Basic
File Permissions in Linux
Basic
File Permissions
This
lesson will cover the following commands:
- chmod ‐ modify file access rights
- su ‐ temporarily become the superuser
- chown ‐ change file ownership
- chgrp ‐ change a file's group ownership
Permission
Groups
Each
file and directory has three user based permission groups:
- owner ‐ The Owner permissions apply only the owner of the file or directory, they will not impact the actions of other users.
- group ‐ The Group permissions apply only to the group that has been assigned to the file or directory, they will not effect the actions of other users.
- all users ‐ The All Users permissions apply to all other users on the system, this is the permission group that you want to watch the most.
Permission
Types Each file or directory has three basic permission types:
- read ‐ The Read permission refers to a user's capability to read the contents of the file.
- write ‐ The Write permissions refer to a user's capability to write or modify a file or directory.
- execute ‐ The Execute permission affects a user's capability to execute a file or view the contents of a directory.
Viewing
the Permissions
You
can view the permissions by checking the file or directory permissions in your
favorite GUI File Manager (which I will not cover here) or by reviewing the
output of the \"ls ‐l\" command while in the terminal and while
working in the directory which contains the file or folder.
The
permission in the command line is displayed as: _rwxrwxrwx 1 owner:group
User
rights/Permissions
- The first character that I marked with an underscore is the special permission flag that can vary.
- The following set of three characters (rwx) is for the owner permissions.
- The second set of three characters (rwx) is for the Group permissions.
- The third set of three characters (rwx) is for the All Users permissions.
Following that grouping since the
integer/number displays the number of hardlinks to the file. The last piece is the Owner and Group
assignment formatted as Owner:Group.
Modifying
the Permissions
When
in the command line, the permissions are edited by using the command chmod. You
can assign the permissions explicitly or by using a binary reference as
described below.
Explicitly
Defining Permissions
To explicitly define permissions you will need
to reference the Permission Group and Permission Types.
The
Permission Groups used are:
- u ‐ Owner
- g ‐ Group
- o or a ‐ All Users
The
potential Assignment Operators are + (plus) and ‐ (minus); these are
used to tell the system whether to add or remove the specific
permissions.
The
Permission Types that are used are:
- r ‐ Read
- w ‐ Write
- x ‐ Execute
So
for an example, lets say I have a file named file1 that currently has the
permissions set to _rw_rw_rw, which means that the owner, group and all users
have read and write permission. Now we want to remove the read and write
permissions from the all users group.
To
make this modification you would invoke the command: chmod a‐rw file1
To
add the permissions above you would invoke the command: chmod a+rw
file1
As
you can see, if you want to grant those permissions you would change the minus
character to a plus to add those permissions.
Using
Binary References to Set permissions
Now
that you understand the permissions groups and types this one should feel
natural. To set the permission using binary references you must first
understand that the input is done by entering three
integers/numbers.
A
sample permission string would be chmod 640 file1, which means that the owner
has read and write permissions, the group has read permissions, and all other user
have no rights to the file.
The
first number represents the Owner permission; the second represents the Group
permissions; and the last number represents the permissions for all other
users. The numbers are a binary representation of the rwx string.
r = 4
w
= 2
x
= 1
You
add the numbers to get the integer/number representing the permissions you wish
to set. You will need to include the binary permissions for each of the three
permission groups.
So
to set a file to permissions on file1 to read _rwxr_____, you would enter chmod
740 file1.
Owners
and Groups
I have made several references to Owners and
Groups above, but have not yet told you how to assign or change the Owner and
Group assigned to a file or directory.
You
use the chown command to change owner and group assignments, the syntax is
simple chown owner:group filename, so to change the owner of file1 to user1 and
the group to family you would enter chown user1:family file1.
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