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Lobo Shell

source code

Overview

Lobo Shell is a simple Unix shell implemented in C that demonstrates the following:

  • Interacting with the operating system via system calls.
  • Tokenization and parsing of command-line input.
  • Command pipelines.
  • Redirection operators (e.g., <, >, >>).
  • Interactive command loop.
graph LR
  Input@{ shape: f-circ, label: "Junction" }
  Lexer[Lexer]
  Parser[Parser]
  Executor[Executor]
  Output@{ shape: f-circ, label: "Junction" }

  Input --> |text| Lexer --> |tokens| Parser --> |commands| Executor --> |output| Output

Key Components

Types

typedef struct Token {
    TokenType type;
    char *text; // lexeme
} Token;
Example 
Token pipe          = { T_PIPE: "|" };
Token redir_in      = { T_REDIR_IN: "<" };
Token redir_out     = { T_REDIR_OUT: ">" };
Token redir_append  = { T_REDIR_APPEND: ">>" };
Token word          = { T_WORD: "everything_else" };

typedef struct TokenStream {
    Token *tokens;
    int size;
} TokenStream;
Example 
input = "grep foo < in.txt | sort > out.txt | uniq >> append.txt";

tokens = {
    { T_WORD: "grep" },
    { T_WORD: "foo" },
    { T_REDIR_IN: "<" },
    { T_WORD: "in.txt" },

    { T_PIPE: "|" },

    { T_WORD: "sort" },
    { T_REDIR_OUT: ">" },
    { T_WORD: "out.txt" },

    { T_PIPE: "|" },

    { T_WORD: "uniq" },
    { T_REDIR_APPEND: ">>" },
    { T_WORD: "append.txt" }
};

Warning

I am refactoring for a new Command struct.

typedef struct Command {
    char **argv;
    char *infile;
    char *outfile;
    bool append;
    bool is_first;
    bool is_last;
} Command;
Example 
input = "grep foo < in.txt | sort > out.txt | uniq >> append.txt";

cmd_1 = {
    argv: [ "grep", "foo", NULL ],
    infile: "in.txt",
    outfile: NULL,
    append: false,
    is_first: true,
    is_last: false
};

cmd_2 = {
    argv: [ "sort", NULL ],
    infile: NULL,
    outfile: "out.txt",
    append: false,
    is_first: false,
    is_last: false
};

cmd_3 = {
    argv: [ "uniq", NULL ],
    infile: NULL,
    outfile: "append.txt",
    append: true,
    is_first: false,
    is_last: true
};

typedef struct Pipeline {
    Command *cmds;
    int size;
} Pipeline;
Example 
input = "grep foo < in.txt | sort > out.txt | uniq >> append.txt";

pipeline = {
    cmds: [ cmd_1, cmd_2, cmd_3 ],
    size: 3
};

Functions

TokenStream lexer(char *line) {
    // <line> --> <tokens>
}

Pipeline parse_tokens(TokenStream ts) {
    // <tokens> --> <commands>
}

void execute_pipeline(Pipeline pl) {
    // execute <commands>
}