# tokio::process::Command - Rust
API documentation for the Rust `Command` struct in crate `tokio`.
Keywords: rust, rustlang, rust-lang, Command

# Struct tokio::process::Command [−] [src]

    pub struct Command { /* fields omitted */ }

This structure mimics the API of `std::process::Command` found in the standard library, but replaces functions that create a process with an asynchronous variant. The main provided asynchronous functions are spawn, status, and output.

`Command` uses asynchronous versions of some `std` types (for example `Child`).

## Implementations

### `impl Command` [src]

#### `pub fn new<S: AsRef<OsStr>>(program: S) -> Command` [src]

Constructs a new `Command` for launching the program at path `program`, with the following default configuration:

- No arguments to the program
- Inherit the current process's environment
- Inherit the current process's working directory
- Inherit stdin/stdout/stderr for `spawn` or `status`, but create pipes for `output`

Builder methods are provided to change these defaults and otherwise configure the process.

If `program` is not an absolute path, the `PATH` will be searched in an OS-defined way.

The search path to be used may be controlled by setting the `PATH` environment variable on the Command, but this has some implementation limitations on Windows (see issue rust-lang/rust#37519).

# Examples

Basic usage:

    use tokio::process::Command;
    let command = Command::new("sh");

#### `pub fn arg<S: AsRef<OsStr>>(&mut self, arg: S) -> &mut Command` [src]

Adds an argument to pass to the program.

Only one argument can be passed per use. So instead of:

    tokio::process::Command::new("sh")
      .arg("-C /path/to/repo");

usage would be:

    tokio::process::Command::new("sh")
      .arg("-C")
      .arg("/path/to/repo");

To pass multiple arguments see `args`.

# Examples

Basic usage:

    use tokio::process::Command;

    let command = Command::new("ls")
            .arg("-l")
            .arg("-a");

#### `pub fn args<I, S>(&mut self, args: I) -> &mut Command where
I: IntoIterator<Item = S>,
S: AsRef<OsStr>,` [src]

Adds multiple arguments to pass to the program.

To pass a single argument see `arg`.

# Examples

Basic usage:

    use tokio::process::Command;

    let command = Command::new("ls")
            .args(&["-l", "-a"]);

#### `pub fn env<K, V>(&mut self, key: K, val: V) -> &mut Command where
K: AsRef<OsStr>,
V: AsRef<OsStr>,` [src]

Inserts or updates an environment variable mapping.

Note that environment variable names are case-insensitive (but case-preserving) on Windows, and case-sensitive on all other platforms.

# Examples

Basic usage:

    use tokio::process::Command;

    let command = Command::new("ls")
            .env("PATH", "/bin");

#### `pub fn envs<I, K, V>(&mut self, vars: I) -> &mut Command where
I: IntoIterator<Item = (K, V)>,
K: AsRef<OsStr>,
V: AsRef<OsStr>,` [src]

Adds or updates multiple environment variable mappings.

# Examples

Basic usage:

    use tokio::process::Command;
    use std::process::{Stdio};
    use std::env;
    use std::collections::HashMap;

    let filtered_env : HashMap<String, String> =
        env::vars().filter(|&(ref k, _)|
            k == "TERM" || k == "TZ" || k == "LANG" || k == "PATH"
        ).collect();

    let command = Command::new("printenv")
            .stdin(Stdio::null())
            .stdout(Stdio::inherit())
            .env_clear()
            .envs(&filtered_env);

#### `pub fn env_remove<K: AsRef<OsStr>>(&mut self, key: K) -> &mut Command` [src]

Removes an environment variable mapping.

# Examples

Basic usage:

    use tokio::process::Command;

    let command = Command::new("ls")
            .env_remove("PATH");

#### `pub fn env_clear(&mut self) -> &mut Command` [src]

Clears the entire environment map for the child process.

# Examples

Basic usage:

    use tokio::process::Command;

    let command = Command::new("ls")
            .env_clear();

#### `pub fn current_dir<P: AsRef<Path>>(&mut self, dir: P) -> &mut Command` [src]

Sets the working directory for the child process.

# Platform-specific behavior

If the program path is relative (e.g., `"./script.sh"`), it's ambiguous whether it should be interpreted relative to the parent's working directory or relative to `current_dir`. The behavior in this case is platform specific and unstable, and it's recommended to use `canonicalize` to get an absolute program path instead.

# Examples

Basic usage:

    use tokio::process::Command;

    let command = Command::new("ls")
            .current_dir("/bin");

#### `pub fn stdin<T: Into<Stdio>>(&mut self, cfg: T) -> &mut Command` [src]

Sets configuration for the child process's standard input (stdin) handle.

Defaults to `inherit` when used with `spawn` or `status`, and defaults to `piped` when used with `output`.

# Examples

Basic usage:

    use std::process::{Stdio};
    use tokio::process::Command;

    let command = Command::new("ls")
            .stdin(Stdio::null());

#### `pub fn stdout<T: Into<Stdio>>(&mut self, cfg: T) -> &mut Command` [src]

Sets configuration for the child process's standard output (stdout) handle.

Defaults to `inherit` when used with `spawn` or `status`, and defaults to `piped` when used with `output`.

# Examples

Basic usage:

    use tokio::process::Command;;
    use std::process::Stdio;

    let command = Command::new("ls")
            .stdout(Stdio::null());

#### `pub fn stderr<T: Into<Stdio>>(&mut self, cfg: T) -> &mut Command` [src]

Sets configuration for the child process's standard error (stderr) handle.

Defaults to `inherit` when used with `spawn` or `status`, and defaults to `piped` when used with `output`.

# Examples

Basic usage:

    use tokio::process::Command;;
    use std::process::{Stdio};

    let command = Command::new("ls")
            .stderr(Stdio::null());

#### `pub fn kill_on_drop(&mut self, kill_on_drop: bool) -> &mut Command` [src]

Controls whether a `kill` operation should be invoked on a spawned child process when its corresponding `Child` handle is dropped.

By default, this value is assumed to be `false`, meaning the next spawned process will not be killed on drop, similar to the behavior of the standard library.

# Caveats

On Unix platforms processes must be "reaped" by their parent process after they have exited in order to release all OS resources. A child process which has exited, but has not yet been reaped by its parent is considered a "zombie" process. Such processes continue to count against limits imposed by the system, and having too many zombie processes present can prevent additional processes from being spawned.

Although issuing a `kill` signal to the child process is a synchronous operation, the resulting zombie process cannot be `.await`ed inside of the destructor to avoid blocking other tasks. The tokio runtime will, on a best-effort basis, attempt to reap and clean up such processes in the background, but makes no additional guarantees are made with regards how quickly or how often this procedure will take place.

If stronger guarantees are required, it is recommended to avoid dropping a `Child` handle where possible, and instead utilize `child.wait().await` or `child.kill().await` where possible.

#### `pub fn uid(&mut self, id: u32) -> &mut Command` [src]

Sets the child process's user ID. This translates to a `setuid` call in the child process. Failure in the `setuid` call will cause the spawn to fail.

#### `pub fn gid(&mut self, id: u32) -> &mut Command` [src]

Similar to `uid` but sets the group ID of the child process. This has the same semantics as the `uid` field.

#### `pub unsafe fn pre_exec<F>(&mut self, f: F) -> &mut Command where
F: FnMut() -> Result<()> + Send + Sync + 'static,` [src]

Schedules a closure to be run just before the `exec` function is invoked.

The closure is allowed to return an I/O error whose OS error code will be communicated back to the parent and returned as an error from when the spawn was requested.

Multiple closures can be registered and they will be called in order of their registration. If a closure returns `Err` then no further closures will be called and the spawn operation will immediately return with a failure.

# Safety

This closure will be run in the context of the child process after a `fork`. This primarily means that any modifications made to memory on behalf of this closure will not be visible to the parent process. This is often a very constrained environment where normal operations like `malloc` or acquiring a mutex are not guaranteed to work (due to other threads perhaps still running when the `fork` was run).

This also means that all resources such as file descriptors and memory-mapped regions got duplicated. It is your responsibility to make sure that the closure does not violate library invariants by making invalid use of these duplicates.

When this closure is run, aspects such as the stdio file descriptors and working directory have successfully been changed, so output to these locations may not appear where intended.

#### `pub fn spawn(&mut self) -> Result<Child>` [src]

Executes the command as a child process, returning a handle to it.

By default, stdin, stdout and stderr are inherited from the parent.

This method will spawn the child process synchronously and return a handle to a future-aware child process. The `Child` returned implements `Future` itself to acquire the `ExitStatus` of the child, and otherwise the `Child` has methods to acquire handles to the stdin, stdout, and stderr streams.

All I/O this child does will be associated with the current default event loop.

# Examples

Basic usage:

    use tokio::process::Command;

    async fn run_ls() -> std::process::ExitStatus {
        Command::new("ls")
            .spawn()
            .expect("ls command failed to start")
            .wait()
            .await
            .expect("ls command failed to run")
    }

# Caveats

## Dropping/Cancellation

Similar to the behavior to the standard library, and unlike the futures paradigm of dropping-implies-cancellation, a spawned process will, by default, continue to execute even after the `Child` handle has been dropped.

The `Command::kill_on_drop` method can be used to modify this behavior and kill the child process if the `Child` wrapper is dropped before it has exited.

## Unix Processes

On Unix platforms processes must be "reaped" by their parent process after they have exited in order to release all OS resources. A child process which has exited, but has not yet been reaped by its parent is considered a "zombie" process. Such processes continue to count against limits imposed by the system, and having too many zombie processes present can prevent additional processes from being spawned.

The tokio runtime will, on a best-effort basis, attempt to reap and clean up any process which it has spawned. No additional guarantees are made with regards how quickly or how often this procedure will take place.

It is recommended to avoid dropping a `Child` process handle before it has been fully `await`ed if stricter cleanup guarantees are required.

# Errors

On Unix platforms this method will fail with `std::io::ErrorKind::WouldBlock` if the system process limit is reached (which includes other applications running on the system).

#### `pub fn status(&mut self) -> impl Future<Output = Result<ExitStatus>>` [src]

Executes the command as a child process, waiting for it to finish and collecting its exit status.

By default, stdin, stdout and stderr are inherited from the parent. If any input/output handles are set to a pipe then they will be immediately closed after the child is spawned.

All I/O this child does will be associated with the current default event loop.

The destructor of the future returned by this function will kill the child if `kill_on_drop` is set to true.

# Errors

This future will return an error if the child process cannot be spawned or if there is an error while awaiting its status.

On Unix platforms this method will fail with `std::io::ErrorKind::WouldBlock` if the system process limit is reached (which includes other applications running on the system).

# Examples

Basic usage:

    use tokio::process::Command;

    async fn run_ls() -> std::process::ExitStatus {
        Command::new("ls")
            .status()
            .await
            .expect("ls command failed to run")
    }

#### `pub fn output(&mut self) -> impl Future<Output = Result<Output>>` [src]

Executes the command as a child process, waiting for it to finish and collecting all of its output.

> Note: this method, unlike the standard library, will unconditionally configure the stdout/stderr handles to be pipes, even if they have been previously configured. If this is not desired then the `spawn` method should be used in combination with the `wait_with_output` method on child.

This method will return a future representing the collection of the child process's stdout/stderr. It will resolve to the `Output` type in the standard library, containing `stdout` and `stderr` as `Vec<u8>` along with an `ExitStatus` representing how the process exited.

All I/O this child does will be associated with the current default event loop.

The destructor of the future returned by this function will kill the child if `kill_on_drop` is set to true.

# Errors

This future will return an error if the child process cannot be spawned or if there is an error while awaiting its status.

On Unix platforms this method will fail with `std::io::ErrorKind::WouldBlock` if the system process limit is reached (which includes other applications running on the system).

# Examples

Basic usage:

    use tokio::process::Command;

    async fn run_ls() {
        let output: std::process::Output = Command::new("ls")
            .output()
            .await
            .expect("ls command failed to run");
        println!("stderr of ls: {:?}", output.stderr);
    }

## Trait Implementations

### `impl Debug for Command` [src]

#### `fn fmt(&self, f: &mut Formatter<'_>) -> Result` [src]

Formats the value using the given formatter. Read more

### `impl From<Command> for Command` [src]

#### `fn from(std: StdCommand) -> Command` [src]

Performs the conversion.

## Auto Trait Implementations

### `impl !RefUnwindSafe for Command` [src]

### `impl Send for Command` [src]

### `impl Sync for Command` [src]

### `impl Unpin for Command` [src]

### `impl !UnwindSafe for Command` [src]

## Blanket Implementations

### `impl<T> Any for T where
T: 'static + ?Sized,` [src]

#### `pub fn type_id(&self) -> TypeId` [src]

Gets the `TypeId` of `self`. Read more

### `impl<T> Borrow<T> for T where
T: ?Sized,` [src]

#### `pub fn borrow(&self) -> &Tⓘ

### Notable traits for &'_ mut F

`impl<'_, F> Future for &'_ mut F where
F: Unpin + Future + ?Sized, type Output = <F as Future>::Output;`
` [src]

Immutably borrows from an owned value. Read more

### `impl<T> BorrowMut<T> for T where
T: ?Sized,` [src]

#### `pub fn borrow_mut(&mut self) -> &mut Tⓘ

### Notable traits for &'_ mut F

`impl<'_, F> Future for &'_ mut F where
F: Unpin + Future + ?Sized, type Output = <F as Future>::Output;`
` [src]

Mutably borrows from an owned value. Read more

### `impl<T> From<T> for T` [src]

#### `pub fn from(t: T) -> T` [src]

Performs the conversion.

### `impl<T, U> Into<U> for T where
U: From<T>,` [src]

#### `pub fn into(self) -> U` [src]

Performs the conversion.

### `impl<T, U> TryFrom<U> for T where
U: Into<T>,` [src]

#### `type Error = Infallible`

The type returned in the event of a conversion error.

#### `pub fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>` [src]

Performs the conversion.

### `impl<T, U> TryInto<U> for T where
U: TryFrom<T>,` [src]

#### `type Error = <U as TryFrom<T>>::Error`

The type returned in the event of a conversion error.

#### `pub fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>` [src]

Performs the conversion.