What three files are always opened by a process

Conceptual level

When you start a process from your shell, the current working directory of the process is the same as the current working directory of your shell. In the context of the shell, the current working directory is the location you are currently "at."

The current working directory of any process is used to interpret relative paths. For example, if your shell's current working directory was /home/rene and you ran ls .. from the shell, the process's current working directory, /home/rene, would be used to resolve .. to /home.

You can see the current working directories of all of the processes running on your system with lsof | grep '\scwd\s' (note that you'll probably need to be root to see other users' processes.) This can give you an idea of how current working directories relate to the processes running on your system.

Program level

The current working directory of the shell is the directory you see and modify with the shell built-ins pwd and cd respectively. These commands call system calls such as getcwd and chdir that work with the current working directory of your shell.

Using bash as an example, the cd built-in eventually runs this line:

if (chdir (nolinks ? newdir : tdir) == 0)

and the pwd built-in eventually runs this line:

the_current_working_directory = getcwd (0, PATH_MAX);

The shell is just a convenient example of the current working directory's use; these same system calls are used by other programs as well. A program can change its current working directory to /usr and any relative paths that the program uses will start from the /usr directory,

Kernel level

The current working directory of a process is stored by the kernel. Linux stores it in the pwd member of a fs_struct pointed to by the fs member of a task_struct. The pwd member is a path struct, which points to information about the mount (vfsmount) and the directory's location in the filesystem (dentry). Each process has a task_struct associated with it.

The chdir and getcwd system calls modify and retrieve information in pwd.

Look at the man page for fork(2). A child process inherits the parent's file descriptors, including standard input and standard output. For a single command, the shell simply lets the child process inherit those descriptors and write its output to the terminal. For a pipeline, it forks each process, sets up a pipe between the output of one and the input of the next, and then executes (exec(2)) each child's executable.