Chmod Calculator

How Unix permissions work

Every file and directory has three sets of permissions: for its owner, its group, and others (everyone else). Each set has three bits - read (4), write (2) and execute (1). Add them up per set to get one octal digit: read + write + execute = 7, read + execute = 5, read + write = 6, read only = 4. So 755 means owner 7, group 5, others 5.

Files vs. directories

The same bits mean different things on a directory, which is why the explanation changes when you switch between File and Directory:

• Read on a directory lets you list the names inside.
• Write lets you create, delete and rename entries - but only together with execute.
• Execute lets you enter the directory and reach files inside by name. Without it, even readable files inside are off-limits.

Special bits

• setuid (4000) - an executable runs with the file owner's privileges (how passwd can update the password file).
• setgid (2000) - on directories, new files inherit the directory's group: ideal for team folders.
• sticky (1000) - in a shared directory, only a file's owner can delete it (/tmp is 1777).

Use chmod -R with care: applying 755 recursively also makes every file executable. A safer pattern is find dir -type d -exec chmod 755 {} + and find dir -type f -exec chmod 644 {} +.

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Common values

• 644 - Standard file
• 755 - Directories and programs
• 777 - Everyone can do everything
• 600 - Private file
• 700 - Private directory
• 400 - Read-only, owner only
• 775 - Shared team directory
• 664 - Group-editable file
• 2775 - Shared directory with setgid
• 1777 - Shared temp directory (sticky)