Master Linux File Permissions: Understanding ls -l, Ownership, and Links

Linux is a multi‑user, multitasking system where file ownership is crucial for privacy. Each user has a UID and GID, which act like a business card to determine read, write, and execute rights.

Running ls -l in the root directory displays seven fields per entry, e.g., lrwxrwxrwx. 1 root root 7 Aug 18 21:27 bin -> usr/bin. These fields represent file type and permissions, link count, owner, group, size (KB), modification time, and name.

The first character indicates the file type: - regular file, d directory, l symbolic or hard link, b block device, c character device, s socket, p named pipe.

The next nine characters are permission bits, grouped in three for owner, group, and others, using r (read), w (write), x (execute), or - if the permission is absent.

The trailing dot may appear when SELinux security labels are present; it disappears if SELinux is disabled.

Symbolic links (symlinks) are special files that store a path to another file or directory, created with ln -s source dest. Hard links point to the same inode and are created with ln source dest. Key differences include:

Both provide alternate paths to a file.

Deleting a symlink breaks the link; deleting a hard link only decrements the link count.

Symlinks can span filesystems and networks; hard links must reside on the same filesystem.

To change file ownership, use chown [-R] username:group filename, where -R applies recursively to directories.

File permissions can be modified using two methods:

Numeric method : Permissions map to numbers (r=4, w=2, x=1). For example, chmod 700 filename grants full rights to the owner and none to group or others.

Symbolic method : Use u (user), g (group), o (others), a (all) with + (add), - (remove), = (set). Example: chmod ug+w filename adds write permission for user and group.

Additional notes:

Hidden files start with a dot ( .) and can be listed with ls -al.

Special directories: . (current), .. (parent), - (previous), ~ (home).

Linux also supports special permission bits s and t, which are less commonly used.

Source: 标点符的《Linux学习笔记：文件的权限》 (http://www.biaodianfu.com/file-permission.html)