UNIX CLASS PPT CHAPTER 6

Accessing files within Downloads is not allowed since execute 11 permissions are not set
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chmod
• The chmod command allows you to change the file permissions for a file that you own
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File Permissions
• A file’s permissions describe which users are allowed to manipulate it • There are 3 types of permissible acts relating to files
Read: access the contents of a file; list the files in a directory - Write: modify the contents of a file; add, remove, and rename files in a directory - Execute: ask the operating system to run this file; access files and subdirectories within the directory
File Security and Advanced UNIX commands
CHAPTER - 6
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Contents
File Permissions in Linux/Unix Ownership in Linux files Absolute mode Symbolic mode Pipes , Grep & Sort
Ownership Permission

The concept of permissions and ownership is crucial in Linux . Here, we will discuss both of them. Let us start with the Ownership.
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chmod ugo=rwx somefile will set read, write, and execute permissions for user, group, and other chmod go-rwx somefile will remove read, write, and execure permissions for group and other

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Permissions
Write The right permission gives you the authority to modify the contents of a file. The write permission on a directory gives you the authority to add , remove and rename files stored in the directory. Consider a scenario where you have write permission on the file but do not have write permission on the directory where the file is stored. You will be able to modify the file contents. But you will not be able rename, move or remove the file from the directory.
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Permissions
Execute In windows an executable program usually has an extension “.exe” and which you can easily run. In Unix/Linux , you cannot run a program unless the execute permission is set. If the execute permission is not set, you might still be able to see/modify the program code (provided read & write permissions are set), but not actually run it.
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chmod 755 sets the permissions for a file to rwx r-x r-x
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Example

Lets see this in action ls –l on terminal gives

Here, we have highlighted ‘-rw-rw-r--‘and this weird looking code is the one that tells us about the permissions given to the owner, user group and the world.
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Ownership in Linux files
Every file and directory in your Unix/Linux system is assigned 3 types of owner , given below User A user is the owner of the file. By default, the person who created a file becomes its owner. Hence , a user is also sometimes called an owner. Group A user- group can contain multiple users. All users belonging to a group will have the same access permissions to the file. Suppose you have a project where a number of people require access to a file. Instead of manually assigning permissions to each user, you could add all users to a group, and assign group permission to file such that only this group members and no one else can read or modify the files.
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File Permissions
• There are 3 different categories of users for which different permissions can be set:
User: The user who owns the file - Group: The group that owns the file - Other: Other users that are not the user or part of the group that owns the file • Here, the user has read, write, and rwx r-x r-user group other

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File Permissions in Linux/Unix

Linux is a clone of UNIX, the multi-user operating system which can be accessed by many users simultaneously. Linux can also be used in mainframes and servers without any modifications. But this raises security concerns as an unsolicited or malign user can corrupt, change or remove crucial data. For effective security , Linux divides authorization into 2 levels
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chmod
• chmod also takes a more compact, numeric way to set the exact permissions for a file • You can think of ‘rwx’ as a 3-bit binary number, where a ‘1’ means that permission is set, and a ‘0’ means it is not
execute permissions • The group has read and execute permissions • Everyone else only has read permissions 10
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ls -l shows file permissions Appending fails because only read permissions are set for favorite_mov ies Listing files within Downloads is allowed since read permissions are set

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Ownership in Linux files