This program is a simple version of bash (Bourne-Again SHell). To use it run the following:
git clone [email protected]:JOAOVIEG/minishell.git minishell
cd minishell
make run
Feel free to compare with the bash terminal.
A shell is simply a macro processor that executes commands. The term macro processor means functionality where text and symbols are expanded to create larger expressions. As a command interpreter, the shell provides the user interface to the rich set of GNU utilities. The programming language features allow these utilities to be combined. Files containing commands can be created, and become commands themselves. These new commands have the same status as system commands in directories such as /bin, allowing users or groups to establish custom environments to automate their common tasks.
Minishell reads its input and divides the input into words and operators, employing the quoting rules to select which meanings to assign various words and characters. It parses these tokens into commands and other constructs, removes the special meaning of certain words or characters, expands others. The parsing is organized into an AST data structure.
Minishell execution process is recursive and it redirects input and output as needed, executes the specified command, waits for the command’s exit status, and makes that exit status available for further inspection or processing.
Depending on the input, the tree can have one or more levels, with nodes corresponding to commands cmd or pipes | .
The first node is the root and the last the tail.
The cmd node type is itself a structure of arrays: args to hold the command and its arguments,
file in case of redirections, heredocfor the special case redirection regarding this minishell.
The simpliest case is a single command.
For example: echo Hello World!
More complex cases involve pipes. A node of type pipe does not have a command but it is itself an empy node with its fds assigned properly.
The child nodes of this root go down left and right. A generic case cmd1 | <in cmd2 | <<hd | >out is parsed as below:
Similliarly to the diagram for the simple commands showed above, cmd2 as arg[0] = cmd2 file[0] = "<" file[1] = "in" and the same for the other commands and its variables in the example, with the respective values.
| Functions | Usage |
|---|---|
| pipe() | Creates a pipe between parent and child processes for inter-process communication. |
| waitpid() | Suspends the calling process until the specified process changes state. Useful for process synchronization. |
| execve() | Replaces the current process image with a new process image. It's used to run a new program. |
| dup2() | Duplicates a file descriptor, making the old and new file descriptors point to the same file. Useful for redirection. |
| signal() | Sets a function to handle a signal. A signal is a software interrupt delivered to a process. |
| sigaction() | Allows to examine and change a signal action. It's more reliable for signal handling than signal(). |
| isatty() | Tests whether a file descriptor refers to a terminal. |
| readline() | Reads a line from the terminal and returns it, useful for interactive command line programs. |
This minishell implementation was inspired mostly by the following documentation:
Minishell: Building a mini-bash
