About this chmod calculator
Unix permissions say who can read, write, and run a file. There are three classes of user, owner, group, and others, and three actions for each. Each action has a number (read is 4, write is 2, execute is 1), and adding them gives one digit per class, so rwxr-xr-x becomes 755. Special bits go in an optional fourth digit at the front: setuid, setgid, and the sticky bit. This calculator keeps the checkboxes and the octal code in step, shows the same permissions as the letters in ls -l, writes the chmod command, and tells you how exposed the settings are.
How to calculate Unix permissions
- Pick a starting point. Press a common permission such as 644, or type an Octal code. The checkboxes follow what you type.
- Adjust the checkboxes. Tick Read, Write, and Execute for Owner, Group, and Others. Tick setuid, setgid, or Sticky bit for special permissions. The octal code updates as you tick.
- Choose the file type. Set File type to Regular file, Directory, or Symbolic link, which changes the first letter of the symbolic form.
- Press Calculate permissions. Copy the chmod command, and read the symbolic form, the ls -l preview, and the exposure rating.
Frequently asked questions
What does 755 mean?
The owner can read, write, and execute (7), while the group and everyone else can read and execute (5 each). It is the usual setting for directories, scripts, and programs that others need to run.
What is the difference between 644 and 600?
With 644 the owner can read and write, and everyone else can read. With 600 only the owner has access. Use 600 for private files such as SSH keys and configuration files with passwords.
What do setuid, setgid, and the sticky bit do?
Setuid and setgid make a program run with the rights of its owner or group instead of the user who started it. On a directory, the sticky bit lets only a file’s owner delete or rename it, as in /tmp.
What do a capital S or T mean in symbolic form?
A capital letter means the special bit is set but the matching execute bit is not. A lower case s or t means both are set. A capital S or T is usually a sign of a mistake.
Why does a directory need execute permission?
On a directory, execute means permission to enter it and reach the files inside. Without it, users cannot open anything in the directory even if they can read the file names.