How It Works

Unix permissions are built on three permission groups—owner (user), group, and others—and three basic capabilities—read, write, and execute. Each capability is represented by a bit. In octal form, those bits are summed for each group to create a digit from 0 to 7.

Permission Values

• Read (r) = 4
• Write (w) = 2
• Execute (x) = 1
• No permission = 0

For example, 7 means 4+2+1 (rwx), 5 means 4+1 (r-x), and 4 means read-only (r--). The calculator applies this logic for each group and displays the equivalent symbolic string and a plain-language explanation.

Key Features

Octal to Symbolic Conversion

Paste a three-digit (e.g., 644) or four-digit (e.g., 2755) octal mode and instantly see the corresponding symbolic permissions, including special bits when present.

Symbolic to Octal Conversion

Enter a symbolic string such as rwxr-x--- and get the exact octal code. This is useful when a tutorial provides symbolic permissions but your deployment process expects numeric modes.

Special Bits Explained

The calculator can interpret setuid, setgid, and the sticky bit. It shows how they affect execution flags (s, S, t, T) and when you should use them.

Human-Friendly Summary

Beyond raw conversion, you’ll get a readable description such as “Owner can read/write/execute; group can read/execute; others can read/execute.” This helps you spot risky modes at a glance.

Copy-Ready Command

The tool generates a ready-to-copy chmod command template (for example, chmod 755 filename) to reduce mistakes when applying permissions.

Use Cases

• Web servers and deployments: Confirm safe defaults for directories (often 755) and files (often 644), then adapt to your hosting environment.
• CI/CD pipelines: Validate modes used in build steps, Docker images, or packaging scripts before they reach production.
• Security reviews: Quickly identify if “others” accidentally gained write access (e.g., 777), which can enable tampering.
• Collaboration on shared systems: Ensure group permissions are correct when multiple users need access without exposing data publicly.
• Education and learning: Practice understanding how rwx maps to octal digits and what each digit actually grants.

In short, a chmod calculator reduces guesswork. It turns permission codes into something you can reason about, making it easier to choose the least-privilege setting that still lets your software run.

Optimization Tips

Prefer Least Privilege

Start with the strictest mode that still works. Grant execute permission only when a file must be executed or a directory must be traversed. Avoid world-writable permissions unless you fully understand the implications.

Differentiate Files vs Directories

Directories require the execute bit to allow access into the directory. A directory with read but without execute can list names but cannot be entered. Many “why can’t I access this folder?” problems are caused by missing the execute bit on directories.

Be Careful with Special Bits

Setuid/setgid can be powerful and dangerous because they change the effective user or group for executed programs. Use them only when you have a clear administrative reason and understand the security model on your system.

Deep Dive: Understanding Execute on Files and Directories

The execute bit is often misunderstood. On a regular file, execute means the operating system is allowed to run it as a program or script (assuming the file content is a valid executable format or has a proper shebang line for scripts). On a directory, execute does not mean “run” — it means “traverse.” If you have execute on a directory, you can access items inside it (subject to the file permissions of those items). If you only have read on a directory but no execute, you may be able to list the filenames, but you cannot cd into the folder or open files within it.

This is why directory modes like 755 and 750 are so common: they allow traversal while restricting who can modify contents. For websites, 755 on public directories is often acceptable, while write access is limited to the owner (or a deployment user). For private application data, 700 or 750 can be safer, depending on whether group collaboration is needed.

Common Permission Patterns

• 644 (rw-r--r--) — typical for text files and configuration files that should be readable but not writable by others.
• 600 (rw-------) — common for secrets like private keys or credential files.
• 755 (rwxr-xr-x) — common for directories and executables that should be runnable by everyone but writable only by the owner.
• 750 (rwxr-x---) — useful when a trusted group needs access but “others” should have none.
• 775 (rwxrwxr-x) — common for collaborative directories where group members can write.

The best choice depends on your threat model and operational needs. When in doubt, choose the smallest set of permissions that allows the application to function, then expand only if you can justify the access.

Troubleshooting Permission Problems

If you’re seeing errors like “Permission denied,” a calculator is only part of the solution. You also need to consider ownership and group membership. Changing permissions does not change who owns a file. In many deployment setups, the fix is to adjust ownership (for example with chown) or to place the process user into the correct group rather than making files world-writable.

Another frequent issue is applying a file-oriented mode to a directory tree. A directory needs execute to be usable. If you apply 644 recursively to folders, you can lock yourself out of traversal. A safer approach is to set directory and file modes separately (directories 755, files 644) or use deployment tools that manage this automatically.

Finally, remember that permission evaluation is cumulative: the system checks permissions based on the first matching class among owner, group, and others. If you are the owner, the “owner” digit is the one that matters, even if group or others are more permissive or restrictive.

FAQ