untitled_shell

A simple shell implemented with abstract syntax tree written in pure C with GNU extensions.

This project is not intended to be a full-featured shell, but rather a simple shell with a simple AST implementation. Note that many features are missing, and the code may crash or behave unexpectedly. Please use it as a learning resource only.

(Expected) Behavior

Parsing

The ast_parse_command() function is responsible for parsing the input string and building the AST. The input string is scanned in the following order:

• If the input string is empty, return NULL.
• If the input string contains ; or &, allocate a AST_LIST, split the input string into two parts and parse each part separately.
• If the input string contains && or ||, allocate a AST_LIST, split the input string and parse it.
• If the input string contains |, allocate a AST_PIPE, split the input string and parse it.
• If the input string contains <, >, << or >>, allocate a AST_REDIRECTION, parse the left side as command and right side as a file.
• If the input string contains " or ', allocate a AST_LITERAL, parse the quoted content as literals.
• Tokenize remaining string with <space>, allocate a AST_COMMAND, and parse as command followed by arguments.
  • argc >= 1
  • arguments[0] is not defined.
  • arguments[n], 1 >= n > argc are pointers to AST_ARGUMENT. AST_ARGUMENT should always to be leaf nodes.

Dumping and Freeing

The ast_print() and ast_free() will dump the content (to stdout) and free the AST, respectively.

Executing

The execution() function accepts a AST and executes it.

• AST_COMMAND fork() (if not forked) and execvpe() the command with AST_ARGUMENTS.
• AST_ARGUMENTS should not be passed into this function.
• AST_LIST fork() and calls the execution() function. waitpid() if specified.
• AST_PIPE open pipes, fork() and calls the execution().
• Other tags are not implemented.

Builtin functions

The shell checks if a function by passing argv[0] to scan_builtin(). If the executable match a builtin command, run_builtin() is called and builtin is executed.

• bye or exit exits the shell.
• cd wraps the chdir() function.
• env dumps the extern char **environ to stdout.
• path sets the PATH environment variable. Parameters are separated by space.

Build

make

or

make dev

For verbose output and printing AST by default.

Credits

Abstract Syntax Tree: An Example in C for the nice example and base Tagged union AST implementation.