subprocess

The subprocess library provides facilities for execution and interaction with external processes and pipelines. It is inspired by Python's subprocess module, adding a Rust spin on top.

Features

The following features are available:

Launching external processes with optional redirection of standard input, output, and error.
Waiting for the process to finish and polling its status: poll, wait, and wait_timeout.
Advanced redirection options, such as connecting standard streams to arbitary files, or merging errors into output like shell's 2>&1 operator.
The communicate method for deadlock-free reading of output while simultaneously providing input to the subprocess.
Connecting multiple commands into OS-level pipelines.
Rustic builder-style API for building commands and pipelines and collecting their results.

The crate has minimal dependencies to third-party crates, only requiring libc, crossbeam, and Win32-related crates on Windows. It is intended to work on Unix-like platforms as well as on recent Windows 7 and later. It is regularly tested on Linux and Windows, and occasionally on FreeBSD and MacOS.

API Overview

The API is separated in two parts: the low-level Popen API directly inspired by Python's subprocess.Popen, and the higher-level convenience API using the builder pattern, in the vein of std::process. The two can be mixed, so it is possible to use builder to create Popen instances, and then to continue working with them directly.

The Popen type offers some functionality currently missing from the Rust standard library. It provides methods for polling the process, waiting with timeout, and the communicate method, useful enough to have been created independently. While the design follows Python's subprocess module, this module was adapted to Rust's style. Some of the changes accommodate the differences between the languages, such as the lack of default and keyword arguments in Rust, and others take advantage of Rust's more advanced type system, or of additional capabilities such as the ownership system and the Drop trait. Python's utility functions such as subprocess.run are not included because they have better alternatives in the form of the builder API.

Working with subprocesses using subprocess::Popen can look like this:

let mut p = Popen::create(&["command", "arg1", "arg2"], PopenConfig {
    stdout: Redirection::Pipe, ..Default::default()
})?;

// Since we requested stdout to be redirected to a pipe, the parent's
// end of the pipe is available as p.stdout.  It can either be read
// directly, or processed using the communicate() method:
let (out, err) = p.communicate(None)?;

// check if the process is still alive
if let Some(exit_status) = p.poll() {
  // the process has finished
} else {
  // it is still running, terminate it
  p.terminate()?;
}

The builder API offers a more Rustic process creation interface, along with convenience methods for capturing output and building pipelines.

let output = Exec::cmd("command").arg("arg1").arg("arg2")
    .stdout(Redirection::Pipe)
    .capture()?
    .stdout_str();

Examples

Note: these examples use the ? operator to show the result. If you are pasting them in a function that doesn't return a Result, such as main, simply replace X? with X.expect("failed to execute"), or with an appropriate pattern match.

Commands

Execute an external command and wait for it to complete:

let exit_status = Exec::cmd("umount").arg(dirname).join()?;

Execute the command using the OS shell, like C's system:

Exec::shell("shutdown -h now").join()?;

Start a subprocess and obtain its output as a Read trait object, like C's popen:

let stream = Exec::cmd("ls").stream_stdout()?;
// call stream.read_to_string, construct io::BufReader(stream), etc.

Capture the output of a command:

let out = Exec::cmd("ls")
  .stdout(Redirection::Pipe)
  .capture()?
  .stdout_str();

Redirect errors to standard output, and capture both in a single stream:

let out_and_err = Exec::cmd("ls")
  .stdout(Redirection::Pipe)
  .stderr(Redirection::Merge)
  .capture()?
  .stdout_str();

Provide input to the command and read its output:

let out = Exec::cmd("sort")
  .stdin("b\nc\na\n")
  .stdout(Redirection::Pipe)
  .capture()?
  .stdout_str();
assert!(out == "a\nb\nc\n");

Connecting stdin to an open file would have worked as well.

Pipelines

Popen objects support connecting input and output to arbitrary open files, including other Popen objects. This can be used to form pipelines of processes. The builder API will do it automatically with the | operator on Exec objects.

Just like in a Unix shell, each command in a pipeline receives standard input from the previous command, and passes standard output to the next command in the pipeline. Optionally, the standard input of the first command can be provided from the outside, and the output of the last command can be captured.

Execute a pipeline and return the exit status of the last command:

let exit_status =
  (Exec::shell("ls *.bak") | Exec::cmd("xargs").arg("rm")).join()?;

Capture the pipeline's output:

let dir_checksum = {
    Exec::cmd("find . -type f") | Exec::cmd("sort") | Exec::cmd("sha1sum")
}.capture()?.output_str();

Interacting with subprocess

Check whether a previously launched process is still running:

let mut p = Exec::cmd("sleep").arg("2").popen()?;
thread::sleep(Duration::new(1, 0))
if p.poll().is_none() {
    // poll() returns Some(exit_status) if the process has completed
    println!("process is still running");
}

Give the process 1 second to run, and kill it if it didn't complete by then.

let mut p = Exec::cmd("sleep").arg("2").popen()?;
if let Some(status) = p.wait_timeout(Duration::new(1, 0))? {
    println!("process finished as {:?}", status);
} else {
    p.kill()?;
    p.wait()?;
    println!("process killed");
}

License

subprocess is distributed under the terms of both the MIT license and the Apache License (Version 2.0). See LICENSE-APACHE for LICENSE-MIT for details. Contributing changes is assumed to signal agreement with these licensing terms.

subprocess 0.1.1