Understand each permission bit
Explore ordinary file and directory modes, inspect their binary groups, and practise choosing access for a task.
Three bits make one octal digit
Read is 4, write is 2, and execute/search is 1. Add the enabled bits within each class, then write the digits in owner, group, others order. This preview does not change any files.
Exactly three digits from 0 to 7, including any leading zeros. Special four-digit modes are taught separately.
Try a mode
chmod 644 ./permission-practice.txt
Copying does not execute this command. If you practise in a terminal, use only a disposable file you own at the example path. The command changes that item's mode; it does not create the item.
For a regular file, read (r) allows reading its contents, write (w) allows changing its contents, and execute (x) permits execution when the file and environment support it. An execute bit does not turn arbitrary text into a program. Access also needs search permission along the path; deleting a file is governed by its parent directory.
This calculator covers the nine ordinary mode bits. The applicable owner, group, or others class is used; permissions are not simply added across all three classes. ACLs, privileges, filesystem rules, and other controls can affect actual access. Special bits and their preservation rules are outside this preview.
Use the ordinary mode bits only. Assume no additional ACLs or special bits; changing a directory mode does not change its children.
The owner can read and write a file. The group can only read it. Others have no permission, and nobody needs execute permission. What ordinary mode expresses this?
Owner: read + write. Group: read. Others: no bits. Add 4, 2, and 1 only when the corresponding permission is needed.
The owner can list, traverse, and change directory entries. The group can list and traverse, but cannot change entries. Others have no permission. Which ordinary mode fits?
For a directory, read lists names, search (x) permits traversal, and changing entries needs write plus search.
The directory owner has rwx. Group and others may traverse the directory to reach a known filename, but may not list names or change entries. Which ordinary mode fits?
Group and others need only the search bit, x. Reading a file also depends on that file's permissions and the rest of its path.
References: GNU numeric modes, GNU permission meanings, and GNU file permissions.