File Ownership

• chown → Change ownership
• chgrp → Change group

Access Control List (ACL)

• setfacl → Set file ACL
• getfacl → Get file ACL

Add Permission to User

setfacl -m u:user:rwx /path/to/file
setfacl -m g:group:rw /path/to/file

Recursive Inheritance from Folder

setfacl -R -m entry /path/to/dir

Remove Specific ACL Entry

setfacl -x u:user /path/to/file

Remove All ACL Entries

setfacl -b /path/to/file

Check ACL

ls -ltr

Example output:

-rw-rw-r--+

Help Commands

• whatis command
• command --help
• man command

Add Text to a File

echo "my text" > myfile    # Overwrite
cat myfile                 # Read the text
echo "hello" >> myfile     # Append

Input and Output Redirects
3 Redirect Types

• stdin → 0 → < or << → Feeding file content to a command
• stdout → 1 → > or >>
• stderr → 2 → 2> or 2>> → Write error output to a file

tee Command

• tee → Output + Save

echo "test" | tee myfile
tee -a file  # Append

Links

Soft Link (Symbolic Link)

• ln -s
• Link will be removed if the original file is removed or renamed.

Hard Link

• ln
• Deleting, renaming, or moving the original file will not affect the hard link.

Diagram

       INODE
        ↑
MY HARD LINK ← MYFILE.TXT → MY-SOFT-LINK

Command to check links:

ls -lTri

File Permissions

Permission Types

Each Permission (R, W, X) Can Be Controlled at Three Levels

• U → User
• G → Group
• O → Others (Everyone)

Permission Strings

Example:

-rwxrwxrwx
 |   |  |
 |   |  └── Others
 |   └───── Group
 └───────── User

To remove write permission from all:

chmod a-w filename

To add read and write permission for user:

chmod u+rw filename

Directory Permissions

Example:

drwxrwxrwx
^   ^  ^
|   |  └── Others
|   └───── Group
└───────── User

Numeric Permission Mapping

Example to set permissions:

chmod 764 file

Breakdown:

• User (7) → rwx
• Group (6) → rw-
• Others (4) → r--

• ls -l → List
• pwd → Print working directory

dr-xr-xr-x
|
directories

-rw-r--r--
|
file

Linux File Types

Inode = Pointer or number of a file on a hard disk.