UNIX Chapter4

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UNIX

Reference:

UNIX Concepts and Applications – 4th Edition

By

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The File System

• The file is a container for storing information.

• We can treat it simply as a sequence of characters.

• Unlike the old DOS files, a UNIX file doesn’t contain the eof (end – of – file) mark.

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The File System

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The File System

• UNIX treats directories and devices as files as well.

• A directory is simply a folder where you store filenames and other directories.

• All physical devices like hard disk, memory, printer, and modem are treated as files.

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The File System

• Ordinary file – also known as regular file. It contains only data as a stream of characters.

• Directory file – it’s commonly said that a directory contains files and other directories, but strictly speaking, it contains their names and a number associated with each name.

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Ordinary (Regular) file

• Divided into two types –

– Text file

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Ordinary (Regular) file

• Text file – contains only printable characters, and you can often view the contents and make sense out of them.

All C, java program sources, shell and perl scripts are text files.

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Ordinary (Regular) file

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Ordinary (Regular) file

• Binary file – contains both printable and unprintable characters that cover the entire ASCII range (0 – 255).

Most UNIX commands are binary files.

The object code and executables that you produce by compiling C programs are also binary files.

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Directory file

• A directory contains no data, but keeps some details of the files and subdirectories that it contains.

• A directory file contains an entry for every file and subdirectory that it houses.

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Directory file

• Each entry has two components:

– The filename

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Directory file

• You can’t write a directory file, but you can perform some action that makes the kernel write a directory.

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Device file

• All devices and peripherals are represented by files.

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Device file

• Device filenames are generally found inside a single directory structure, /dev.

• A device file is indeed special; it’s not a stream of characters.

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Device file

• The operation of a device is entirely governed by the attributes of its associated file.

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What’s in (File)name?

• In UNIX, a filename can consists of up to 255 characters.

• Files may or may not have extensions, can consists of practically any ASCII character except the / and the NULL character (ASCII value 0).

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What’s in (File)name?

• Example of valid filenames

– last_time

– list.

– ^V^B^D-++bcd

– {}[]

– @#$%*abcd

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What’s in (File)name?

• Recommended that only the following characters be used in the filenames:

– Alphabetic characters and numerals

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

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The parent – child relationship

• All files in UNIX are “related” to one another.

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The parent – child relationship

• There is a top, which serve as the reference point for all files.

• This top is called root and is represented by a / (front slash).

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The parent – child relationship

• It is conceptually different from the user-id root used by the system administrator to log in.

• Every file, apart from root, must have a parent.

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HOME variable

• When you log on to the system, UNIX automatically places you in a directory called the home directory.

• It is created by the system when a user account is opened.

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HOME variable

• The shell variable HOME know your home directory.

$ echo $HOME /home/directory

• What we see above is an absolute pathname.

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HOME variable

• The home directory is determined by the system administrator at the time of opening a user account.

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HOME variable

• Its often convenient to refer to a file dd located in your home directory as $HOME/dd.

• Most shells (except Bourne) use the ~ symbol for this purpose.

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HOME variable

• The ~ symbol is a little tricky to use because it can refer to any user’s home directory and not just your own.

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HOME variable

• The principle is this:

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pwd : Checking your current

cd : Changing the current directory

• You an move around in the file system using the cd (change directory) command.

• When used with an argument, it changes the current directory to the directory specified as argument.

$ pwd

/home/dd

$ cd progs progs must be in current dir $ pwd

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cd : Changing the current directory

Alternatively we can do

$ pwd

/home/dd

$ cd /home/dd/progs $ pwd

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cd : Changing the current directory

$ pwd

/home/dd/progs $ cd /bin

$ pwd /bin

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cd : Changing the current directory

cd can be used without any arguments:

$ pwd

/home/dd/progs $ cd

$ pwd

/home/dd

cd used without arguments reverts to the home directory.

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mkdir : Making directories

• Directories are created with mkdir (make directories) command.

• The command is followed by names of the directories to be created.

$ mkdir patch

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mkdir : Making directories

• You can create a number of subdirectories with one mkdir command

$ mkdir patch1 dbs doc

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mkdir : Making directories

• Following command creates a directory tree

$ mkdir pis pis/progs pis/data

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mkdir : Making directories

$ mkdir pis/progs pis/data pis

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mkdir : Making directories

• Sometimes, the system refuses to create a directory. The reasons are as follows:

– The directory may already exist

– There may be an ordinary file by that name in the current directory

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rmdir : Removing directories

• The rmdir (remove directory) command removes directories.

$ rmdir pis

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rmdir : Removing directories

• rmdir can also delete more than one directory

in one shot.

$ rmdir pis/data pis/progs pis

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rmdir : Removing directories

• Two important rules when deleting directories

– You can’t delete a directory with rmdir unless it is empty.

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mkdir & rmdir

• The mkdir and rmdir commands work only in directories owned by the user.

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How files and directories are

created and removed

• A file (ordinary or directory) is associated with a name and a number, called inode number.

• When a directory is created, an entry comprising these two parameters is made in the file’s parent directory.

• The entry is removed when the directory is removed.

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Absolute pathnames

$ cat /home/dd/login.sql

If the first character of a pathname is /, the file’s location must be determined with respect to root (the first /).

Such a pathname, as one above, is called an

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Absolute pathnames

Since date resides in /bin (or /usr/bin), you can also use the absolute pathname:

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Relative pathnames

A relative pathname is just a path that starts from your current directory, and thus, doesn't start with a slash (/) character.

• Any example relative path is: homeworks/test1

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Using . and .. in relative pathnames

• UNIX offers a shortcut to the relative pathname – that uses either the current or parent directory as reference, and specifies a path relative to it.

– A single dot (.) represents the current directory

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Using . and .. in relative pathnames

$ pwd

/home/kumar/progs/data/text $ cd .. (moves one level up)

$ pwd

/home/kumar/progs/data

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Using . and .. in relative pathnames

$ pwd

/home/kumar/pis

$ cd ../.. (moves two levels up)

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Using . and .. in relative pathnames

$ cp ../sharma/.profile .

A filename can begin with a .

(This copies the file .profile to the current directory (.))

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ls : Listing directory contents

To list the contents of the current directory:

$ ls

README bago.hlp helpex.hlp hyperhelp app-defaults helpex hoh.hlp

• This lists the contents of the current directory.

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ls : Listing directory contents

To list each file's type and inode number:

$ ls -Fi

6752 Mail/ 102574 SGML/ 147215 UNIX_tips/

172802 Xapps/ 51200 bin/ 44801 dead.letter

44802 mbox 44809 test* 102733 tmp/

• This lists the contents of the current directory.

• Directories are shown with a trailing '/';

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ls : Listing directory contents

To list the contents of a directory using wildcard characters:

$ ls *.txt

udi1.txt wais_x500_www.txt

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ls : Listing directory contents

To list files in another directory:

$ ls ../Mail

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ls : Listing directory contents

• To list each subdirectory and its contents, repeating this recursion:

$ ls -R uunet

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ls : Listing directory contents

Option Action

-a list hidden files

-d list the name of the current directory -F show directories with a trailing '/' executable files with a trailing '*'

-g show group ownership of file in long listing

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ls : Listing directory contents

Option Action

-l long listing giving details about files and directories

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The UNIX file system

• /bin and /usr/bin

• /sbin and /usr/sbin

• /etc

• /dev

• /lib and /usr/lib

• /usr/include

• /usr/share/man

• /tmp

• /var

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The UNIX file system

• /bin and /usr/bin: here all the commonly used UNIX

commands are found. PATH variable always shows these directories in its list.

• /sbin and /usr/sbin: if there is a command that you

can not execute but the system administrator can, then it would probably be in one of these directories.

• /etc: this directory contains the configuration files of

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The UNIX file system

• /dev: this directory contains all device files. These

files don’t occupy space on disk. There could be more subdirectories like pts, dsk and rdsk in this directory.

• /lib and /usr/lib: contains all library files in binary

form. You will need to link your c programs with files in this directory.

• /usr/include: contains the standard header files used

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The UNIX file system

• /usr/share/man: here the man pages are stored.

• /temp: here the users are allowed to create

temporary files. These files are wiped away regularly by the system.

• /var: the variable part of the file system. Contains all

your print jobs and your outgoing and incoming mails.

• /home: on many systems the users are housed here.