The program that starts programs

Processes Threads and the Kernel · Open board

The kernel handing each program its own walled-off memory and its own turn on the chip. This lesson builds the ordinary program that asks the kernel for those walls, one command at a time: the shell.

Before you start. A process is one running program, walled off so nothing else can touch its memory. The kernel is the one program that can make new ones (Processes, Threads & the Kernel). Picture each process with numbered doors: when it opens a file, the kernel hands it a number, and the program reads and writes through that door. The numbers are file descriptors (I/O, Interrupts & the Bus). Part one of this build made a small machine that runs instructions (Build a stack machine). This part builds the program that starts other programs.

The idea in 30 seconds

A shell is an ordinary program that runs a loop: read a line, ask the kernel to copy the shell (fork), have the copy replace itself with the command (exec), and wait for it to end.
fork returns twice: in the original it returns the new process’s id (the number the kernel gives each process), and in the copy it returns 0. That is how each knows which one it is.
cd and exit must run inside the shell itself. A separate program can change only its own working directory, and that change ends with it.
>, < and | work by repointing a child’s input and output (file descriptors 0 and 1, the numbers it reads and writes through) after fork and before exec, so the command never knows where its input and output go.
A pipe is a one-way channel between two programs. Its reader sees end of input only when every process holding the end it is written into has closed it, so a shell that forgets to close its own leaves the reader waiting forever.
Ctrl-C makes the kernel send a signal, a numbered note whose default effect is to end the process, to the shell and the running command together. The shell ignores it. Each child must switch back to the default before it replaces itself with the command (exec), because an ignored signal stays ignored.

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