UNIX / LINUX File Permission / Access Modes

Directory Access Modes

Directory access modes are listed and organized in the same manner as any other file. There are a few differences that need to be mentioned: Read Access to a directory means that the user can read the contents. The user can look at the filenames inside the directory. Write Access means that the user can add or delete files from the directory. Execute Executing a directory doesn't really make sense, so think of this as a traverse permission. A user must have execute access to the bin directory in order to execute the ls or the cd command.

SUID and SGID File Permission part 2

The SUID and SGID bits will appear as the letter "s" if the permission is available. The SUID "s" bit will be located in the permission bits where the owners' execute permission normally resides.

Changing Group Ownership

The chgrp command changes the group ownership of a file. The basic syntax is as follows: $ chgrp group filelist The value of group can be the name of a group on the system or the group ID (GID) of a group on the system. Following example helps you understand the concept − $ chgrp special testfile $ Changes the group of the given file to special group.

Changing Ownership

The chown command changes the ownership of a file. The basic syntax is as follows: $ chown user filelist The value of the user can be either the name of a user on the system or the user id (uid) of a user on the system. The following example will help you understand the concept: $ chown amrood testfile $ Changes the owner of the given file to the user amrood. NOTE − The super user, root, has the unrestricted capability to change the ownership of any file but normal users can change the ownership of only those files that they own.

Using chmod in Symbolic Mode

The easiest way for a beginner to modify file or directory permissions is to use the symbolic mode. With symbolic permissions you can add, delete, or specify the permission set you want by using the operators in the following table: 1. + Adds the designated permission(s) to a file or directory. 2. - Removes the designated permission(s) from a file or directory. 3. = Sets the designated permission(s).

Group permissions

The group's permissions determine what actions a user, who is a member of the group that a file belongs to, can perform on the file.

Owner permissions

The owner's permissions determine what actions the owner of the file can perform on the file.

Other (world) permissions

The permissions for others indicate what action all other users can perform on the file.

File Access Modes

The permissions of a file are the first line of defense in the security of a Unix system. The basic building blocks of Unix permissions are the read, write, and execute permissions, which have been described below: Read Grants the capability to read, i.e., view the contents of the file. Write Grants the capability to modify, or remove the content of the file. Execute User with execute permissions can run a file as a program.

Using chmod with Absolute Permissions

The second way to modify permissions with the chmod command is to use a number to specify each set of permissions for the file. Each permission is assigned a value, as the following table shows, and the total of each set of permissions provides a number for that set. 0. No permission --- 1. Execute permission --x 2. Write permission. -w- 3. Execute and write permission: 1 (execute) + 2 (write) = 3 -wx 4. Read permissionr. -- 5. Read and execute permission: 4 (read) + 1 (execute) = 5 r-x 6. Read and write permission: 4 (read) + 2 (write) = 6. rw- 7. All permissions: 4 (read) + 2 (write) + 1 (execute) = 7. rwx

Changing Permissions

To change the file or the directory permissions, you use the chmod(change mode) command. There are two ways to use chmod — the symbolic mode and the absolute mode.

Changing Owners and Groups

While creating an account on Unix, it assigns a owner ID and a group ID to each user. All the permissions mentioned above are also assigned based on the Owner and the Groups. Two commands are available to change the owner and the group of files: chown − The chown command stands for "change owner" and is used to change the owner of a file. chgrp − The chgrp command stands for "change group" and is used to change the group of a file.

The Permission Indicators

While using ls -l command, it displays various information related to file permission as follows: Here, the first column represents different access modes, i.e., the permission associated with a file or a directory. The permissions are broken into groups of threes, and each position in the group denotes a specific permission, in this order: read (r), write (w), execute (x) − ----The first three characters (2-4) represent the permissions for the file's owner. For example, -rwxr-xr-- represents that the owner has read (r), write (w) and execute (x) permission. ----The second group of three characters (5-7) consists of the permissions for the group to which the file belongs. For example, -rwxr-xr-- represents that the group has read (r) and execute (x) permission, but no write permission. ----The last group of three characters (8-10) represents the permissions for everyone else. For example, -rwxr-xr-- represents that there is read (r) only permission.