magalhaesm / minishell

This project is about creating a simple shell.

Introduction

The 8th project of 42 cursus syllabus asks students to implement a simplified shell. It's about minishell, as beautiful as a shell.

As the project's subject states, the existence of shells is linked to the very existence of IT. At the time, all developers agreed that communicating with a computer using aligned 1/0 swiches was seriously irritating. It was only logical that they came up with the idea of creating a software to communicate with a computer using interactive lines of commands in a language somewhat close to the human language.

How to compile and run the project

1) Copy this repository to your local workstation

git clone https://github.com/magalhaesm/minishell.git

2) Install the required libraries to run the functions from readline library

sudo apt-get install -y libreadline6 libreadline6-dev

3) Compile the project with Makefile

make

4) Launch the executable file

./minishell

If you want to run the executable program automatically with valgrind flags to check for leaks, just write make checks and press enter.

Implemented builtins

Builtin	Command description
echo with -n	Displays a line of text to the standard output
cd	Changes the working directory of the current shell execution environment
pwd	Prints the full filename of the current working directory
export name[=word]	Sets the export attribute for variables
unset	Unsets values and attributes of variables and functions
env	Sets each name to value in the enviroment and run command
exit	Causes the shell to exit with exit status specified

About how we implemented the project

There are two main files at projects's root directory:

• include: contains all the necessary headers for the project
• grammar: the implemented grammar, which creates and checks the sentences
• libft: our own C library with some helpful auxiliary functions, such as ft_strlen
• tests: contains unit tests created with Criterion testing framework to evaluate functions in src
• src: contains the whole project implementation
  • builtins: implements the required builtins by the project's subject
  • exec: consists of functions that executes user's input
  • expansion: implements shell expansors, such as quote marks and wildcards
  • helpers: contains utils function used on the project as a whole
  • parser: consists of functions that implement the minishell's grammar and create a tree of a given input by the user
  • scanner: implements a scanner to create tokens of a given input by the user
  • signals: consists of functions that handle user events, such as ctrl-C, ctrl-D and ctrl-\
  • table: implements a hash table to store enviroment variables

Project flowchart

In a nutshell, the project flowchart is as follows:

graph LR;
    INPUT-->scanner;
    scanner-->parser;
    parser-->expansor;
    expansor-->executor;
    executor-->OUTPUT

As an example, if given the input true && ls || echo the program generates the tree below to be executed:

graph TD;
    OR-->AND;
    OR-->echo;
    AND-->true;
    AND-->ls;

References

• General references:

  • CS 61: Systems Programming and Machine Organization (2022), by Harvard University
  • NYSTROM, Robert. Crafting interpreters. 2020
  • AHO, Alfred. Compiladores: princípios, técnicas e ferramentas. 2007
  • Playlist about compilers, by Judson Santiago
  • Playlist about Programming Languages and Compilers, by Rafael Ivo
  • Subshell: types and origins, by prog.shell.linux

• Useful tools for grammar generation:

  • The context free grammar checker, by University of Calgary
  • Grammophone: transform context-free grammars

• About hash table implementation

  • FNV Hash, by Landon Curt Noll

• About parser:

  • Recursive Descendant Parsing, by hhp3

• About syntax analyzer:

  • 08: Análise Sintática Top Down: Parser Descendente Recursivo and 10: Análise Preditiva (Parser Recursivo Preditivo, by Andre Du Bois

• About bonus implementation

  • Wildcard Matching with Dynamic Programming, by Tushar Roy

• The same project done by fellows 42SP's students

  • https://github.com/sgkhusal/minishell/
  • https://github.com/SouzaSA/42sp_minishell.git

• Automated tester that helped us to find some bugs:

  • https://github.com/LucasKuhn/minishell_tester

minishell's Issues

Redirect output in append mode with >>

Interactive mode: ctrl-\ does nothing

Handle $? which should expand to the exit status of the most recently executed foreground pipeline

Implement lexical analyzer

Should receive a command line and return a list of tokens.

Redirect output with >

Here-docs mode: handle ctrl+\

Must be done after working here-docs feature.

Implement pipes

Here-documents with <<

Builtin: exit with no options

Have a working history

feat(list): implement AND and OR operators

fix(list): return code is not propagated between commands

Implement wildcard algorithm

Implement parser

Handle environment variables

Adds functions along the lines of libc to handle environment variables

fix(unset): segfault when unsetting the first env

Handle single quote

Display a prompt when waiting for a new command

Builtin: env with no options or arguments

Describe shell grammar using BNF

Builtin: cd with only a relative or absolute path

Handle Ctrl+C, Ctrl+D and Ctrl+\

Builtin: echo with option -n

Expand environment variables

fix(setenv): segfault when receive null value

Review function export.

Here-docs mode: handle ctrl+D

Must be done after working here-docs feature.

Interactive mode: ctrl-D exits the shell

feat(subshell): parenthesis for priorities

Search and launch the right executable

feat(subshell): implement parenthesis for priorities

Here-docs mode: handle ctrl+C

Must be done after working here-docs feature.

Builtin: pwd with no options

fix(signals): double prompt when terminating a child process

Handle EXIT_STATUS from interactive shell and here-docs mode

This task must only be done after we have a structure/global variable to handle shell data.

Builtin: unset with no options

Invalid read of length 1 when it has unmatched strings

Problem found while looking for leaks.

==5169== Invalid read of size 1
==5169==    at 0x109E19: is_at_end (scanner_utils.c:47)
==5169==    by 0x109F87: string (token_word.c:43)
==5169==    by 0x109EB9: token_word (token_word.c:29)
==5169==    by 0x109CB0: scan_double_char_token (scanner.c:66)
==5169==    by 0x109B2D: scan_token (scanner.c:38)
==5169==    by 0x109332: print_tokens (minishell.c:47)
==5169==    by 0x10926F: main (minishell.c:30)

Handle double quote

Builtin: export with no options

Interactive mode: ctrl-C displays a new prompt on a new line

Create hash table for executable paths.

Redirect input with <