EP8: File Permission & chmod Explained

🎯 What You’ll Learn

In this episode, we’ll break down:

• What file permissions are 🔒

• How to read them 📖

• How to change them using chmod 🛠

• Real-world analogies that actually make sense 👷‍♂️

All based on RHCSA exam objectives.

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📦 Why Permissions Matter?

Imagine Linux as an apartment building:

• Every file or folder is like a room.

• Each user is a tenant.

• Permissions control who can enter, modify, or even see what's inside the room.

Without the correct permissions, users may:

• Get denied access

• Accidentally modify system files 😬

• Or worse — cause security issues 🚨

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🧾 Understanding the Permission Format

Run this command:

Copyls -l

You’ll see something like:

Copy-rw-r--r--. 1 alice developers 1234 Jul 7 12:00 report.txt

Breakdown of -rw-r--r--:

🔑 Permissions are always in this order:
User | Group | Others

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🧠 Real-World Analogy

For directories, x means:

• You can enter (cd into) the directory

• Without it, even if you can read files, you can’t enter the folder

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🛠 How to Change Permissions: chmod

✳️ Symbolic Method

Copychmod [who]+/-[permission] filename

✅ Examples:

Copychmod u+x script.sh    # Add execute to user
chmod g-w file.txt     # Remove write from group
chmod o+r report.txt   # Add read for others
chmod a-x script.sh    # Remove execute for everyone

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🔢 Numeric Method (Octal)

Permissions also have numeric values:

You add them up:

✅ Example:

Copychmod 755 script.sh

Breakdown:

• 7 = user → rwx

• 5 = group → r-x

• 5 = others → r-x

Another one:

Copychmod 644 file.txt

Means:

• User: read+write (6)

• Group: read only (4)

• Others: read only (4)

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🗂 Bonus: Default Permissions & umask

Every time you create a file/folder, Linux assigns default permissions, which are affected by a value called umask.

✅ Check umask:

Copyumask

Common default is 0022, meaning:

• Files default to 644

• Directories default to 755

To permanently change umask, modify:

• ~/.bashrc or /etc/bashrc for global changes

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🔍 Verify File Ownership

Use ls -l to check the owner and group:

Copy-rw-r--r-- 1 alice developers 1234 Jul 7 12:00 file.txt

To change ownership:

Copysudo chown bob:designers file.txt

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🧪 Most Common RHCSA Scenarios

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✅ Summary / Key Takeaways

🔑 Remember:

• Use symbolic method for readability

• Use numeric (octal) for scripting or when you know exact values

• File vs directory permissions behave slightly differently

• Practice both methods for RHCSA exam

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🎬 Final Words

Understanding permissions is essential for system security and file control. Once you grasp the concept, chmod becomes your daily command as a sysadmin!