Permissions are the unix way of controlling access to resources such as files, directories and devices and may be specified for an owner, group, or all users.
Permissions on Unix-like systems are managed in three distinct classes. These classes are known as user, group, and others. In effect, Unix permissions are a simplified form of access control lists (ACLs).
There are three specific permissions on Unix-like systems that apply to each class:
- The read permission, which grants the ability to read a file. When set for a directory, this permission grants the ability to read the names of files in the directory (but not to find out any further information about them such as contents, file type, size, ownership, permissions, etc.)
- The write permission, which grants the ability to modify a file. When set for a directory, this permission grants the ability to modify entries in the directory. This includes creating files, deleting files, and renaming files.
- The execute permission, which grants the ability to execute a file. This permission must be set for executable binaries (for example, a compiled C++ program) or shell scripts (for example, a Perl program) in order to allow the operating system to run them. When set for a directory, this permission grants the ability to traverse its tree in order to access files or subdirectories, but not see the content of files inside the directory (unless read is set).
When a permission is not set, the rights it would grant are denied. Unlike ACL-based systems, permissions on a Unix-like system are not inherited. Files created within a directory will not necessarily have the same permissions as that directory.
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