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How Contexts Affect Users

From the perspective of a user, contexts are generally unimportant as long as they do not want a program to survive past the end of their login session.

Contexts do become a problem for the administrator, however. For example, if the administrator upgrades sshd by killing the old version, starting the new one, and logging out, strange things could happen since the context in which sshd was running no longer exists.

Contexts also pose an issue for users running background jobs with nohup or users detaching terminal sessions using screen. There are times when it is perfectly reasonable for a program to survive past logout, but by default, this does not occur.

There are three basic ways that a user can get around this. In the case of daemons, they can modify the startup scripts to start the application. On restart, the application will be started in the startup context. This is not very practical if the computer in question is in heavy use, however. Fortunately, there are other ways to start services in a startup context.

The second way to run a service in the startup context is to use ssh to connect to the computer. Since sshd is running in the startup context, programs started from an ssh session also register themselves in the startup context. (Note that a user can safely kill the main sshd process without being logged out. The user just needs to be careful to kill the right one.)

The third way is to log in as the console user (>console), which causes LoginWindow to exit and causes init to spawn a getty process on the console. Since init spawns getty, which spawns login, which spawns the user’s shell, any programs started from the text console will be in the startup context.

More generally, any process that is the child of a process in the startup context (other than those inherited by init because their parent process exited) is automatically in the startup context. Any process that is the child of a process in the login context is, itself, in the login context. This means that daemons can safely fork children at any time and those children will be in the startup context, as will programs started from the console (not the Console application). This also means that any program started by a user in a terminal window, from Finder, from the Dock, and so on, will be in the currently logged in user’s login context, even if that user runs the application using su or sudo.

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