Chmod Calculator
Calculate Unix file permissions visually — get octal, symbolic notation and the chmod command
File permissions
Special bits
What is chmod?
chmod (change mode) is a Unix command that sets read, write, and execute permissions on files and directories for three classes: owner, group, and others. Permissions control who can access or modify a file on the filesystem.
Understanding octal notation
Each permission class is represented as a 3-bit value: read=4, write=2, execute=1. Adding the bits gives an octal digit per class. For example, rwx=7, rw-=6, r-x=5, r--=4. A full permission set like 755 means owner rwx (7), group rx (5), others rx (5).
Common permission patterns
644 is standard for web files (owner can write, others read). 755 is used for directories and executables. 600 keeps files private to the owner. 777 should be avoided as it allows anyone to modify the file — a security risk on shared systems.
Frequently asked questions
Everything about Unix/Linux file permissions and chmod
chmod stands for "change mode". It is a Unix/Linux command used to change the access permissions of files and directories. The "mode" refers to the permission bits that define who can read, write, or execute a file.
644 (rw-r--r--) is the standard permission for regular files: the owner can read and write, while group and others can only read. 755 (rwxr-xr-x) adds execute permission for the owner and read+execute for group and others — used for directories and executable scripts.
Execute permission (x) is required to run a file as a program or script. For directories, execute permission means the ability to enter the directory (cd into it) and access its contents. Without execute permission on a directory, even read permission won't allow you to list its files.
SUID (Set User ID) causes a program to run with the file owner's privileges instead of the caller's — used by commands like sudo and passwd. SGID (Set Group ID) similarly elevates to the file's group. The sticky bit on a directory prevents users from deleting files owned by others, commonly set on /tmp.