execution.md

Execution Engine

The execution module is the heart of Minishell, responsible for coordinating process creation, data flow between commands, and handling external binaries.

Command Execution Flow

When a command is submitted, the execution engine performs the following:

1. Redirection Handling: It processes redirections (<, >, <<, >>) first to set up the appropriate file descriptors for input and output.
2. Built-in Check: It checks if the command is a built-in. If so, it executes it directly in the current process.
3. External Command Execution: If the command is not a built-in, it forks a new process and uses execve to search for and execute the binary using the PATH environment variable.

Handling Pipelines

Pipelines (|) allow the output of one command to be used as the input for another.

• Minishell creates a pipe between each command in a pipeline.
• The stdout of the preceding command is redirected to the write-end of the pipe.
• The stdin of the succeeding command is redirected to the read-end of the pipe.
• All processes in a pipeline are forked and executed concurrently.

Redirections

• Input Redirection (<): Reading input from a file instead of stdin.
• Output Redirection (>): Directing output to a file instead of stdout (overwrites).
• Append Redirection (>>): Directing output to a file instead of stdout (appends).
• Heredoc (<<): Providing input directly in the shell until a specified delimiter is reached.

Process Management

Minishell carefully manages child processes:

• It waits for all commands in a pipeline to finish using waitpid.
• It captures and updates the exit status (accessible via $?).
• It properly closes file descriptors after use to prevent leaks and exhausted resources.