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

2. 08/20/24 Files, types & access permissions 2
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

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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

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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

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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

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

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

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File types
(d) directory fileSpecial 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

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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)

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File Access Permissions
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:

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File Access Permissions
OperationEffect on a fileEffect 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

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File Access Permissions
executecan 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

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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

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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.

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File permissions

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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.

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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