Unix Permission Calculator (chmod)
Select read, write, and execute permissions or enter an octal code like 755 in MiniWebtool’s Unix Permission Calculator to generate chmod commands and view security risk assessments.
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Type a three-digit octal value or tick the read, write and execute boxes for owner, group and others. The result shows the octal and symbolic forms and the chmod command.
Moderate Exposure
Others can read or execute. Ensure this is intentional.
-rwxr-xr-x 1 user group 4096 Jan 27 12:00 my_file.txt
About Unix Permission Calculator (chmod)
About Unix Permission Calculator
This Unix Permission Calculator (chmod calculator) helps you understand and calculate Linux/Unix file permissions. It provides an interactive visual interface to set permissions and instantly see the corresponding octal notation, symbolic notation, and ready-to-use chmod commands.
File permissions in Unix/Linux systems control who can read, write, or execute files and directories. Understanding these permissions is essential for system security, web server configuration, and proper file management.
At a Glance
Unix Permission Calculator (chmod) turns read, write and execute choices into octal and symbolic permissions and a chmod command. Click the permission grid or enter an octal code such as 755.
| Feature | What you get | Good for |
|---|---|---|
| Permission grid | Toggle read (4), write (2) and execute (1) for owner, group and others, or type an octal value such as 755 to fill the grid. | Working out a mode without doing the arithmetic |
| Octal and symbolic | Shows both forms together, for example 755 and rwxr-xr-x, plus a ready-to-copy chmod command. | Translating between numeric and letter notation |
| Special permissions | Switch on SetUID, SetGID and the sticky bit, which add a leading digit of 4, 2 or 1. | Shared directories such as /tmp and executables that run as their owner |
| File type | Choose regular file, directory or symbolic link. | Seeing the full ls -l style string for the right object |
| Terminal preview | Displays what the permissions look like in ls -l output. | Checking a setting before running it on a server |
| Risk assessment | Rates the chosen permissions with a visual indicator, and preset buttons load common settings such as 644, 755 and 600. | Reviewing web server file and folder permissions |
Understanding Unix Permissions
Permission Types
- Read (r = 4): View file contents or list directory contents
- Write (w = 2): Modify file contents or create/delete files in directory
- Execute (x = 1): Run file as program or access directory
User Categories
- Owner (User): The user who owns the file
- Group: Users belonging to the file's group
- Others: All other users on the system
Octal vs Symbolic Notation
| Octal | Binary | Symbolic | Description |
|---|---|---|---|
0 | 000 | --- | No permissions |
1 | 001 | --x | Execute only |
2 | 010 | -w- | Write only |
3 | 011 | -wx | Write and execute |
4 | 100 | r-- | Read only |
5 | 101 | r-x | Read and execute |
6 | 110 | rw- | Read and write |
7 | 111 | rwx | Full permissions |
Common Permission Settings
| Octal | Symbolic | Use Case |
|---|---|---|
644 | rw-r--r-- | Standard files (HTML, CSS, images) |
755 | rwxr-xr-x | Directories and executable scripts |
600 | rw------- | Private files (SSH keys, passwords) |
700 | rwx------ | Private directories and scripts |
664 | rw-rw-r-- | Group-collaborative files |
775 | rwxrwxr-x | Group-collaborative directories |
777 | rwxrwxrwx | Full access (NOT recommended) |
Special Permissions
SetUID (4xxx)
When set on an executable file, the program runs with the file owner's privileges instead of the user running it. Example: /usr/bin/passwd runs as root to modify /etc/shadow.
SetGID (2xxx)
On executables, runs with the group's privileges. On directories, new files inherit the directory's group instead of the creator's primary group.
Sticky Bit (1xxx)
On directories, prevents users from deleting files they don't own. Essential for shared directories like /tmp.
Security Best Practices
- Avoid 777: Never use world-writable permissions unless absolutely necessary
- Minimal access: Grant only the permissions required for proper operation
- Protect sensitive files: Use 600 or 400 for private keys and configuration files
- Web directories: Use 755 for directories, 644 for files served by web servers
- Review SetUID/SetGID: Audit programs with special permissions regularly
How to Use This Calculator
- Select the file type (regular file, directory, or symbolic link)
- Click the permission cells to toggle Read, Write, and Execute for each user category
- Or enter an octal value directly (like 755) to see the corresponding permissions
- Enable special permissions (SetUID, SetGID, Sticky Bit) if needed
- Review the security assessment and terminal preview
- Copy the chmod command to use in your terminal
Frequently Asked Questions
What is chmod in Unix/Linux?
chmod (change mode) is a Unix/Linux command used to change the access permissions of files and directories. It controls who can read, write, or execute a file. Permissions are set for three categories: owner (user who owns the file), group (users in the file's group), and others (everyone else).
What do the numbers in chmod mean (like 755 or 644)?
In chmod, each digit represents permissions for owner, group, and others respectively. Each digit is the sum of: 4 (read), 2 (write), and 1 (execute). For example, 755 means owner has full access (7=4+2+1), while group and others can read and execute (5=4+1).
What is the difference between symbolic and octal notation?
Octal notation uses numbers (like 755) where each digit represents combined permissions. Symbolic notation uses letters: r (read), w (write), x (execute), and - (no permission). For example, rwxr-xr-x is the symbolic form of 755.
What are SetUID, SetGID, and Sticky Bit?
These are special permissions: SetUID (4) allows a program to run with the owner's privileges. SetGID (2) runs with group privileges or inherits directory group. Sticky Bit (1) on directories prevents deletion of files by non-owners.
Why is chmod 777 considered dangerous?
chmod 777 gives everyone full read, write, and execute permissions. This is dangerous because any user can modify or delete the file, potentially leading to security vulnerabilities, malware injection, or data loss.
Reference this content, page, or tool as:
"Unix Permission Calculator (chmod)" at https://MiniWebtool.com/unix-permission-calculator/ from MiniWebtool, https://MiniWebtool.com/
by miniwebtool team. Updated: Jan 27, 2026
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