This was a group project and it was done with Joao Rema.

42 - MINISHELL

SUMMARY

Minishell is a minimal Unix shell implemented in C, designed to replicate core shell functionalities and provide a hands-on understanding of shell. It supports command parsing, execution, pipelines, redirections, environment variable management, and several built-in commands.

We divided the project into 2 big parts parsing/execution and builtin-in commands (cd, echo, pwd, env, unset, exit, export).

I implemented the built-in commands and adjusted the execution flow to incorporate these commands.

REPOSITORY STRUCTURE

• inc/: Contains the header file minishell.h and Mylib/, which contains a custom library for utility functions.
• src/: Contains the source code for the program.
• Makefile: Tells make how to compile and link the program.
• README.md: Project overview and usage instructions.

KEY FEATURES

Syntax Check

It detects unclosed quotes, misplaced pipes/redirections, usage of unsuppported operators (&& and ||);
Reports error message and avoid running on unecessary inputs.

Tokenization

It keeps track of quotes to avoid splitting words;
It splits input into tokens: words, pipes, redirections;
Returns all possible tokens at the end and skips spaces.

Binary Tree (AST) Building

Recursive Descent Parsing :
  Pipes: parse_pipe recursively splits the token stream at pipe tokens, creating a binary tree where each pipe is a node.
  Redirections: parse_redirect handles redirection tokens (<, >, >>, <<), building subtrees for each redirection.
  Commands: parse_command collects consecutive WORD tokens as command arguments and creates command nodes.

Node Types:

  Each node (t_tree) has a type (PIPE, REDIRECT_L, REDIRECT_R, APPEND, HEREDOC, WORD, etc.).
  Arguments: Command nodes store argument arrays, in position 0 is the cmd.
  Redirection Info: Redirection nodes store file info and heredoc filenames.

Binary tree example:

Input line: cat file.txt | grep hello > out.txt

      PIPE
      "|"
     /    \
 CMD       REDIR_OUT
"cat"         ">"
 ARG         /   \
file.txt    CMD   FILENAME
         "grep"  "out.txt"
           ARG
          "hello"

Executing the tree:

1 Heredoc Execution

It detects how many heredocs are there in the input line;
Detects the EOF,
Creates a temporary file,
All heredocs are handled  in a child process,
Expandes variables inside heredoc if EOF not quoted.

2 Expansions

Checks the tree for nodes that need expanding.
Replaces with necessary info from variables.
It handles for exemple : "$VAR", "$?", "$0";

3 Pipe Handling

  Set up pipes between commands when a PIPE node is encountered.
  Uses fork(), pipe(), and dup2() to connect child processes;

4 Redirection Handling

  Apply input/output redirections as specified by REDIRECT nodes.
  It performs the command linked to the redirect token te correct stdin/stdout;

5 Built-in Command Execution

  Detect and execute built-in commands directly in the shell process.

6 External Command Execution

  Fork child processes and use execve to run external cmds.

7 Process Synchronization

  Wait for all child processes to finish and collect their exit statuses.

Signal Handling

SIGINT (Ctrl+C)
SIGQUIT (Ctrl+):
Safe Handling during heredoc, builtins, execution phases.

Exit Code

Set the correct exit code;
By expanding $? exit codes can be checked;

USAGE

1. Compile the program:

make

2. Run the program with no arguments:

./minishell