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subprocess

The subprocess library provides facilities for execution of and interaction with external processes and pipelines, inspired by Python's subprocess module. subprocess is hosted on crates.io, with API Documentation on docs.rs.

Features

This library is about launching external processes with optional redirection of standard input, output, and error. It covers similar ground as the std::process standard library module, but with additional functionality:

• The communicate family of methods for deadlock-free capturing of subprocess output/error to memory, while simultaneously feeding data to its standard input. Capturing supports optional timeout and read size limit.

• Connecting multiple commands into OS-level pipelines.

• Flexible redirection options, such as connecting standard streams to arbitrary files, or merging output streams like shell's 2>&1 and 1>&2 operators.

• Non-blocking and timeout methods to wait on the process: poll, wait, and wait_timeout.

The crate has minimal dependencies to third-party crates, only requiring libc on Unix and winapi on Windows. It is intended to work on Unix-like platforms as well as on reasonably recent Windows. It is regularly tested on Linux, MacOS and Windows.

API Overview

The API is separated in two parts: the low-level Popen API similar to Python's subprocess.Popen, and the higher-level API for convenient creation of commands and pipelines. The two can be mixed, so it is possible to use builder to create Popen instances, and then to continue working with them directly.

While Popen loosely follows Python's subprocess module, it is not a literal translation. 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 can be better expressed using the builder pattern.

The high-level API offers an elegant process and pipeline creation interface, along with convenience methods for capturing their output and exit status.

Examples

Spawning and redirecting

Execute an command and wait for it to complete:

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

To prevent quoting issues and injection attacks, subprocess will not spawn a shell unless explicitly requested. To execute a command using the OS shell, like C's system, use Exec::shell:

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("find /").stream_stdout()?;
// Call stream.read_to_string, construct io::BufReader(stream) and iterate it
// by lines, etc...

Capture the output of a command:

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

Redirect standard error to standard output, and capture them in a string:

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

Provide some 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_eq!(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.

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::shell("find . -type f") | Exec::cmd("sort") | Exec::cmd("sha1sum")
}.capture()?.stdout_str();

The low-level Popen type

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()?;
}

Querying and terminating

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 and LICENSE-MIT for details. Contributing changes is assumed to signal agreement with these licensing terms.