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chmod

file permissionslinux

The chmod command is one of the most frequently used commands in Linux/Unix-like operating systems. chmod Change file mode bits (permissions)

See Examples View Syntax

Quick Reference

Command Name:

chmod

Category:

file permissions

Platform:

linux

Basic Usage:

chmod 755 filename.sh

Common Use Cases

Syntax

chmod [OPTION]... MODE[,MODE]... FILE...
chmod [OPTION]... OCTAL-MODE FILE...
chmod [OPTION]... --reference=RFILE FILE...

Options

Option	Description
-c, --changes	Like verbose but report only when a change is made
-f, --silent, --quiet	Suppress most error messages
-v, --verbose	Output a diagnostic for every file processed
--no-preserve-root	Do not treat '/' specially (the default)
--preserve-root	Fail to operate recursively on '/'
--reference=RFILE	Use RFILE's mode instead of specifying a MODE value
-R, --recursive	Change files and directories recursively
--help	Display help message and exit
--version	Output version information and exit

Permission Modes:

Mode	Description
u	User who owns the file
g	Group that owns the file
o	Others (not owner or group)
a	All (user, group, others)
r	Read permission
w	Write permission
x	Execute permission
s	Set user or group ID on execution
t	Sticky bit
+	Add the specified permissions
-	Remove the specified permissions
=	Set the specified permissions and remove all others

Examples

How to Use These Examples

The examples below show common ways to use the chmod command. Try them in your terminal to see the results. You can copy any example by clicking on the code block.

#

Basic Examples:

# Give the owner read, write, and execute permissions
chmod u+rwx filename.sh

# Remove write permission for group and others chmod go-w important_file.txt

# Set read and execute for all (user, group, others) chmod a+rx script.sh

# Use octal notation to set permissions (rwx for owner, rx for group and others) chmod 755 program.bin

# Add execute permission to a script chmod +x install.sh

Advanced Examples:

# Set permissions recursively for a directory and its contents
chmod -R 755 /var/www/html

# Add sticky bit to a directory (only file owners can delete their files) chmod +t /home/shared # Add setuid bit (execute as file owner) chmod u+s /usr/bin/passwd # Add setgid bit (execute as file group) chmod g+s /var/www/uploads # Copy permissions from one file to another chmod --reference=reference.txt target.txt # Change permissions but don't follow symbolic links chmod -h 644 symlink.txt # Set exact permissions (remove any other bits) using symbolic notation chmod =rw file.txt # Only change permissions if they match a specific pattern chmod --from=u=r,g=r,o= u+w,g+w data.txt

Try It Yourself

Practice makes perfect! The best way to learn is by trying these examples on your own system with real files.

Understanding Syntax

Pay attention to the syntax coloring: commands, options, and file paths are highlighted differently.

Notes

Key Features:

The chmod command is one of the most important Linux/Unix commands, used to change the permissions (or "mode") of files and directories:

Control who can read, write, or execute files
Set special permission bits (setuid, setgid, sticky bit)
Use either symbolic or octal notation to specify permissions
Apply changes recursively to directory structures
Match permissions between files using reference files

Permission Notation:

Symbolic Notation:

Consists of three parts: who, operation, and permissions

Who: u (user/owner), g (group), o (others), a (all)
Operation: + (add), - (remove), = (set exactly)
Permissions: r (read), w (write), x (execute), s (setuid/setgid), t (sticky bit)

Octal (Numeric) Notation:

Uses three or four digits to represent permissions

First digit (optional): Special permissions (setuid, setgid, sticky bit)
- 4 = setuid
- 2 = setgid
- 1 = sticky bit
Second digit: Owner permissions
Third digit: Group permissions
Fourth digit: Others permissions
For each of the last three digits:
- 4 = read
- 2 = write
- 1 = execute
- 0 = no permission

Common Permission Values:

777 (rwxrwxrwx): Full permissions for everyone (rarely appropriate)
755 (rwxr-xr-x): Owner can read/write/execute, others can read/execute
644 (rw-r--r--): Owner can read/write, others can read
700 (rwx------): Owner has full access, no access for others
600 (rw-------): Owner can read/write, no access for others
4755 (rwsr-xr-x): Same as 755 but with setuid bit set

Special Permission Bits:

Setuid (s): When set on an executable, it runs with the permissions of the owner
Setgid (s): When set on an executable, it runs with the permissions of the group. When set on a directory, new files created in the directory inherit the directory's group
Sticky bit (t): When set on a directory, files within that directory can only be deleted by their owners, the directory owner, or root

Important Considerations:

You must be the owner of a file or have appropriate privileges to change its permissions
Be careful with recursive changes, especially on system directories
Use the --preserve-root option to prevent accidental permission changes to the root directory
The default umask on your system affects the initial permissions of new files
Directory permissions work differently than file permissions:
- r: List directory contents
- w: Create/delete files in the directory
- x: Access files and subdirectories within the directory

Related Commands

These commands are frequently used alongside chmod or serve similar purposes:

Explore All Commands

Use Cases

Learn By Doing

The best way to learn Linux commands is by practicing. Try out these examples in your terminal to build muscle memory and understand how the chmod command works in different scenarios.

See Examples Try Lab Exercises

$ chmod

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