UNIX FILE SYSTEM

File Systems in Unix

Contents

1 Introduction

In Unix, the files are organized into a tree structure with a root named by the character ’/’. The first few

levels of the tree look like this:

/

|

---------------------

/ | | | \

etc bin usr tmp dev

| |

------ --------

/ \ / \

ls .. csh ucb ... lib

Your own files form a subtree of this tree. For example, in many systems the user files are subdirectories

of a directory named ‘home’ within ‘usr’; if we had users Jack and Jill, for example, Jack’s home directory

would be /usr/home/jack, and all his files would be within that subtree, and the analogous statement would

hold for Jill.

Suppose Jill’s directory looks like this:

jill

|

/ | | | \

hill water pail story misc

| |

/ \ |

fresh salt rocks

File names can be given either in relative terms, or with full path names. Look at the file ‘salt’ above. If we

are in the directory ‘water’, we can refer to this file as simply

salt

If we are in the directory above, i.e. the one named ‘jill’, then we must write

water/salt

If we are in the directory ‘misc’, we can write either

../salt

or

˜/water/salt

If we are not in any of Jill’s directories, we can write

˜jill/water/salt

In any case, the full pathname will work:

/usr/home/jill/water/salt

2 File Types

There are four types of files in the Unix file system.

2.1 Ordinary Files

An ordinary file may contain text, a program, or other data. It can be either an ASCII file, with each of its

bytes being in the numerical range 0 to 127, i.e. in the 7-bit range, or a binary file, whose bytes can be of all

possible values 0 to 255, in the 8-bit range.

2.2 Directory Files

Suppose that in the directory x I have a, b and c, and that b is a directory, containing files u and v. Then b

can be viewed not only as a directory, containing further files, but also as a file itself. The file b consists of

information about the directory b; i.e. the file b has information stating that the directory b has files u and v,

how large they are, when they were last modified, etc.

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2.3 Device Files

In Unix, physical devices (printers, terminals etc.) are represented as “files.” This seems odd at first, but it

really makes sense: This way, the same

read()

and

write()

functions used to read and write real files can

also be used to read from and write to these devices.

2.4 Link Files

Suppose we have a file X, and type

ln X Y

If we then run

ls

, it will appear that a new file, Y, has been created, as a copy of X, as if we had typed

cp X Y

However, the difference is the

cp

does create a new file, while

ln

merely gives an alternate name to an old

file. If we make Y using

ln

, then Y is merely a new name for the same physical file X.

3 Obtaining Information About the Files in a Given Directory

The ‘a’ (“all”) and ‘l’ (“long”) options of the

ls

command will give us a lot of information about files in a

specified directory (if we don’t specify a directory, then the current directory is assumed). Here is a sample

output from typing

ls -al

drwxr-xr-x 6 ecs4005 1024 Apr 22 13:30 ./

drwxr-xr-x 74 root 1536 Mar 24 12:51 ../

-rw------- 1 ecs4005 188 Apr 13 15:53 .login

-rw------- 1 ecs4005 6 Mar 24 11:29 .logout

-rw------- 1 ecs4005 253 Apr 10 12:50 .xinitrc

-rw-r--r-- 1 ecs4005 516 Apr 10 13:00 .twmrc

-rw-r--r-- 1 ecs4005 1600 Apr 22 10:59 test2.out

The

ls

command gets its information about the directory b by reading the file b.

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The output is separated into six columns:

1st column - access permissions (see below)

2nd column - number of file entries (in the case of directory files)

3rd column - owner

4th column - size in bytes

5th column - date and time of last modification

6th column - name

4 File Access Control

In Unix, all files are protected under some access control mechanism, so that the owner of a file can deny

access of his files to other users. The first column of the long directory list shows the access characteristics

of a file, in te form of 10  ags, e.g. drwxr-xr-x.

The meanings of the  ags are shown below:

Position 1 file type: d (directory)

- (ordinary file)

l (symbolic link)

Position 2-4 permissions for the owner: r (read)

w (write)

x (execute)

Position 5-7 permissions for other users in the same group

Position 8-10 permissions for all other users

Note that a hyphen (‘-’) denotes lack of the given permission type. For example, r-x would mean that read

and execute permission are granted, but not write permission.

In order to remove a file, you must have write permission for it.

In order to view the contents of a directory, i.e. see what files are there, you need read permission for that

directory. In order to actually access a file (read from it, write to it, or execute it) in the directory, you need

execute permission for the directory.

5 Some File Commands

5.1 chmod

You can use this command to change the access permissions of any file for which you are the owner. The

notation used is:

u user (i.e. owner)

g group

o others

+ add permission

- remove permission

r read

w write

x execute

As an example, the command

chmod ugo+rw .login

would add read and write permission for all users to the .login file of the person issuing this command.

In some cases it is useful for a user to deny himself/herself permission to write to a file, e.g. to make sure

he/she doesn’t remove the file by mistake.

5.2 du and df

The du command displays the sizes in kilobytes of all files in the specified directory, and the total of all

those sizes; if no directory is specified, the current directory is assumed.

The df command displays the amount of unused space left in your disk systems.

5.3 diff

This command displays line-by-line differences between two ASCII files. If for example, you have two

versions of a C source file but don’t remember how the new version differs from the old one, you could type

diff oldprog.c newprog.c

6 Wild Cards

These will save you a lot of typing!

There are two wild-card characters in Unix, ‘*’ and ‘?’.

The wildcard ‘*’ will matches with any string of characters. For example,

rm *.c

would delete all files in the current directory whose names end with ‘.c’.

The wildcard ‘?’ will match with any single character. For example,

rm x?b.c

would delete all files whose names consisted of five characters, the first of which was ‘x’ and the last three

of which were ‘b.c’. Example: rm prog?.c will delete all the files (in the current directory) The files x3b.c

and xrb.c would be deleted, while the file xuvb.c would not.

In addition,

[0-9] matches character from ‘0’ through ‘9’

[a-z] matches character from ‘a’ through ‘z’

For instance,

rm test[1-3].c

would remove test1.c, test2.c and test3.c but not test4.c.