chmod Calculator & Linux Permissions

What is chmod?

chmod (change mode) is a Unix/Linux command used to change file and directory permissions. Permissions control who can read, write, and execute files. They are represented as a three-digit octal number (e.g., 755) or symbolic notation (e.g., rwxr-xr-x).

Understanding Permission Bits

Each octal digit represents permissions for Owner (User), Group, and Others:

• 4 (r) — Read permission
• 2 (w) — Write permission
• 1 (x) — Execute permission

Add these numbers for each category. For example, 7 = 4+2+1 = read+write+execute (rwx).

Common Permission Examples

• 644 — rw-r--r-- (files readable by all, writable by owner)
• 755 — rwxr-xr-x (directories and executables)
• 777 — rwxrwxrwx (all permissions for everyone)
• 600 — rw------- (user only, no group/others)
• 700 — rwx------ (user only, with execute)

What is Umask?

Umask is a system setting that defines the default permissions for newly created files and directories. It works by subtracting the umask value from the base permissions. A common umask is 022, which results in 644 for files (666 - 022 = 644) and 755 for directories (777 - 022 = 755).

Frequently Asked Questions

How do I make a file executable?

Use chmod 755 filename to make a file executable by the owner and readable/executable by group and others. For user-only execution, use chmod 700 filename.

How do I make a file read-only?

Use chmod 444 filename to make a file read-only for all users. Use chmod 400 to make it read-only for the owner only.

What's the difference between 644 and 755?

644 (rw-r--r--) allows the owner to write but the group and others cannot. 755 (rwxr-xr-x) adds execute permission for all, typically used for directories and scripts.

Is my file data secure with this calculator?

Yes. Everything runs in your browser. No data is sent to any server. Your chmod calculations are performed entirely on your device.