Day 6 File Permissions and Access Control Lists

1. Understanding File Permissions

2. Writing an article about file permission

3. Access control list

4. Additional task

5. Understanding sticky bit, SUID, SGID

   

1. Understanding File Permissions

Create a Simple File and Check Permissions

# Create a simple file
touch myfile.txt

# List files with detailed permissions
ls -ltr

Change Ownership and Group

• Change ownership with chown:

    chown username:groupname myfile.txt
  

• Change group with chgrp:

    chgrp groupname myfile.txt
  

Change User Permissions

• Change permissions with chmod:

    chmod 755 myfile.txt  # Example: full access to owner, read/execute for group and others
  

• Note the changes after running ls -ltr again:

     ls -ltr myfile.txt
  

2. Writing an Article about File Permissions

Sample Article Title: Understanding File Permissions in Linux

Content: In Linux, file permissions determine who can access or modify files and directories. Each file has three categories of users:

• Owner: The individual who created the file, possesses the highest level of permissions.

• Group: Users who belong to a specific group assigned to the file, with shared permissions.

• Others: All other users on the system who do not fall into the previous two categories.

Each of these categories can have three types of permissions:

• Read (r): Allows viewing the contents of a file.

• Write (w): Allows modifying or deleting the file.

• Execute (x): Allows running the file as a program.

Permissions can be modified using the following commands:

• chown: Change ownership of a file.

• chgrp: Change the group ownership of a file.

• chmod: Change the permissions assigned to the owner, group, and others.

Understanding and managing file permissions is essential for maintaining system security and proper access control.

3. Access Control Lists (ACL)

Reading about ACL

• Get ACL permissions:

    getfacl myfile.txt
  

• Set ACL permissions:

    setfacl -m u:username:rwx myfile.txt  # Granting rwx permissions to a specific user
  

Create a Directory and Set ACL

# Create a directory
mkdir mydirectory

# Set specific ACL permissions
setfacl -m u:username:rwx mydirectory  # Set rwx permissions for a specific user
setfacl -m g:groupname:rx mydirectory  # Set rx permissions for a group

# Verify permissions
getfacl mydirectory

4. Additional Tasks

Script to Change Permissions of Multiple Files

#!/bin/bash

# Script to change permissions of multiple files in a directory
if [ "$#" -ne 2 ]; then
    echo "Usage: $0 <directory_path> <permissions>"
    exit 1
fi

DIRECTORY=$1
PERMISSIONS=$2

# Change permissions for all files in the specified directory
chmod -R "$PERMISSIONS" "$DIRECTORY"

echo "Permissions changed to $PERMISSIONS for all files in $DIRECTORY."

Usage:

chmod +x change_permissions.sh
./change_permissions.sh /path/to/directory 755

Script to Set ACL Permissions

#!/bin/bash

# Script to set ACL permissions for a user on a given file
if [ "$#" -ne 3 ]; then
    echo "Usage: $0 <file_path> <username> <permissions>"
    exit 1
fi

FILE=$1
USER=$2
PERMISSIONS=$3

# Set ACL permissions for the specified user
setfacl -m u:"$USER":"$PERMISSIONS" "$FILE"

echo "ACL permissions set for user $USER on $FILE."

Usage:

chmod +x set_acl.sh
./set_acl.sh myfile.txt username rwx

5. Understanding Sticky Bit, SUID, and SGID

Sticky Bit Example

The sticky bit is typically set on directories to allow only the file's owner to delete or rename the file.

# Set sticky bit on a directory
mkdir sticky_dir
chmod +t sticky_dir

SUID Example

The SUID (Set User ID) allows a user to execute a file with the permissions of the file owner.

# Create a script with SUID
touch myscript.sh
chmod 4755 myscript.sh  # Set SUID

SGID Example

The SGID (Set Group ID) allows a user to execute a file with the permissions of the group owner.

# Create a directory and set SGID
mkdir sgid_dir
chmod g+s sgid_dir  # Set SGID

6. Backup and Restore Permissions

Backup Current Permissions

#!/bin/bash

# Script to backup permissions of files in a directory
if [ "$#" -ne 2 ]; then
    echo "Usage: $0 <directory_path> <backup_file>"
    exit 1
fi

DIRECTORY=$1
BACKUP_FILE=$2

# Backup permissions to a file
ls -l "$DIRECTORY" > "$BACKUP_FILE"

echo "Permissions backed up to $BACKUP_FILE."

Restore Permissions

#!/bin/bash

# Script to restore permissions from a backup file
if [ "$#" -ne 2 ]; then
    echo "Usage: $0 <directory_path> <backup_file>"
    exit 1
fi

DIRECTORY=$1
BACKUP_FILE=$2

# Restore permissions from the backup file
while read -r line; do
    # Extract filename and permissions
    PERMISSIONS=$(echo "$line" | awk '{print $1}')
    FILENAME=$(echo "$line" | awk '{print $9}')
    chmod "$PERMISSIONS" "$DIRECTORY/$FILENAME"
done < "$BACKUP_FILE"

echo "Permissions restored from $BACKUP_FILE."

Summary

This comprehensive guide includes practical tasks and scripts to help you understand and manage Linux file permissions and ownership, ACLs, and advanced concepts like sticky bit, SUID, and SGID. Each script is designed to automate common tasks in a DevOps environment, enhancing efficiency and control over file systems.