How the Shell Executes Programs: Fork, Exec, and Environment Variables
Preamble
A shell process is a command-line interpreter — the program that reads your input, locates the right program or builtin command, and orchestrates its execution. Whether you use Bourne Shell, Bash, Korn Shell, or another flavor, the underlying mechanics are remarkably similar: the shell forks a child process, the child replaces its image with the target program via exec, and the parent waits for completion. In this post, we'll trace that fork-exec-wait cycle step by step, explore how environment variables are passed to programs, and look at the shell's capabilities as an interpreter — including loops, conditionals, and history expansion. This is the second in a three-part series. The first post covers UNIX terminal devices and line discipline, and the third explores writing portable C code.