chmod and File Permissions

🛡️ chmod and File Permissions in Linux

chmod (change mode) is used to manage file permissions in Linux. Permissions control who can read, write, or execute a file or directory.

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🔤 Permission Types

Symbol	Meaning	Applies To
r	Read	View content
w	Write	Modify content
x	Execute	Run as program

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👥 Permission Levels

Level	Description
u	User (owner)
g	Group
o	Others (everyone else)
a	All (user + group + others)

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🔢 Numeric (Octal) Representation

Each permission is a number:

Permission	Binary	Octal
r	100	4
w	010	2
x	001	1

Add values to combine permissions:

Access	Octal
r--	4
rw-	6
rwx	7

Example: chmod 755 file Means:

• User: rwx (7)

• Group: r-x (5)

• Others: r-x (5)

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🔧 Common Usage

Set Permissions with Numbers

chmod 644 file.txt     # rw-r--r--
chmod 755 script.sh    # rwxr-xr-x

Add or Remove Permissions Symbolically

chmod +x script.sh     # Add execute for all
chmod u+x script.sh    # Add execute for user
chmod go-w file.txt    # Remove write for group/others

Recursive Change (for folders)

chmod -R 755 /path/to/dir

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🔍 View File Permissions

Use ls -l to list file permissions:

ls -l file.txt

Example output:

-rw-r--r-- 1 user group  1234 May 15 14:00 file.txt

• First char: - (file) or d (directory)

• Next 9: permissions for user, group, others

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🔐 Special Bits (Optional)

Bit	Octal	Description
setuid	4	Run as file owner's user ID
setgid	2	Run as group's ID / sticky group
sticky	1	Only owner can delete (e.g. /tmp)

Set using chmod:

chmod 4755 file      # setuid
chmod 2755 dir       # setgid
chmod 1755 dir       # sticky

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✅ Examples

Make script executable

chmod +x my_script.sh

Remove all access for others

chmod o-rwx file.txt

Set folder and contents to rwxr-xr-x

chmod -R 755 my_folder

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