Custom Unix Shell

Tru Bui

A custom Unix shell written in C/C++. Supports running programs, redirection, pipes, background jobs, line editor, wildcard expansion, and path completion. Built using Flex and Bison.

Key Features

• Lex/Yacc-based parser supporting complex command grammars
• Execution engine using fork(), execvp(), and waitpid()
• Full support for I/O redirection: >, >>, <, 2>, >&, >>&
• Pipe operator
• Background job execution (&) and process management
• Signal handling: Ctrl-C behavior, SIGCHLD zombie cleanup
• Built-in commands: cd, exit, setenv, unsetenv, printenv, source
• Subshells
• Environment variable expansion
• Tilde expansion & wildcarding (* and ? with regex)
• Custom line editor (arrow keys, backspace, delete, home/end)
• Command history with up/down arrows

Implementation Details

The project is split into several core components:

• Lexer (Flex): tokenizes commands, handles quotes, escapes, subshell extraction
• Parser (Bison): builds a full AST of Command and SimpleCommand objects
• Executor: performs process creation, piping, redirection, job control
• Signal subsystem: manages Ctrl-C and zombie processes safely
• Line editor: implements raw-terminal input, cursor movement, dynamic insertion
• Wildcard engine: converts shell wildcards to regex using regcomp
• Subshell mechanism: uses two pipes + yy_unput() to inject subshell output into lexer

Contact me to see source code!