umask

Display the current mask in octal notation

$ umask

$ umask -S

$ umask [a+r]

$ umask [077]

umask [options] [mode]

-S
    Produce symbolic output (e.g., 'u=rwx,g=rx,o=rx').

-p
    Output in a format that can be reused as input.

[mode]
    An octal number or symbolic expression representing the new umask.

The umask command sets or displays the file mode creation mask. This mask determines which permissions will be removed from newly created files and directories. The mask is represented in octal notation. When creating a file, the system takes the requested permissions (typically 666 for files and 777 for directories) and then removes the permissions specified by the umask. For example, a umask of 022 means that write permission for group and others will be removed. The command allows users to control default permissions for their files, enhancing security. Without arguments, umask displays the current mask. With an octal number, it sets the mask. Using symbolic notation it can also set the mask based on existing permissions. The command is essential for managing file access control and maintaining a secure system.

The umask only affects permissions at the time of file creation. Changing the umask after a file is created will
not alter the file's existing permissions. The umask is process-specific, meaning changes affect only the current shell and its children.

The umask can also accept symbolic modes similar to chmod. These include specifying user (u), group (g), others (o), and all (a) combined with operators like '+', '-', and '=' to add, remove, or set permissions. For example, 'u=rwx,g=rx,o=' removes all permissions for others.

The umask command is often placed in shell startup files like .bashrc or .profile to set a user's default file creation mask across all sessions.

The umask command has been a standard part of Unix-like operating systems since early versions, providing a way to control file permissions at creation time. It's designed to ensure that files are created with reasonable default security settings, reducing the risk of unintentionally granting overly permissive access to sensitive data. Its importance has persisted due to the fundamental role it plays in managing file access control.

chmod(1), chown(1), setfacl(1)