Which command lets you alter the default permissions in a shell?

The umask command is used to set default permissions for newly created files and directories in a shell. When a file or directory is created, the umask value dictates what permissions will be masked or denied from the default set, allowing you to control the permissions for users, groups, and others. By setting a specific umask value, you can restrict or modify the permissions that are automatically assigned when a new file or directory is created.

For instance, if the system defaults to allow read and write permissions for the owner, read permissions for the group, and no permissions for others (often represented as 664 for files and 775 for directories), you can use umask to change these defaults, resulting in different permissions upon file creation.

The other commands listed serve different purposes. chmod is utilized for changing permissions on existing files and directories rather than setting default permissions. lsmod and insmod relate to module management in the Linux kernel, specifically for loading and unloading kernel modules, and are unrelated to file or directory permissions.