2 Manual & Filestore

Introduction

• Using the manual• The UNIX filestore• File permissions

The on-line manual• man <command> gives detailed

information about UNIX commands and other facilities

• Details include command format, description, examples, known problems, related files and commands

• Sections for commands, programming, admin and others

• man intro for the introduction• man man (of course)

UNIX Filestore

• Files and directories

• The filesystem hierarchy

• File handling commands

• Permissions

Files• From the user’s point of view, all

information on the computer is stored in files

• Files may contain many kinds of information, including programs, data and documents

• Like paper files, they have a name (chosen by the user) and some content

• By convention, the filename suffix suggests the type of content

• In some operating systems (for example Unix) it is optional, while in some others (such as Windows) it is a requirement

Example files

• Me.sh = Shell script• Me.pl = Perl script• Me.txt = Text file• Me.jpg, Me.png, Me.gif = Image files• Me.html = HTML web page file• Me.zip, Me.gz Me.bz2 = Compressed file• Me.tar = Archived file• Me.log = Log fileThese are just conventions and are not enforced by

the operating system!http://filext.com/alphalist.php?extstart=%5EA

Directories

• Files are stored in directories (folders in Windows)

• Each directory may contain many files and also other directories

• By convention, directory names do not usually have suffixes (why not?)

• A file should have an extension (why?)

The filestore

• There is a single hierarchical filestore that is shared by all users

• It might be spread over the network, may involve many disks on may different computers and may even be linked to other filestores

• Each user has their own personal home directory (~)

• There are also common areas for programs, administration, etc.

• The filestore starts at the root directory (/)

• A user is always in a current working directory (CWP) from which they give commands to access files

• When users log on, the CWP is set to their home directory

The filestore

(root)

staff usr bin stud etc

Research Teaching Private

pgugitmastersxxxgtrxxx

CUA

Coursework1.txt

CUA

xxx02u xxx04u

MVR

Lecture1.ppt

Lecture2.doc

.profile

The filestore

Absolute and Relative Pathnames

• There are two ways of specifying filenames– from the current directory - relative

pathnames– from the root directory - absolute pathnames

• Simple pathnames consist of sequences of names separated by ‘/’ characters

• An example of a relative pathname:../myDoc.txt

• An example of an absolute pathname:/stud/ug/xxx04u/Documents/myDoc.txt

Where am I in the File Structure?

• pwd prints the pathname of the current working directory

• cd pathname changes current directory– with no argument it goes to the home

directory

Where am I in the File Structure? (2)

robin$ pwd/stud/ug/xxx04urobin$ cd USTrobin$ pwd/stud/ug/xxx04u/UST

robin$ pwd/stud/ug/xxx04u/USTrobin$ cd ..robin$ pwd/stud/ug/xxx04u/

(root)

staff usr bin stud etc

Research Teaching Private

pgugitmastersxxxgtrxxx

CUA

Coursework1.txt

UST

xxx02u xxx04u

MVR

Lecture1.ppt

Lecture2.doc

.profile

File handling commands – Viewing File Contents

• The cat [filename…] command displays the contents of the named files

• It reads the contents of the file(s) and outputs to the shell window

• with no arguments cat simply echoes back what you type at the keyboard

• there is no scrolling with the cat command

• more [filename…] lets you scroll through a file– also less [filename…]

Viewing File Contents (2)

• head displays the first 10 lines of a file

• tail displays the last 10 lines of a file

• ls [pathname…] lists the contents of the named directories– with no argument, the current directory is

listed• mv pathname1 pathname2 moves a file

from pathname1 to pathname2– if pathname2 already exists, then its

previous contents are lost– if not, then it is created

Viewing and Changing Directory Contents

• cp pathname1 pathname2 copies the contents of pathname1 to pathname2

– if pathname2 already exists, – then its previous contents are lost– if not, then it is created

• rm pathname removes files but not directories (be careful)

Viewing and Changing Directory Contents (2)

• mkdir pathname ... creates a new directory– it fails if they already exist

Manipulating Directories

robin$ rmdir fredrmdir: directory “fred”: Directory not empty

robin$ mkdir fredrobin$ mkdir fredmkdir: failed to make directory “fred”; File exists

• rmdir pathname removes directories– it fails if they are not empty

Printing

• lpr pathname prints files– it goes to your default printer– you can specify a printer with the -P flag, for

example lpr -Phet fred.ps• do not send text files and program listings to laser

printers• do not send postscript files to line printers!

• Printing not configured on unnc-cslinux• Most of these commands have many flags

for specifying different options

More about pathnames• Special symbols can be used in pathnames.. the directory one level above this../.. the directory two levels above this. the current directory~ this user’s home directory~user a specific user’s home directory* wildcard matching any string? wildcard matching any single

character

Relative and Absolute Pathnames Revisited

(root)

staff usr bin stud etc

Research Teaching Private

pgugitmastersxxxgtrxxx

CUA

Coursework1.txt

CUA

xxx02u xxx04u

MVR

Lecture1.ppt

Lecture2.doc

.profile

/

../../.. /stud

You are here!

/stud/ug/xxx04u..

/stud/ug../..

../../../..

~ ~xxx04u

Relative Absolute

. /stud/ug/xxx04u/CUA

Security and file permissions

• A shared filestore needs a security mechanism to prevent the unauthorised reading and writing of files

• UNIX associates a permissions list with each file saying who can do what to it

• Each directory also has a permissions list

• How you can access a file depends upon its permissions list and those of all its parent directories

Permissions lists• Specify who can do what• There are three kinds of who:

u the user (owner)g members of the user’s groupo others - anyone else

• There are three kinds of what:r readw writex execute

Permissions lists (2)

• Use ls -l to see permissions lists• (think of –l as ‘long’)

drwxr--r-- 1 gtr staff 173568 Sep 21 15:41 Labsdrwxr--r-- 1 gtr staff 202752 Sep 21 17:43 Lectures-rw-r--r-- 1 gtr staff 2638848 Oct 6 18:58 Outline.doc-rw-r----- 1 gtr staff 87552 Oct 6 18:19 Intro.ppt

Permissions lists (3)

type user group other

- rw- r-- r--

d rwx --- ---

- rw- rw- rw-

Each permissions list is shown as ten characters:

Group’s permissions(read only)

User’s permissions

(read, write and execute)

Others’ permissions

(read only)File/Directory name

File size

Group that user is in

UserDate file was last modified/created

Directory

-rwxr--r-- 1 gtr staff 5173568 Sep 21 15:41 cve_userdrwxr--r-- 1 gtr staff 202752 Sep 21 17:43 Reports-rw-r--r-- 1 gtr staff 2638848 Oct 6 18:58 Yr1Report.doc-rwxr--r-- 1 gtr staff 5587552 Oct 6 18:19 plod_node_mgr

The effect of directory permissions

• Execute lets you change (cd) into that directory– (i.e. you are searching the directory)

• Read lets you list files in the directory– (i.e. you are reading the directory contents)

• Write lets you create and delete files– (i.e. you are writing to the directory contents)

• To use a file at all you must have execute permission on all of its parents (otherwise it is as if it doesn’t exist)

other parents up to the root

file

parent directory

execute permission controls whether any files and directories below this point can be accessed at all

permissions control whether files can be accessed at all, listed, created and deleted

permissions control specific actions on this file

Setting and changing permissions -chmod

• chmod mode pathname is used to alter permissions lists

• Mode specifies a sequence of changes, each of the form who operation permission– who is u, g or o– operation is + (grant) or - (revoke)– permission is r, w or x

• Examples:chmod o-r g-r plan.docchmod u+rwx Admin

The mode can also be a three digit octal number that is interpreted as a sequence of nine bits to set the whole permissions list at once:– chmod 644 progress.txt

644 is 110 100 100 which is interpreted as rw- r-- r--

– chmod 777 progress.txt777 is 111 111 111 which is interpreted rwx rwx rwx

– chmod 400 progress.txt400 is 100 000 000 which is interpreted as r-- --- ---

Setting and changing permissions – chmod (2)

Setting and changing permissions – chmod (3)

• Files are created with a default permission– usually -rw- r-- r--– depends upon the command used to

create the file– set using the umask command

Setting and changing permissions – chmod (3)

• Files are created with a default permission– usually -rw- r-- r--– depends upon the command used to

create the file– set using the umask command

Nice quote 1

• At a recent computer expo (COMDEX), Bill Gates reportedly compared the computer industry with the auto industry and stated, "If GM had kept up with technology like the computer industry has, we would all be driving $25.00 cars that got 1,000 miles to the gallon."

(also size – you could fit a Rolls Royce on the head of a pin. )

Nice quote 2

• In response to Bill's comments, General Motors issued a press release stating: If GM had developed technology like Microsoft, we would all be driving cars with the following characteristics:

• 1. For no reason whatsoever, your car would crash twice a day.

Next Lecture

• UNIX commands for handling files• Regular Expressions and Searching

files• Redirecting output• Bash facilities

Summary

• Manual Pages

• The UNIX filestore

• Pathnames

• Security and file permissions