$ cargo watch

Cargo Watch watches over your project's source for changes, and runs Cargo commands when they occur.

If you've used nodemon, gulp, guard, watchman, or similar others, it will probably feel familiar.

• In the public domain / licensed with CC0.
• Maintenance status: best-effort / passive.
• Uses Caretaker Maintainership.
• Minimum Supported Rust Version: 1.51.0.

Install

Pre-built binaries are available on the Github Releases tab.

$ cargo install cargo-watch

To upgrade:

$ cargo install --force cargo-watch

With cargo-binstall:

$ cargo binstall cargo-watch

Or clone and build with $ cargo build then place in your $PATH.

Usage

By default, it runs check (which is available since Rust 1.16). You can easily override this, though:

$ cargo watch [-x command]...

A few examples:

# Run tests only
$ cargo watch -x test

# Run check then tests
$ cargo watch -x check -x test

# Run run with arguments
$ cargo watch -x 'run -- --some-arg'

# Run an arbitrary command
$ cargo watch -s 'echo Hello world'

# Run with features passed to cargo
$ cargo watch --features "foo,bar"

There's a lot more you can do! Here's a copy of the help:

USAGE:
    cargo watch [FLAGS] [OPTIONS]

FLAGS:
    -c, --clear             Clear the screen before each run
        --debug             Display debug output
    -h, --help              Display this message
        --ignore-nothing    Ignore nothing, not even target/ and .git/
        --no-gitignore      Don’t use .gitignore files
        --no-ignore         Don’t use .ignore files
        --no-restart        Don’t restart command while it’s still running
        --poll              Force use of polling for file changes
        --postpone          Postpone first run until a file changes
    -q, --quiet             Suppress output from cargo-watch itself
    -V, --version           Display version information
        --watch-when-idle   Ignore events emitted while the commands run

OPTIONS:
    -x, --exec <cmd>...         Cargo command(s) to execute on changes [default: check]
    -s, --shell <cmd>...        Shell command(s) to execute on changes
    -d, --delay <delay>         File updates debounce delay in seconds [default: 0.5]
        --features <features>   List of features passed to cargo invocations
    -i, --ignore <pattern>...   Ignore a glob/gitignore-style pattern
    -w, --watch <watch>...      Watch specific file(s) or folder(s) [default: .]

ARGS:
    <cmd:trail>...    Full command to run. -x and -s will be ignored!

Cargo commands (-x) are always executed before shell commands (-s). You can use
the `-- command` style instead, note you'll need to use full commands, it won't
prefix `cargo` for you.

By default, your entire project is watched, except for the target/ and .git/
folders, and your .ignore and .gitignore files are used to filter paths.

On Windows, patterns given to -i have forward slashes (/) automatically
converted to backward ones (\) to ease command portability.

Ignore files

.gitignore files are used by default to ignore paths to watch and trigger runs. To stop honouring them, pass --no-gitignore.

.ignore files in the same syntax are also used by default. This file can be used to specify files that should be ignored by cargo watch but checked into git, without constantly adding --ignore abc options on the command-line. Do note that .ignore files may also be used by other programs, like ripgrep. To stop honouring these, pass --no-ignore.

Cargo watch also has an internal list of default ignores on top of those specified in files, like target/ and .git/ and various other common types (logs, editor swap files, lockfiles, etc).

To skip absolutely all ignores, use the --ignore-nothing flag.

Ignore syntax

See the glob::Pattern docs for a more detailed specification of the glob matching syntax used for --ignore.

On Windows, patterns should be specified with Windows-style (\\) separators. Unix-style separators (/) would not match Windows paths, which could be confusing and give the appearance of commandline ignores not working.

From Cargo Watch 7.0.0, / in commandline ignores are automatically translated to \\ when running on Windows, but one should still try to write the correct patterns for the platform, as there may be more subtle differences.

From Cargo Watch 7.3.0, --ignore patterns were fixed to provide better experience with directory matching. Previously, ignoring a folder would need unyieldy -i folder/** patterns; now that is handled internally, and only -i folder is needed for the same effect.

Reloading servers seamlessly

Cargo Watch pairs very well with systemfd/Catflap, tools for Unixy platforms that lets one spawn a socket before the watcher runs that Rust servers can then bind to, avoiding request-dropping and the infamous ADDRINUSE error. For example:

$ systemfd --no-pid -s http::5000 -- cargo watch -x run

Of course, if you don't need to guard against these issues or don't want to modify your program to grab sockets instead of ports, you can use Cargo Watch as-is: it will happily just restart your server normally.

Restarting an application only if the build/check succeeds

Brought up by @LeDominik, here's a pattern that may be very useful: you're working on a server or app, but want it to keep running while you're writing a new feature or fixing a bug, potentially causing the code not to compile anymore in the meantime.

In this case, you can use this strategy: run a first cargo watch with check, build, test, or whatever you want, and append -s 'touch .trigger (or equivalent for your platform). Then, run a second cargo watch simultaneously that only watches that .trigger file. For example:

$ cargo watch -x check -s 'touch .trigger'

and

$ cargo watch --no-gitignore -w .trigger -x run

The --no-gitignore flag ensures that you can safely add .trigger to your .gitignore file to avoid mistakenly committing it.

Troubleshooting

Please note that this project (and its upstream, watchexec)'s maintainership is at a low. Response to issues is on a best-effort basis only. Feature and improvement work is stalled indefinitely. Pull requests will be reviewed and merged as time allows. If you wish to take on more active maintainership, even for a limited time, enquire within (open an issue or send me mail).

---

In all cases, start by checking your version with cargo watch --version and, if necessary, upgrading to the latest one.

RLS is slow while using cargo watch, or vice versa, or it's waiting for the project lock a lot

Cargo builds (and checks, and clippy, and tests because the tests have to be built) take out a lock on the project so two cargo instances don't run at the same time. That may include language servers like RLS (and possibly RA)! So your editor might be fighting with cargo watch for the lock when you save.

There's not really a way around this. Either stop using cargo watch, or stop using RLS/RA, or accept that whenever you save there may be some slowdown for a little while. While I'm investigating ways to make this less of an issue, there's not going to be a quick fix anytime soon. Rust Analyzer is itself working on a different solution in conjunction with the compiler.

On Windows 7 (or lower): "failed to add to job object: Access denied (OS Error 5)"

Cargo Watch versions 5.0.0 and up (and Watchexec versions 1.3.0 and up) do not support Windows 7 or lower. There are no plans at the moment to add such support.

You can downgrade to the last version which did support Windows 7 (and lower), but do keep in mind that many bug fixes and features are missing there:

$ cargo install --force --vers 4.0.3 cargo-watch

I want to run cargo-watch directly, without going through cargo

You can! But you'll have to specify the watch subcommand as the first argument, like so:

$ /path/to/cargo-watch watch -x build

I want to run cargo-watch outside of a Cargo project

That's not supported. If you have a good reason to use a Cargo-specific tool outside a Cargo project, please open an issue! Otherwise, you'll probably be best served with using Watchexec.

If file updates seems to never trigger

Try using --poll to force the polling fallback.

If that still doesn't work, and you're using an editor that does "safe saving", like IntelliJ / PyCharm, you may have to disable "safe saving" as that may prevent file notifications from being generated properly.

Linux: If it fails to watch some deep directories but not others / "No space left on device"

You may have hit the inotify watch limit. Here's a summary of what this means and how to increase it.

If you want to only recompile one Cargo workspace

Cargo workspaces are not natively supported yet.

However, as you can run "arbitrary commands" with the -s option, you can write workspace-aware commands manually.

If it runs repeatedly without touching anything

That can happen when watching files that are modified by the command you're running.

If you're only running compiles or checks (i.e. any command that only affects the target/ folder) and you're using -w, you might be confusing the target-folder-ignorer. Check your options and paths.

You can also use the --watch-when-idle flag to ignore any event that happens while the command is running.

If it runs repeatedly only touching ignored files

Make sure the files you ignored are the only ones being touched. Use the --debug option to see exactly which files were modified and triggered the restart (or were ignored). Some programs and libraries create temporary files that may not match a simple ignore pattern.

As above, you can also use the --watch-when-idle flag to help.

I don't have colour in my cargo output / for cargo test

This sometimes happens on some terminal configurations or for test harnesses. A quick workaround (instead of going down the rabbit hole of debugging your console settings) is to pass --color=always to the command. E.g.

$ cargo watch -x 'check --color=always'

For test (and bench) commands, you'll need to pass the flag to the underlying program instead of cargo:

$ cargo watch -x 'test -- --color=always'

I want to compile my build with additional features

$ cargo watch --features foo,bar

will run cargo check --features foo,bar on every watched change.

The --features will be passed to every supported cargo subcommand.

$ cargo watch --features foo,bar -x build -x doc

will run both build and doc with the foo and bar features.

Something not covered above / I have a feature request

Please open an issue, or look through the existing ones. You may also want to look through issues for the Notify library this tool depends on, or the issues for the Watchexec tool that we use under the covers.

If you want more verbose output, try running with the --debug flag. Note that this will also enable debug mode for watchexec. When filing an issue, make sure to include a log with --debug enabled so problems can be diagnosed.

I want to embed Cargo Watch in my own (Rust) tool

It is not recommended to do that directly. You may of course call cargo-watch as any other program, and technically it exposes an (undocumented) library that could be directly / statically embedded. If you have no other option, that may be your best bet.

However, for most cases, consider building on top of Watchexec instead. That is itself built on Notify, and both of these can be used as Rust libraries.

• If you want to build a tool that runs, restarts, and otherwise manages commands in response to file changes, you'll most probably want to use Watchexec.

• If you want to build a tool that responds to file changes, but does not need to run commands, or does so in a way that is not well-supported by Watchexec, then Notify is your ticket.

Wait, is this just a wrapper on top of watchexec?

Kind of! Watchexec does a really good job of watching files and running commands and all the details that go with this. Cargo Watch uses the Watchexec library interface and calls it with its own custom options, defaults, and particularities, so you can just run cargo-watch in your project and be in business.

When asking questions and/or filing bugs, keep in mind that Cargo Watch and Watchexec share the same maintainer at the moment (but Notify does not, anymore)!

About

Created by Félix Saparelli and awesome contributors.