PowerPoint Presentation Microsoft notes .ppt

08/20/24 Files, types & access permissions 1
Files, File types and Access
Permissions

Introduction
 Every file on a Linux system is owned
by a user, and users cannot change
or even read each others' files
without being given permission
 Because of this, and because
unprivileged users do not normally
operate with root-level access, Linux
is less susceptible to viruses

Users and groups
 Each user is identified uniquely by a
username and UID, and they have a
home directory
 The user has full access to their home
directory – they can create, move,
and remove files & directories in
anyway subject to limitations such as
disk quotas

Users and groups
 A users ability to gain access to files
& directories depends on the
permissions of the file, uid, & group
affiliations
 Users may be assigned to groups, &
group permissions may then be
applied to files & directories for
collective access

Users and groups
 Groups are uniquely identified by
group names & GIDs
 By default, every user belongs to at
least one group (user private group,
named after their user name) & they
are the only member of that group

File types
 Any information stored on disk is
stored in files.
 UNIX and Linux supports different file
types, some common ones are:

File types
File type meaning
(-) a normal file Stores collection of bytes of
information. Formats include
GIF, word document, etc.
The command file can be
used to determine the type
of a file

File types
(d) directory file Special type of file that
contains names of files and
their i-nodes
(b) block device
file
Special file type that
provides access to a
character device
(c) character
device file
Special type of file that
provides access to block
device

File Access Permissions
 Every file & directory has permissions
set that determine who can access it
 Permissions are set for the owner
(user – u) of the file, the group
members (g), and all others
 Permissions that are set on a file are
read (r), write (w) & execute (x)

 The owner of the file is allowed to set
these permissions on the file.
 These permissions mean the following
when applied to files or directories:

Operation Effect on a file Effect on a
directory
read Can display
(cat, less)
contents of file
can list (ls)
contents of file
write can delete and
modify the file
can create &
delete files in
that directory

execute can run the file or
program if its an
executable file
can access a file
in that directory,
can do a long
listing (ls -al) of
the directory

Users, groups and others
 Processes wishing to access a file on
a UNIX/Linux system are placed into
one of 3 categories:
 User - the individual user who owns
the file
 Group - the collection of people that
belong to the group that owns the file
 Other - anybody who doesn't fall into
the first two categories

File permissions
 Each user category (user, group &
other) has a separate set of file
permissions, which control what file
operations each particular user
category can perform
 File permissions consist of 4 fields:
 file type, user permissions, group
permissions, and other permissions.

File permissions

File permissions
-rw- r-- r-- 1 rahman users 0 2007-
03-08 22:34 sanya1
 In the above output,
 owner can read and write the file (rw-)
 group can only read the file (r- -)
 other can also only read the file (r- -)
 A – represents no permission.

Symbolic and Numeric
permissions
 Symbolic Permissions - uses the
symbols r, w, and x to represent
permissions on a file, hence rwxr-x-
w- is referred to as symbolic
permissions.
 The numeric or absolute permissions
represents file permissions using
numbers

Symbolic and Numeric permissions
 The OS itself doesn't use symbols to
represent permissions, instead it uses
numbers
 The numeric method uses the
numbers 4, 2, 1 for read, write and
execute permissions, respectively.
 Research on how to convert from
symbolic to numeric permissions

Changing File Permissions
 UNIX/Linux provide commands for
users to change the permissions
associated with a file
 chmod – change permissions for a file
 chown – change the user owner of a file
 umask – set the default file permissions
for files to be created
 chgrp – change the group owner of a file

chmod
 The chmod command is used to
change a file's permissions
 Only the user who owns the file and
root can change permissions of a file
 Its format is:
#chmod [-R] operation files

chmod
 The optional switch -R causes chmod
to recursively descend any
directories, changing all file
permissions as it goes
 Example:
#chmod 770 mamanya.file
 will change the permissions of the file
mamanya.file to the numeric
permissions 770

Examples
 chmod u+rwx temp.dat
 Add rwx permission for owner of the file
 chmod go-rwx temp.dat
 Remove all permissions for the group
and other categories
 chmod -R a-rwx /etc
 Turn off all permissions, for all users, for
all files in the /etc directory

Examples
 chmod -R a= /
 Turn off all permissions for everyone for
all files
 chmod 770 temp.dat
 Allow the user and group read, write
and execute, and others no access

chown
 Used to change ownership of a file
 Used only by root for two reasons:
 In a fs with quotas, a user could avoid
the quota system by giving away the
ownership of a file to another user
 Security - if anyone can give ownership
of a file to root, they could create a
program that is setuid to the owner of
the file and then change the owner of
the file to root

chown
 Format: chown [-R] owner files
 owner is either a numeric user
identifier or a username
 file is a list of files of which you
wish to change the ownership

chown
 Some systems allow owner in the
chown command to take the format
owner.group. This allows you to
change the owner and the group
owner of a file with one command

Examples
 chown david /home/david
 Change the owner of the directory
/home/david to david
 chown -R root /
 Change the owner of all files to root
 chown david.users /home/david
 Change ownership of file /home/david to
user david and group users

chgrp
 Used to change group owner of a file
 Any user can use chgrp to change
any file they are the owner of
 However, you can only change the
group owner of a file to a group to
which you belong

chgrp
 Format: chgrp [-R] group files
 group is either a numeric group
identifier or a group name
 files is a list of files of which you
wish to change the ownership

Example
 chgrp users /home/david
 Change the group owner of the
directory /home/david to the group
users

Default permissions
 When you create a new file it
automatically receives a set of file
permissions, e.g
 rahman@linux:~$ touch sanya1
 rahman@linux:~$ ls -l sanya1
 -rw-r--r-- 1 rahman users 0 2007-03-
08 22:34 sanya1

Default permissions
 Above, the command touch has been
used to create an empty file sanya1
 The file has been given the default
permissions rw-rw-r–
 Any file created on this machine will
receive the same default permissions

Default permissions
 A new directory also receives the
same set of file permissions with the
addition of execute permissions in
order to make it accessible
 rahman@linux:~$ mkdir sanya
 rahman@linux:~$ ls -ld sanya
 drwxr-xr-x 2 rahman users 72 2007-
03-08 22:34 sanya

Default permissions
 By default all users are able to see
the contents of the directory but only
the owner can add or remove files
from the directory
 Any new directory created on this
machine will receive these default
permissions: drwxr-xr-x

umask
 The built-in shell command umask is
used to specify and view what the
default file permissions are
 Running umask without any
arguments displays the current
default permissions
 The -S option displays the default
permissions in symbolic format

umask
 rahman@linux:~$ umask
 0002
 By default, umask uses the numeric
format for permissions
 It returns a number that specifies
which permissions are turned off
when a file is created

umask
 In the example above:
 user and group have the value 0,
meaning that by default no
permissions are turned off for them
 other has the value 2 – by default
the write permission is turned off
 Read: how to use umask to set
default permissions

File and Directory Commands
 cd - change directory
 ls - list file/directory names
 cp - copy files and directories
 mv - move and/or rename files and
directories
 rm - remove files
 mkdir - make directory

File and Directory Commands
 rmdir - remove an empty directory
 rm -r - recursively remove a directory
and its contents
 file - determine type of file content
 cat - view file content
 less - view text page by page
 find – find a file

Exercise
 See hand out for exercise on file and
directory permissions

PowerPoint Presentation Microsoft notes .ppt

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