devops-notes/02_Linux_Permissions.md

Linux – File Permissions (Day 2)

What are File Permissions?

File permissions control who can read, write, or execute a file or folder in Linux.

They protect the system from:

• accidental deletion
• unauthorized access
• hacking

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Why Permissions are Important for DevOps?

DevOps engineers manage live servers. Wrong permissions can:

• break applications
• expose data
• allow hackers access

So permissions are critical.

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Permission Types

There are 3 permissions:

• r → read
• w → write
• x → execute

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Permission Groups

Permissions are set for 3 types of users:

• u → user (owner)
• g → group
• o → others

Order is always:

user | group | others

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Checking Permissions

ls -l
-rwxr-xr--
Meaning:

user → rwx

group → r-x

others → r--

Add values to set permissions.

Examples:

7 = rwx (4+2+1)

6 = rw- (4+2)

5 = r-x (4+1)

4 = r--

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Setting Permissions

chmod 755 file.txt

Meaning:

user → 7 (rwx)

group → 5 (r-x)

others → 5 (r-x)

Symbolic Method

chmod u+x file.sh
chmod g-w file.sh
chmod o+r file.sh

Common Permission Examples

chmod 755 file.txt
chmod 644 file.txt
chmod 777 file.txt
chmod 666 file.txt
chmod 700 file.txt
chmod 600 file.txt

Execute Permission (Very Important)

To run a script:

./script.sh

If permission denied:

chmod +x script.sh

Folder Permissions Rule

r → list files
w → create/delete files
x → enter folder

Without x, you cannot cd into folder.

Execute Permission (Very Important)

To run a script:

./script.sh

If permission denied:

chmod +x script.sh

Folder Permissions Rule

r → list files

w → create/delete files

x → enter folder

Without x, you cannot cd into folder.

Mini Practice Task

mkdir test
touch test/app.sh
ls -l test
chmod 755 test/app.sh
ls -l test

Common Mistakes

• Giving 777 permissions (very dangerous)
• Not understanding folder execute permission
• Changing permissions blindly on production servers

Changing Ownership

To change the owner of a file:

sudo chown username file.txt

To change both owner and group:

sudo chown username:groupname file.txt

To change the group ownership:

sudo chgrp groupname file.txt

Recursive Changes

Apply permissions or ownership to a directory and all its contents:

chmod -R 755 folder_name/
chown -R username:groupname folder_name/

Checking Current Permissions

To see detailed permissions for all files in a directory:

ls -la