chmod Calculator

How to use chmod Calculator

1. Tick Read, Write and Execute for Owner, Group and Others, and any Special bits (setuid, setgid, sticky). Or type a mode into Octal (755, 0755, 4755) or Symbolic (rwxr-xr-x, or an ls -l string like drwxrwxrwt). All views stay in sync.
2. Tick It's a directory when the mode is for a directory. It changes the ls -l type character and how X behaves in expressions.
3. Copy the ls -l form, the chmod command in octal or symbolic form, or the 4-digit octal. Warnings appear for risky modes such as 777, world-writable files and setuid.
4. Pick a Preset such as 644, 755, 600 or 1777, or type a chmod expression under Apply a symbolic mode (for example u+x,g-w,o=r or a+rX) and press Use this mode to take the result.
5. Enter a umask in the umask calculator to see the default mode of New files and New directories, and press Use to load one.

How it works

A Unix mode is 12 bits: three bits (read 4, write 2, execute 1) each for owner, group and others, plus setuid (4000), setgid (2000) and sticky (1000). Octal is those bits written in base 8. In the symbolic form a special bit replaces the matching execute position: s or t when execute is also set, capital S or T when it isn’t. When you paste an ls -l string, the first character is read as the file type (- d l c b p s) and a trailing ., + or @ (SELinux, ACL or extended attributes) is ignored.

Expressions follow POSIX and GNU chmod rules. Each comma-separated clause is [ugoa][+-=][rwxXst], applied in order to the current mode. X adds execute only for a directory or when someone already has execute, permissions can be copied from another class (o=g, go=u-w), and = on a directory keeps its setgid bit, as GNU chmod does. A clause that is only digits replaces the whole mode.

The umask calculator removes the umask’s bits from 666 for files and 777 for directories. Everything is calculated instantly in your browser.

Limits

• Octal modes are 1 to 4 digits from 0 to 7, with an optional 0o prefix. Symbolic input must be exactly 9 characters, or 10 (11 with a trailing ., + or @) for an ls -l string.
• In Apply a symbolic mode, a clause without u, g, o or a (such as +x) applies to everyone and is not reduced by the umask field, whereas real chmod would mask it with your shell’s umask.
• Copying from a class takes one class at a time (o=g, not o=gu) and can’t be mixed with r, w or x in the same operator; t only affects others, so u+t changes nothing.
• Only the permission bits are modelled. Owner and group, ACLs and extended attributes aren’t, and the result isn’t checked against any real file.

Privacy

Everything is calculated in your browser and nothing is uploaded or saved; the site’s Content Security Policy blocks requests to other servers. Copy share link puts the mode, the directory setting, the expression and the umask in the link’s # fragment, which browsers don’t send to servers.

Frequently asked questions

What is the difference between 755 and 644?

755 (rwxr-xr-x) lets the owner read, write and execute, and everyone else read and execute; it suits programs and directories. 644 (rw-r--r--) has no execute bit and suits ordinary files.

What does a capital S or T mean in ls -l output?

The setuid, setgid or sticky bit is set but the matching execute bit isn’t. For setuid on a file that isn’t executable by its owner this has no effect, and the tool warns about it.

Why does a 4-digit mode like 4755 start with 4?

The first digit holds the special bits: 4 for setuid, 2 for setgid and 1 for sticky, added together. 4755 is 755 plus setuid, and 1777 is the usual mode for /tmp.

What does capital X do in a chmod expression?

X adds execute only if the target is a directory or someone already has execute. a+rX makes a tree readable without making ordinary files executable. Tick It's a directory to see the directory result.

Why is 777 a bad idea?

Every user on the system can change or replace the file. Use 755, or 775 for a directory shared by a group. For shared writable directories, add the sticky bit (1777) so users can only delete their own files.

How does umask decide the default permissions?

New files start from 666 and new directories from 777, and every bit set in the umask is removed. With the common 022, files get 644 and directories 755; with 077 only the owner has access.