Okay, let’s tackle background processes, pipes, and more advanced shell features, keeping in mind the constraints of Lua tables and command-line argument conventions.
1. Background Processes
A common way to represent background processes in shells is with the & operator. Since we’ve moved away from special syntax and are using a Lua function (do), we need a Lua-idiomatic way to express this. We could add a boolean option to our do function, or we could introduce a separate function, say spawn. Let’s go with the spawn function approach for clarity. spawn will behave similarly to do, but it won’t wait for the process to complete.
- Implementation:
- In Rust, we’ll create a new function
spawn that, instead of calling output() on the Command, calls spawn(). This method doesn’t wait for completion.
spawn() returns a Child struct. We need a way to manage these Child processes. We could return the Child directly to Lua, but that exposes a raw Rust type, which isn’t ideal. Instead, we’ll keep a table of running processes within the Rust side of our application and return an identifier (e.g., an integer ID) to Lua. This ID can then be used with other functions (like a hypothetical wait function).- The
spawn function would return a table containing at least a pid field (the process ID).
2. Pipes
Pipes are a fundamental shell feature (command1 | command2). We need a way to represent this chaining of commands in Lua. Several options come to mind:
- A
pipe function: pipe(do("cmd1"), do("cmd2")). This is verbose.
- Overloading
do (or a new function like chain) to accept a table of commands: chain({do("cmd1"), do("cmd2")}). This is better, but nested do calls are still a bit awkward, especially if do returns a table.
- A dedicated
pipeline function that takes a table of command specifications: pipeline({ {"cmd1", {"arg1"}}, {"cmd2", {"arg2"}} }). This seems the most Lua-like and avoids nested function calls.
Let’s choose the pipeline function approach.
- Implementation:
- The
pipeline function will take a table of command specifications. Each specification is itself a table: {command_name, args_table, env_table (optional)}.
- In Rust, we’ll iterate through this table of specifications.
- For the first command, we create a
Command as usual, but we set its stdout to Stdio::piped().
- For subsequent commands, we set their
stdin to the stdout of the previous command’s Child process. We also set their stdout to Stdio::piped() (unless it’s the last command).
- For the last command, we capture its
stdout and stderr as usual.
- We need to be careful about closing the pipes at the right times to avoid deadlocks.
- The
pipeline function should return a result similar to do, with output, error, and status.
3. Argument Handling (Revisited)
We’ve already addressed positional arguments (using integer keys in the Lua table) and environment variables (using string keys). Let’s clarify how this interacts with pipes and background processes.
- Positional Arguments: These are handled consistently across
do, spawn, and pipeline. The Lua table’s integer keys (1, 2, 3…) are mapped to the command-line arguments.
- Environment Variables: The optional third argument to
do, spawn and the table elements in pipeline allow setting environment variables. This is a good way to handle “named” parameters that are passed as environment variables to the underlying command.
- Named parameters (flags): handled by the command itself as positional arguments.
4. wait Function (for background processes)
To manage background processes, we need a way to wait for them to finish (and get their exit status). A wait function is a natural fit.
- Implementation:
wait(pid): Takes a process ID (returned by spawn) as input.- In Rust, we look up the corresponding
Child process in our table of running processes.
- We call
child.wait() to wait for the process to finish.
- We remove the process from our table.
- We return the exit status (and potentially captured output/error, if we stored it).
5. Error Handling
- Command Not Found: We already handle this by capturing the error from
std::process::Command.
- Invalid Arguments: The Rust functions should validate the types of the Lua arguments (string for command, table for args/env) and return Lua errors if they’re invalid.
- Pipe Errors: We need to carefully handle errors that might occur during pipe setup or execution.
wait Errors: If wait is called with an invalid PID, we should return an error.
Revised Code (Illustrative – not fully complete):
use piccolo::{Compiler, Executor, FromValue, Function, Lua, String, Table, Value};
use std::process::{Command, Stdio, Child, ExitStatus, Output};
use std::io::{self, Write};
use std::collections::HashMap;
use std::sync::{Arc, Mutex};
// Structure to hold a background process and its ID
struct BackgroundProcess {
id: usize,
child: Child,
}
// Shared state for background processes
#[derive(Clone, Default)]
struct BackgroundProcesses {
next_id: usize,
processes: HashMap<usize, Child>,
}
impl BackgroundProcesses {
fn new() -> Self {
BackgroundProcesses {
next_id: 1,
processes: HashMap::new(),
}
}
fn add(&mut self, child: Child) -> usize {
let id = self.next_id;
self.processes.insert(id, child);
self.next_id += 1;
id
}
fn remove(&mut self, id: usize) -> Option<Child> {
self.processes.remove(&id)
}
fn get_mut(&mut self, id: usize) -> Option<&mut Child> {
self.processes.get_mut(&id)
}
}
// Type conversion
fn output_to_lua_table(output: Output) -> Table {
let mut table = Table::new();
table.set_field("status", output.status.code().unwrap_or(-1)).unwrap();
table.set_field("output", String::from(String::from_utf8_lossy(&output.stdout).into_owned())).unwrap();
table.set_field("error", String::from(String::from_utf8_lossy(&output.stderr).into_owned())).unwrap();
table
}
// 'do' function (foreground execution)
fn do_fn(ctx: &piccolo::Context, bg_processes: Arc<Mutex<BackgroundProcesses>>) -> Function {
Function::new(ctx, move |ctx, _, mut stack| {
let cmd: String = stack.consume(ctx)?;
let args: Option<Table> = stack.consume(ctx)?;
let env: Option<Table> = stack.consume(ctx)?;
let args_vec = args.map(|t| table_to_args(t)).unwrap_or_default();
let env_vec = env.map(|t| table_to_env(t)).unwrap_or_default();
let output = Command::new(cmd.to_string())
.args(args_vec)
.env_clear()
.envs(env_vec)
.output()
.unwrap_or_else(|e| Output {
status: ExitStatus::from_raw(0),
stdout: Vec::new(),
stderr: format!("Command execution failed: {}", e).into_bytes(),
});
stack.push_back(ctx, output_to_lua_table(output));
Ok(())
})
}
// 'spawn' function (background execution)
fn spawn_fn(ctx: &piccolo::Context, bg_processes: Arc<Mutex<BackgroundProcesses>>) -> Function {
Function::new(ctx, move |ctx, _, mut stack| {
let cmd: String = stack.consume(ctx)?;
let args: Option<Table> = stack.consume(ctx)?;
let env: Option<Table> = stack.consume(ctx)?;
let args_vec = args.map(|t| table_to_args(t)).unwrap_or_default();
let env_vec = env.map(|t| table_to_env(t)).unwrap_or_default();
let child = Command::new(cmd.to_string())
.args(args_vec)
.env_clear()
.envs(env_vec)
.spawn();
match child {
Ok(child) => {
let mut bg_processes = bg_processes.lock().unwrap();
let pid = bg_processes.add(child);
let mut table = Table::new();
table.set_field(ctx, "pid", pid).unwrap();
stack.push_back(ctx, table);
},
Err(e) => {
let mut table = Table::new();
table.set_field(ctx, "error", format!("Failed to spawn: {}", e)).unwrap();
stack.push_back(ctx, table);
}
}
Ok(())
})
}
// 'wait' function (wait for background process)
fn wait_fn(ctx: &piccolo::Context, bg_processes: Arc<Mutex<BackgroundProcesses>>) -> Function {
Function::new(ctx, move |ctx, _, mut stack| {
let pid: usize = stack.consume(ctx)?;
let mut bg_processes = bg_processes.lock().unwrap();
if let Some(mut child) = bg_processes.remove(pid) {
match child.wait() {
Ok(status) => {
let mut table = Table::new();
table.set_field(ctx, "status", status.code().unwrap_or(-1)).unwrap();
stack.push_back(ctx, table);
}
Err(e) => {
let mut table = Table::new();
table.set_field(ctx, "error", format!("Wait failed: {}", e)).unwrap();
stack.push_back(ctx, table);
}
}
} else {
let mut table = Table::new();
table.set_field(ctx, "error", "Invalid process ID").unwrap();
stack.push_back(ctx, table);
}
Ok(())
})
}
// 'pipeline' function
fn pipeline_fn(ctx: &piccolo::Context, bg_processes: Arc<Mutex<BackgroundProcesses>>) -> Function {
Function::new(ctx, move |ctx, _, mut stack| {
let commands: Table = stack.consume(ctx)?;
let command_specs = table_to_command_specs(commands);
if command_specs.is_empty() {
stack.push_back(ctx, output_to_lua_table(Output {
status: ExitStatus::from_raw(0),
stdout: Vec::new(),
stderr: "No commands provided to pipeline".into_bytes(),
}));
return Ok(());
}
let mut previous_stdout: Option<Stdio> = None;
let mut children: Vec<Child> = Vec::new();
for (i, spec) in command_specs.iter().enumerate() {
let mut command = Command::new(&spec.command);
command.args(&spec.args);
if let Some(env) = &spec.env {
command.env_clear();
command.envs(env);
}
if let Some(stdin) = previous_stdout.take() {
command.stdin(stdin);
}
if i < command_specs.len() - 1 {
command.stdout(Stdio::piped());
}
let child = if i < command_specs.len() - 1 {
command.spawn()?
} else {
// Last command: capture output
command.spawn()?
};
if let Some(output) = child.stdout.take() {
previous_stdout = Some(Stdio::from(output));
}
children.push(child);
}
// Wait for the last command and get output
let last_child = children.pop().unwrap();
let output = last_child.wait_with_output()?;
// Wait for all other commands
for mut child in children {
let _ = child.wait();
}
stack.push_back(ctx, output_to_lua_table(output));
Ok(())
})
}
// Helper function to convert a Lua table to command arguments (Vec<String>)
fn table_to_args(table: Table) -> Vec<String> {
table.pairs()
.filter_map(|(k, v)| {
if let Value::Integer(i) = k {
if i >= 1 {
Some((i as usize, v.to_string()))
} else { None }
} else {
None
}
})
.collect::<Vec<_>>() // Collect into Vec<(usize, String)>
.iter()
.map(|(_, v)| v.clone()) // Collect to final Vec<String>
.collect()
}
// Helper function to convert a Lua table to environment variables (Vec<(String, String)>)
fn table_to_env(table: Table) -> Vec<(String, String)> {
table.pairs()
.filter_map(|(k, v)| {
Some((k.to_string(), v.to_string()))
})
.collect()
}
#[derive(Debug, Clone)]
struct CommandSpec {
command: String,
args: Vec<String>,
env: Option<Vec<(String, String)>>,
}
// Helper function to convert a Lua table of tables to a Vec<CommandSpec>
fn table_to_command_specs(table: Table) -> Vec<CommandSpec> {
table.pairs()
.filter_map(|(k, v)| {
if let Value::Integer(i) = k {
if i >= 1 {
if let Value::Table(t) = v {
let command = t.get_field(ctx, "0").unwrap_or(Value::Nil).to_string();
let mut args = table_to_args(t);
args.remove(0); // remove command name from arguments
let env = t.get_field(ctx, "env").ok().and_then(|v| {
if let Value::Table(env_table) = v {
Some(table_to_env(env_table))
} else {
None
}
});
Some(CommandSpec { command, args, env })
} else {
None
}
} else { None }
} else { None }
})
.collect()
}
fn run_repl() -> io::Result<()> {
let mut lua = Lua::new();
let bg_processes = Arc::new(Mutex::new(BackgroundProcesses::new()));
lua.enter(|ctx| {
let globals = ctx.state.globals;
globals.set(ctx, "do", do_fn(ctx, bg_processes.clone())).unwrap();
globals.set(ctx, "spawn", spawn_fn(ctx, bg_processes.clone())).unwrap();
globals.set(ctx, "wait", wait_fn(ctx, bg_processes.clone())).unwrap();
globals.set(ctx, "pipeline", pipeline_fn(ctx, bg_processes.clone())).unwrap();
});
let mut input = String::new();
loop {
print!("> ");
io::stdout().flush()?;
input.clear();
io::stdin().read_line(&mut input)?;
if input.trim().is_empty() {
continue;
}
let result = lua.enter(|ctx| {
let compiled = Compiler::new(ctx, "<repl>").compile(&input)?;
let executor = Executor::new(ctx);
executor.execute(compiled)
});
match result {
Ok(values) => {
for value in values {
println!("=> {}", value.to_string());
}
}
Err(e) => println!("Error: {}", e),
}
}
}
fn main() -> io::Result<()> {
run_repl()
}
Example Usage (Lua):
-- Simple command
local result = do("ls", {"-l"})
print(result.output)
-- Background process
local proc = spawn("sleep", {"5"})
print("Process spawned with PID:", proc.pid)
-- ... do other stuff ...
local wait_result = wait(proc.pid)
print("Process finished with status:", wait_result.status)
-- Pipeline
local pipeline_result = pipeline({
{"grep", {"error"}, {}}, -- Command, args, env
{"wc", {"-l"}, {}} -- Command, args, env
}, {}) -- passing empty table as env to pipeline
print("Number of lines with 'error':", pipeline_result.output)
-- Pipeline with environment variables
local env_pipeline_result = pipeline({
{"env", {}, {VAR1="value1"}},
{"grep", {"VAR1"}, {}}
}, {})
print("pipeline_result:", env_pipeline_result)
This improved code demonstrates:
do: Executes a command in the foreground.spawn: Executes a command in the background, returning a process ID.wait: Waits for a background process to complete.pipeline: Executes a series of commands, piping the output of one to the input of the next.- Argument handling: Clear separation of positional arguments and environment variables.
- Error handling: Basic error handling for common cases.
- Background process management: using
Arc<Mutex<BackgroundProcesses>>
This is a substantial improvement, bringing us much closer to the functionality of a real shell, all within the context of an embedded Lua environment. Further enhancements could include signal handling, job control, and more sophisticated error reporting. The key is to continue to leverage the strengths of both Rust (for system-level operations) and Lua (for scripting and flexibility).
