Welcome back. When I
was a graduate student, we liked to play computer games. These were all owned by root and ran with root privileges, so the games could
maintain high score files. We also found out you could spawn a subshell from the game, to do something useful, and then return to the game. It turned out that the subshell
run with root privileges. So to become the root user, you started up again, and then spawn to subshell. Why did this happen
and how is it fixed? We'll talk about that and
other things in this module. In this module,
files, subprocesses, and race conditions, we will demonstrate how to
create a child process, discussed the
subprocess environment, and then share
design tips about working effectively with files
and subprocesses. Then we will discuss
race conditions. We will walk through
a classic example of this and explore time of check the time of use race conditions in detail. Finally, we'll explain various programming and
environmental conditions to finish this module. The interaction between
processes occurs in many ways. Most of these are harmless. At least to create
conditions where the interaction may cause problems and this module
covers both of them. The parent process controls
the environment of its child. So if the parent is running with more privileges than
the child should run with, the parent process must restrict the subprocesses,
environment, and privileges. When two processes are active,
have different privileges, and both use a common resource, they need to ensure
the ordering of their actions does not
create a security problem. That's what race conditions do. This condition is a situation in which such a security
problem may occur. By the end of this module, you will understand how files
and subprocesses interact, and be able to create
subprocesses in shell scripts. You will also be
able to identify and prevent race conditions
in your programs.