Master Linux File Permissions: Decode ls -l, chmod, and Links

Linux is a multi‑user, multitasking operating system where each user has a UID and GID that act like a personal ID card; file ownership and permissions determine who can read, write, or execute a file.

Understanding ls -l Output

Running ls -l in the root directory shows entries such as lrwxrwxrwx. 1 root root 7 Aug 18 21:27 bin -> usr/bin. The seven fields represent: lrwxrwxrwx. – file type and permission bits 1 – number of hard links root – owner root – group 7 – size in kilobytes Aug 18 21:27 – last modification time bin -> usr/bin – file name (and target for links)

File Type Characters

-

– regular file d – directory l – symbolic or hard link b – block device c – character device s – socket p – named pipe (FIFO)

Permission Bits

The next nine characters are grouped in triples for owner, group, and others, using r (read), w (write), and x (execute). A dash ( -) indicates the absence of that permission.

The trailing dot ( .) signals an SELinux security label; it disappears when SELinux is disabled.

Symbolic vs. Hard Links

Symbolic link (symlink) : a special file containing a path to another file or directory, similar to a Windows shortcut. Create with ln -s source target.

Hard link : another directory entry that points to the same inode as the original file. Multiple names can refer to the same data; the file persists until the last hard link is removed. Create with ln source target.

Key differences :

Both provide alternate paths to the same content.

Removing the target of a symlink invalidates it; removing a hard link only decrements the link count.

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

Changing Ownership

Use chown [-R] username:group filename to change the owner and/or group. The -R flag applies the change recursively to directories.

Modifying Permissions

1. Numeric Method

Permission bits are represented by octal values: read = 4, write = 2, execute = 1. Combine them for each class (owner, group, others). Example: chmod 700 filename grants full rights to the owner and none to group or others.

2. Symbolic Method

Introduce the letters u (user/owner), g (group), o (others), a (all) and the operators + (add), - (remove), = (set exactly). Example: chmod ug+w filename adds write permission for both owner and group.

Additional Topics

Hidden files : Files whose names start with a dot ( .) are not shown by default; use ls -al to list them.

Special directories : . – current directory .. – parent directory - – previous working directory ~ – current user's home directory

Special permission bits : s (setuid/setgid) and t (sticky bit) exist beyond the basic rwx set, but are rarely used in everyday operations.