Chmod Calculator — Unix File Permissions

What do chmod 755, 644, and 600 mean?

Each digit combines read (4), write (2), and execute (1). Mode 755 gives the owner all three permissions and everyone else read and execute. Mode 644 gives the owner read and write, while group and others can only read. Mode 600 limits read and write access to the owner. These are examples, not a universal recommendation for every file.

Octal to symbolic permissions

The nine-character symbolic result groups permissions as owner, group, and others. For example, 0755 becomes rwxr-xr-x. The leading octal digit represents special bits; this calculator always shows all four digits to make those bits explicit. The symbolic output omits the file-type character shown by ls -l.

Setuid, setgid, and the sticky bit

Special bits use 4 for setuid, 2 for setgid, and 1 for sticky. Lowercase s or t means the corresponding execute bit is also set; uppercase S or T means it is not. Their effects depend on file type and operating system. On directories, setgid commonly controls group inheritance and sticky restricts who may delete or rename entries.

File access and directory access differ

Directory execute permission allows traversal, read allows listing names, and write controls changes to entries together with other required permissions. Actual access also depends on ownership, parent directories, ACLs, and system policy. This tool calculates mode bits only; it does not inspect files, set ownership, or execute chmod. Avoid 777 as a blanket solution to permission errors.

Reference: GNU file permissions documentation.