierarchically above the one you have chosen to remove.

E.g.

            rmdir patch      Directory must be empty

            rmdir pis pis/progs pis/data     Shows error as pis is not empty. However rmdir

silently deletes the lower level subdirectories progs

and data.

9. The PATH environment variable

Environmental variables are used to provide information to the programs you use. We have already seen one such variable called HOME.

A command runs in UNIX by executing a disk file. When you specify a command like date, the system will locate the associated file from a list of directories specified in the PATH variable and then executes it. The PATH variable normally includes the current directory also.

Whenever you enter any UNIX command, you are actually specifying the name of an executable file located somewhere on the system. The system goes through the following steps in order to determine which program to execute:

1. Built in commands (such as cd and history) are executed within the shell.

2. If an absolute path name (such as /bin/ls) or a relative path name (such as ./myprog),   the system executes the program from the specified directory.

3. Otherwise the PATH variable is used.

10. ls - list directory contents

The command to list your directories and files is ls. With options it can provide information about the size, type of file, permissions, dates of file creation, change and access.

Syntax

ls [options] [argument]

Common Options

When no argument is used, the listing will be of the current directory. There are many very useful options for the ls command. A listing of many of them follows. When using the command, string the desired options together preceded by "-".

-a Lists all files, including those beginning with a dot (.).

-d Lists only names of directories, not the files in the directory

-F Indicates type of entry with a trailing symbol: executables with *, directories with / and

     symbolic links with @

-R Recursive list

-u Sorts filenames by last access time

-t Sorts filenames by last modification time

-iDisplays inode number

-l Long listing: lists the mode, link information, owner, size, last modification (time). If the file is a symbolic link, an arrow (-->) precedes the pathname of the linked-to file.

The mode field is given by the -l option and consists of 10 characters. The first character is one of the following:

CHARACTER            IF ENTRY IS A

d                      directory

-                       plain file

b                      block-type special file

c                      character-type special file

l                       symbolic link

s                       socket

The next 9 characters are in 3 sets of 3 characters each. They indicate the file access permissions: the first 3 characters refer to the permissions for the user, the next three for the users in the Unix group assigned to the file, and the last 3 to the permissions for other users on the system.

Designations are as follows:

r read permission

w write permission

x execute permission

- no permission

Examples

1. To list the files in a directory:

$ ls

2. To list all files in a directory, including the hidden (dot) files:

$ ls -a

3. To get a long listing:

$ ls -al

total 24

drwxr-sr-x 5 workshop acs 512 Jun 7 11:12 .

drwxr-xr-x 6 root sys 512 May 29 09:59 ..

-rwxr-xr-x 1 workshop acs 532 May 20 15:31 .cshrc

-rw------- 1 workshop acs 525 May 20 21:29 .emacs

-rw------- 1 workshop acs 622 May 24 12:13 .history

-rwxr-xr-x 1 workshop acs 238 May 14 09:44 .login

-rw-r--r-- 1 workshop acs 273 May 22 23:53 .plan

-rwxr-xr-x 1 workshop acs 413 May 14 09:36 .profile

-rw------- 1 workshop acs 49 May 20 20:23 .rhosts

drwx------ 3 workshop acs 512 May 24 11:18 demofiles

drwx------ 2 workshop acs 512 May 21 10:48 frank

drwx------ 3 workshop acs 512 May 24 10:59 linda

11. The UNIX File System

The root directory has many subdirectories. The following table describes some of the subdirectories contained under root.

  

Basic File Attributes

The UNIX file system allows the user to access other files not belonging to them and without infringing on security. A file has a number of attributes (properties) that are stored in the inode. In this chapter, we discuss,

•         ls –l to display file attributes (properties)

•         Listing of a specific directory

•         Ownership and group ownership

•         Different file permissions

Listing File Attributes

            ls command is used to obtain a list of all filenames in the current directory. The output in UNIX lingo is often referred to as the listing. Sometimes we combine this option with other options for displaying other attributes, or ordering the list in a different sequence. ls look up the file’s inode to fetch its attributes. It lists seven attributes of all files in the current directory and they are:

•         File type and Permissions

•         Links

•         Ownership

•         Group ownership

•         File size

•         Last Modification date and time

•         File name

The file type and its permissions are associated with each file. Links indicate the number of file names maintained by the system. This does not mean that there are so many copies of the file. File is created by the owner. Every user is attached to a group owner. File size in bytes is displayed. Last modification time is the next field. If you change only the permissions or ownership of the file, the modification time remains unchanged. In the last field, it displays the file name.

For example,

$ ls –l

total 72

-rw-r--r--                1  kumar  metal  19514  may  10  13:45          chap01

-rw-r--r--                1  kumar  metal    4174  may  10  15:01          chap02

-rw-rw-rw-            1  kumar  metal       84   feb   12   12:30         dept.lst

-rw-r--r--                1  kumar  metal    9156  mar  12    1999         genie.sh

drwxr-xr-x             2  kumar  metal      512  may   9   10:31          helpdir

drwxr-xr-x             2  kumar  metal      512  may   9   09:57          progs

Listing Directory Attributes

ls -d will not list all subdirectories in the current directory

For example,

ls –ld helpdir progs

drwxr-xr-x  2  kumar  metal      512  may   9   10:31 helpdir

drwxr-xr-x  2  kumar  metal      512  may   9   09:57 progs

Directories are easily identified in the listing by the first character of the first column, which here shows a d. The significance of the attributes of a directory differs a good deal from an ordinary file. To see the attributes of a directory rather than the files contained in it, use ls –ld with the directory name. Note that simply using ls –d will not list all subdirectories in the current directory. Strange though it may seem, ls has no option to list only directories.

File Ownership

When you create a file, you become its owner. Every owner is attached to a group owner. Several users may belong to a single group, but the privileges of the group are set by the owner of the file and not by the group members. When the system administrator creates a user account, he has to assign these parameters to the user:

The user-id (UID) – both its name and numeric representation

The group-id (GID) – both its name and numeric representation

File Permissions

UNIX follows a three-tiered file protection system that determines a file’s access rights. It is displayed in the following format:

Filetype owner (rwx) groupowner (rwx) others (rwx)

For Example:

-rwxr-xr--  1   kumar   metal   20500  may 10 19:21  chap02

               r w x                             r - x                          r - -

owner/user                   group owner                            others

The first group has all three permissions. The file is readable, writable and executable by the owner of the file. The second group has a hyphen in the middle slot, which indicates the absence of write permission by the group owner of the file. The third group has the write and execute bits absent. This set of permissions is applicable to others.

            You can set different permissions for the three categories of users – owner, group and others. It’s important that you understand them because a little learning here can be a dangerous thing. Faulty file permission is a sure recipe for disaster

Changing File Permissions

A file or a directory is created with a default set of permissions, which can be determined by umask. Let us assume that the file permission for the created file is -rw-r--r--. Using chmod command, we can change the file permissions and allow the owner to execute his file. The command can be used in two ways:

In a relative manner by specifying the changes to the current permissions

In an absolute manner by specifying the final permissions

Relative Permissions

chmod only changes the permissions specified in the command line and leaves the other permissions unchanged. Its syntax is:

            chmod category operation permission filename(s)

chmod takes an expression as its argument which contains:

      user category (user, group, others)

      operation to be performed (assign or remove apermission)

      type of permission (read, write, execute)

Category                    operation                    permission

u - user                                    + assign                       r - read

g - group                      - remove                      w - write

o - others                     = absolute                    x - execute

a - all (ugo)

Let us discuss some examples:

Initially,

-rw-r--r--          1          kumar    metal  1906   sep      23:38   xstart

chmod u+x xstart

-rwxr--r--         1          kumar    metal  1906   sep      23:38   xstart

The command assigns (+) execute (x) permission to the user (u), other permissions remain unchanged.

chmod ugo+x xstart     or

chmod a+x xstart         or

chmod +x xstart

-rwxr-xr-x        1          kumar    metal  1906   sep      23:38   xstart

chmod accepts multiple file names in command line

chmod u+x note note1 note3

Let initially,

-rwxr-xr-x        1   kumar   metal   1906   sep 23:38    xstart

chmod go-r xstart

Then, it becomes

-rwx--x--x        1   kumar   metal   1906   sep 23:38    xstart

Absolute Permissions

Here, we need not to know the current file permissions. We can set all nine permissions explicitly. A string of three octal digits is used as an expression. The permission can be represented by one octal digit for each category. For each category, we add octal digits. If we represent the permissions of each category by one octal digit, this is how the permission can be represented:

·         Read permission – 4 (octal 100)

·         Write permission – 2 (octal 010)

·         Execute permission – 1 (octal 001)

Octal               Permissions                Significance

0                                  - - -                  no permissions

1                                  - - x                  execute only

2                                  - w -                 write only

3                                  - w x                write and execute

4                                  r - -                  read only

5                                  r - x                  read and execute

6                                  r w -                 read and write

7                                  r w x                read, write and execute

            We have three categories and three permissions for each category, so three octal digits can describe a file’s permissions completely. The most significant digit represents user and the least one represents others. chmod can use this three-digit string as the expression.

Using relative permission, we have,

chmod a+rw xstart

Using absolute permission, we have,

             chmod 666 xstart

             chmod 644 xstart

             chmod 761 xstart

will assign all permissions to the owner, read and write permissions for the group and only execute permission to the others.

777 signify all permissions for all categories, but still we can prevent a file from being deleted. 000 signifies absence of all permissions for all categories, but still we can delete a file. It is the directory permissions that determine whether a file can be deleted or not. Only owner can change the file permissions. User can not change other user’s file’s permissions. But the system administrator can do anything.

The Security Implications

Let the default permission for the file xstart is

                        -rw-r--r--

chmod u-rw,go-r xstart                                               or

chmod 000 xstart

            ----------

This is simply useless but still the user can delete this file

On the other hand,

chmod a+rwx xstart

chmod 777 xstart

                        -rwxrwxrwx

The UNIX system by default, never allows this situation as you can never have a secure system. Hence, directory permissions also play a very vital role here

We can use chmod Recursively.

chmod -R a+x shell_scripts

This makes all the files and subdirectories found in the shell_scripts directory, executable by all users. When you know the shell meta characters well, you will appreciate that the * doesn’t match filenames beginning with a dot. The dot is generally a safer but note that both commands change the permissions of directories also.

Directory Permissions

It is possible that a file cannot be accessed even though it has read permission, and can be removed even when it is write protected. The default permissions of a directory are,

                       

                             rwxr-xr-x (755)

A directory must never be writable by group and others

Example:

mkdir c_progs

ls –ld c_progs

drwxr-xr-x       2  kumar  metal  512  may  9  09:57  c_progs

            If a directory has write permission for group and others also, be assured that every user can remove every file in the directory. As a rule, you must not make directories universally writable unless you have definite reasons to do so.

Changing File Ownership

Usually, on BSD and AT&T systems, there are two commands meant to change the ownership of a file or directory. Let kumar be the owner and metal be the group owner. If sharma copies a file of kumar, then sharma will become its owner and he canmanipulate the attributes

chownchanging file owner and chgrp changing group owner

On BSD, only system administrator can use chown

On other systems, only the owner can change both

chown

Changing ownership requires superuser permission, so use su command

            ls -l note

            -rwxr----x        1 kumar  metal  347  may  10  20:30  note

chown sharma note; ls -l note

            -rwxr----x        1 sharma  metal  347  may  10  20:30  note

            Once ownership of the file has been given away to sharma, the user file permissions that previously applied to Kumar now apply to sharma. Thus, Kumar can no longer edit note since there is no write privilege for group and others. He can not get back the ownership either. But he can copy the file to his own directory, in which case he becomes the owner of the copy.

chgrp

This command changes the file’s group owner. No superuser permission is required.

                        ls –l dept.lst

-rw-r--r--          1  kumar  metal  139  jun  8  16:43  dept.lst

                        chgrp dba dept.lst; ls –l dept.lst

-rw-r--r--          1  kumar  dba  139  jun  8  16:43  dept.lst

            In this chapter we considered two important file attributes – permissions and ownership. After we complete the first round of discussions related to files, we will take up the other file attributes.