minishell

A small Unix shell in C, with the command line parsed by a lex scanner and a yacc grammar rather than by hand-rolled string splitting.

What it supports

ls -l | grep '^d' | wc -l        # pipelines of arbitrary length
sort < input.txt > output.txt    # stdin and stdout redirection
make >& build.log                # stderr redirection
sleep 30 &                       # background execution
echo $HOME/$USER                 # environment variable expansion
ls fil?.txt                      # single-character wildcard
cd /tmp                          # builtin
umask 022                        # builtin

How it works

Parsing. scanner.l tokenises the line into words and the four operators (|, <, >, &); parser.y holds the grammar. Every reduction calls back into the shell — command() appends an argument, pipeline() closes the current command and starts the next — so the parser fills the argument vectors directly instead of building a tree that would then have to be walked.

Redirections are collected into a three-slot filev array, one per stream, and the grammar rejects a second redirection of the same stream at parse time rather than at execution time.

Execution. The shell forks one child per pipeline stage and creates a pipe between each pair. Each child rewires its ends with dup2, closes every descriptor it does not need — the part that is easy to get wrong, because a single leaked write end leaves the next stage blocked on a read that never returns — and calls execvp.

For a foreground pipeline the shell waits for the children; for a background one it does not, and prints the process group so the job can be found later.

Expansion. Variable and wildcard expansion happen after parsing and before execution, so a $VAR holding spaces expands to one argument rather than being re-tokenised.

Signals. SIGINT and SIGQUIT are ignored in the shell itself and restored to their defaults in the children, so Ctrl-C kills the running command and leaves the prompt alive.

Build and run

Requires gcc, flex and bison (or lex and yacc).

make
./msh

Layout

File	Contents
main.c	Execution: fork, pipes, redirection, expansion, builtins
scanner.l	Lexer — words and operators
parser.y	Grammar and the callbacks that fill the argument vectors
Makefile	Provided with the assignment; not to be modified

Licence

See LICENCIA — Licencia de Proyecto Educativo Práctico (LPEP). This was an assignment at ETSIINF, Universidad Politécnica de Madrid, built on a skeleton authored by the course staff.