How Linux permissions map to bits

Every file and directory has three permission classes: the owner, the group, and everyone else (others). Each class carries three permission bits:

read (r) has the value 4
write (w) has the value 2
execute (x) has the value 1

Add the values for a class to get its octal digit. Read and write is 4 + 2 = 6, read and execute is 4 + 1 = 5, and all three is 4 + 2 + 1 = 7. Line up the digits for owner, group, and others and you get the familiar three digit mode, such as 644 or 755.

Symbolic notation shows the same information as nine characters, three per class, in the order read, write, execute. A dash means the bit is off, so rw-r--r-- is the symbolic form of 644.

Common permission modes

Mode	Symbolic	Typical use
644	rw-r–r–	Regular files that the owner edits and everyone reads
755	rwxr-xr-x	Directories and scripts: owner full access, others read and run
600	rw——-	Private files such as SSH keys and credentials
700	rwx——	Private directories only the owner may enter
777	rwxrwxrwx	Full access for everyone, which is rarely safe. See what 777 means before using it

Special bits

A fourth, leading digit sets the special bits:

setuid (4) runs an executable with the owner’s privileges
setgid (2) makes new files in a directory inherit the group, and runs executables with the group’s privileges
the sticky bit (1) on a shared directory lets only a file’s owner delete or rename it, which is how /tmp works

When a special bit is set, its execute position in the symbolic output changes: a lowercase s or t means execute is also on, while an uppercase S or T means execute is off. For the full command syntax and recursive changes, see the chmod command guide .

Toggle read, write, and execute for owner, group, and others
Class	Read	Write	Execute
owner
group
others

How Linux permissions map to bits #

Common permission modes #

Special bits #

How Linux permissions map to bits

Common permission modes

Special bits