Unix File Permissions Calculator (chmod)

How Unix File Permissions and chmod Octal Values Work

Every file and directory on a Unix or Linux system carries nine permission bits, arranged as three permissions across three classes of user. The permissions are read, write and execute. The classes are the file owner, the group the file belongs to, and everyone else, conventionally called others. This calculator lets you tick the boxes you want and immediately shows the octal number, the symbolic string and the ready-to-run chmod command. Sysadmins fixing a permission-denied error, developers deploying a web application, and anyone who has been told their SSH key is too open all need to translate between those three representations quickly. The octal maths is simple addition and worth learning once. Read is worth 4, write is worth 2, execute is worth 1. Add the values you want for a class and you get a single digit from 0 to 7, because 4 plus 2 plus 1 is 7 and there is no combination that overflows. Read plus write plus execute is 7, read plus write is 6, read plus execute is 5, read alone is 4, and nothing at all is 0. Three digits, one per class in the order owner, group, others, describe the whole file. The symbolic form shows exactly the same information as nine characters: three groups of three where a letter means granted and a dash means denied, so rwxr-xr-x and 755 are two spellings of one thing. Execute means something different on a directory, incidentally: it grants the right to enter it and access its contents rather than to run it. Work two real values. For a shell script you want to run yourself and let others run but not modify, tick read, write and execute for owner giving 4 plus 2 plus 1 equals 7, then read and execute for group giving 4 plus 1 equals 5, and the same for others. The result is 755, symbolic rwxr-xr-x, and the command chmod 755 deploy.sh. Now take a private configuration file holding a database password. The owner needs read and write, which is 4 plus 2 equals 6; the group gets read only, which is 4; others get nothing, which is 0. That gives 640, symbolic rw-r-----, and chmod 640 .env. Notice the third digit being zero is the entire security decision, and it is a single character. Where this bites in practice. A developer uploads a PHP application and gets a 403 error because the directory lacks the execute bit the web server needs to traverse it. Someone copies a private key onto a new laptop and SSH refuses to use it until the file is chmod 600, because SSH deliberately rejects keys that any other user could read. A CI pipeline fails because a build script is not executable, fixed by adding the execute bit for the owner. An operations engineer audits a server and finds an uploads directory left at 777 from a debugging session. The pitfall is 777, and it deserves stating bluntly: it grants read, write and execute to everyone on the system, which on a shared or public-facing server means any other user or any compromised process can rewrite your files. It is the permission people reach for when something does not work, and it fixes the symptom by removing the protection. The right move is to work out which class actually needs the access. Two related notes: permissions interact with ownership, so chmod alone will not help if the wrong user owns the file and you need chown instead; and the setuid, setgid and sticky bits form an optional fourth leading digit that this calculator covers as a separate concern. Everything runs in your browser and nothing is uploaded.

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