0666

0666 is an octal notation frequently used in Unix-like systems to specify file permission modes. The leading zero signals that the number is in base-8 (octal). The digits 666 indicate read and write permissions for the user (owner), the group, and others, with no execute permission. In symbolic form, this corresponds to rw-rw-rw-.

In practice, 0666 is used as a mode or mask when creating files. The actual permissions that result when a file is created depend on the process’s umask. For example, with a typical umask of 0022, opening or creating a file with mode 0666 yields 0644 (rw-r--r--). If the umask were 0000, the resulting permissions would be 0666. Executable bits are not granted by 0666 itself.

Numerically, 0666 in octal equals 438 in decimal. It represents the absence of the execute bit, distinguishing it from 0777 (read, write, and execute for all). The three octal digits map directly to user, group, and other permissions.

In programming and system administration, leading zeros to denote octal literals are common in languages such as C and C++, where 0666 may appear in code that creates files. Some modern languages and tools prefer explicit octal syntax (for example, 0o666 in Python 3). In shell and chmod usage, 666 or 0666 is understood as world-readable and writable permissions, subject to the system’s umask and security policies. Caution is advised, as world-writable files can pose security risks in multi-user or publicly accessible environments.