In Linux and Unix-like operating systems, file permissions are used to control access to files and directories. Each file and directory has a set of permission bits that determine who can read, write, or execute the file.
There are nine permission bits for each file or directory:
Owner permissions: These permissions apply to the owner of the file or directory. The owner is the user who created the file or directory.
Group permissions: These permissions apply to the group that the file or directory belongs to. A group is a collection of users who are allowed to access the file or directory.
Other permissions: These permissions apply to all other users who are not the owner or a member of the group associated with the file or directory.
Each of the three sets of permissions (owner, group, and other) has three bits:
Read (r): This permission allows a user to read the contents of the file or list the contents of a directory.
Write (w): This permission allows a user to modify the contents of a file or add or delete files from a directory.
Execute (x): This permission allows a user to execute a file or search for a file in a directory.
The permission bits are usually represented as a string of nine characters, with the first three characters representing the owner permissions, the next three representing the group permissions, and the last three representing the other permissions.
For example, a file with the permission bits "rw-rw-r--" would have read and write permissions for the owner, read and write permissions for the group, and read-only permissions for other users.
You can use the chmod command to change the permission bits on a file or directory. For example, to give read, write, and execute permissions to the owner and read and execute permissions to the group and other users, you could use the command chmod 755 filename.