Tick the permissions, or type a mode such as 755 or rwxr-xr-x, and get the octal value, the symbolic form and the chmod command.
Permission bits
Class
Readr = 4
Writew = 2
Executex = 1
Special
Owneru
Groupg
Otherso
What this mode allows
Command with the octal mode
Command with the symbolic mode
Common modes
The owner can change the file and everyone can read it. This is the usual mode of a regular file.
Only the owner can read and change the file. Use it for private files such as SSH keys.
The owner can change it, and everyone can read and execute it. This is the usual mode of programs and directories.
Only the owner can read, change and execute it. Use it for private directories and scripts.
The owner and the group can change it, and everyone can read and execute it. Use it for directories that a group shares.
The owner and the group can change the file, and everyone can read it.
Only the owner can read the file, and no class has write permission.
Everyone can create files, but the sticky bit stops users from deleting each other's files. This is the mode of /tmp.
About this tool
How a mode is built
Unix keeps three sets of permissions for every file: one for the owner, one for the group and one for everyone else. Each set has three bits, read (4), write (2) and execute (1), and the sum of the bits that are on is one octal digit. In 755 the owner has 4 + 2 + 1 = 7 and the group and the others have 4 + 1 = 5, which ls -l prints as rwxr-xr-x.
The symbolic field also accepts the full ten-character string from ls -l, such as -rw-r--r-- or drwxr-xr-x. The first character is the file type, not a permission, so the tool ignores it.
What the bits mean on a directory
On a directory, read lets you list the names inside it, and execute lets you enter it and reach the files inside. Write lets you create, delete and rename entries, but only together with execute. That is why a directory usually has execute wherever it has read. It also means that deleting a file depends on the permissions of the directory that contains it, not on the permissions of the file.
setuid, setgid and the sticky bit
A fourth digit in front holds three special bits: setuid (4), setgid (2) and sticky (1). ls -l shows them in the execute position as s or t. A capital S or T means that the special bit is set while the execute bit under it is not.
The two commands that this tool prints set the bits you ticked, but chmod does not always remove a special bit that the file already has. GNU chmod, which most Linux distributions use, keeps the setuid and setgid bits of a directory unless the mode names them, even with chmod 755: write chmod 00755 or add a-s to clear them. The chmod of macOS keeps the sticky bit through a clause such as o=rx, and chmod -t removes it. The sticky bit is printed as a separate +t clause because both implementations accept that form.
Questions
What does chmod 755 mean?
The owner can read, write and execute; the group and everyone else can read and execute but not write. It is the usual mode for programs, scripts and directories that others may use but only the owner may change.
What permissions should an SSH private key have?
Use 600, so that only the owner can read and write the file. OpenSSH refuses to use a private key that other users can read.
Why should I avoid chmod 777?
Mode 777 lets every user on the system change or replace the file, and add or delete files if it is a directory. If a program fails with a permission error, find out which user it runs as and give that user or its group the access it needs.
What does a capital S or T mean in ls -l?
It means that setuid, setgid or the sticky bit is set, but the execute bit in the same position is not. A lower-case s or t means that both are set.
Is anything I type sent to a server?
No. The conversion is done by JavaScript in your browser tab. The tool makes no network request with what you type, and the site's security policy blocks requests to any other host.