rust-parallel 0.2.3

# rust-parallel

Command-line utility to execute commands in parallel and aggregate their output.

Similar to [GNU Parallel](https://www.gnu.org/software/parallel/parallel_examples.html) or [xargs](https://man.openbsd.org/xargs) with `-n1` option but implemented in rust and [tokio](https://tokio.rs).

Being written in asynchronous rust it is quite fast - see [benchmarks](https://github.com/aaronriekenberg/rust-parallel/wiki/Benchmarks).

[![Crates.io][crates-badge]][crates-url]

[crates-badge]: https://img.shields.io/crates/v/rust-parallel.svg
[crates-url]: https://crates.io/crates/rust-parallel

# Usage:
```
Run commands in parallel

Usage: rust-parallel [OPTIONS] [COMMAND_AND_INITIAL_ARGUMENTS]...

Arguments:
  [COMMAND_AND_INITIAL_ARGUMENTS]...  Optional command and initial arguments to run for each input line

Options:
  -i, --input <INPUT>   Input file or - for stdin.  Defaults to stdin if no inputs are specified
  -j, --jobs <JOBS>     Maximum number of commands to run in parallel, defauts to num cpus [default: 8]
  -0, --null-separator  Use null separator for reading input instead of newline
  -h, --help            Print help information
  -V, --version         Print version information
```

# Installation:
1. [Install Rust](https://www.rust-lang.org/learn/get-started)
2. Install the latest version of this app from [crates.io](https://crates.io/crates/rust-parallel):
```
$ cargo install rust-parallel   
```

# Demos:

1. Small demo of 5 echo commands.  With `-j5` all 5 commands are run in parallel.  With `-j1` commands are run sequentially:

```
$ cat >./test <<EOL
echo hi
echo there
echo how
echo are
echo you
EOL

$ cat test | rust-parallel -j5
are
hi
there
how
you

$ cat test | rust-parallel -j1
hi
there
how
are
you
```

2. Specifying command and intial arguments on command line:

```
$ head -100 /usr/share/dict/words | rust-parallel md5 -s
MD5 ("aal") = ff45e881572ca2c987460932660d320c
MD5 ("A") = 7fc56270e7a70fa81a5935b72eacbe29
MD5 ("aardvark") = 88571e5d5e13a4a60f82cea7802f6255
MD5 ("aalii") = 0a1ea2a8d75d02ae052f8222e36927a5
MD5 ("aam") = 35c2d90f7c06b623fe763d0a4e5b7ed9
MD5 ("aa") = 4124bc0a9335c27f086f24ba207a4912
MD5 ("a") = 0cc175b9c0f1b6a831c399e269772661
MD5 ("Aani") = e9b22dd6213c3d29648e8ad7a8642f2f
MD5 ("Aaron") = 1c0a11cc4ddc0dbd3fa4d77232a4e22e
MD5 ("aardwolf") = 66a4a1a2b442e8d218e8e99100069877
```

3. Working on a set of files from `find` command.  The `-0` option works nicely with `find -print0` to handle filenames with newline or whitespace characters:

```
$ find . -type f -print0 | rust-parallel -0 gzip -f -k
```

4. Using `awk` to form complete commands:

```
$ head -100 /usr/share/dict/words | awk '{printf "md5 -s %s\n", $1}' | rust-parallel
MD5 ("Abba") = 5fa1e1f6e07a6fea3f2bb098e90a8de2
MD5 ("abaxial") = ac3a53971d52d9ce3277eadf03f13a5e
MD5 ("abaze") = 0b08c52aa63d947b6a5601ee975bc3a4
MD5 ("abaxile") = 21f5fc27d7d34117596e41d8c001087e
MD5 ("abbacomes") = 76640eb0c929bc97d016731bfbe9a4f8
MD5 ("abbacy") = 08aeac72800adc98d2aba540b6195921
MD5 ("Abbadide") = 7add1d6f008790fa6783bc8798d8c803
MD5 ("abb") = ea01e5fd8e4d8832825acdd20eac5104
```

5. Using as part of a shell pipeline.  stdout and stderr from each command run are copied to stdout/stderr of the rust-parallel process.

```
$ head -100 /usr/share/dict/words | rust-parallel md5 -s | grep -i abba
MD5 ("Abba") = 5fa1e1f6e07a6fea3f2bb098e90a8de2
MD5 ("abbacomes") = 76640eb0c929bc97d016731bfbe9a4f8
MD5 ("abbacy") = 08aeac72800adc98d2aba540b6195921
MD5 ("Abbadide") = 7add1d6f008790fa6783bc8798d8c803
```

6. By default `rust-parallel` reads input from stdin only.  The `-i` option can be used 1 or more times to override this behavior.  `-i -` means read from stdin, `-i ./test` means read from the file `./test`:

```
$ cat >./test <<EOL
foo
bar
baz
EOL

$ head -5 /usr/share/dict/words | rust-parallel -i - -i ./test echo
A
aalii
aa
a
aal
bar
foo
baz
```

7. Set environment variable `RUST_LOG=debug` to see debug output.

```
$ head -10 /usr/share/dict/words | RUST_LOG=debug rust-parallel md5 -s
```

# Features:
* Use only safe rust.
* Use only asynchronous operations supported by [tokio](https://tokio.rs), do not use any blocking operations.  This includes writing to stdout and stderr.
* Support arbitrarily large number of input lines, avoid `O(number of input lines)` memory usage.  In support of this:
  * [`tokio::sync::Semaphore`](https://docs.rs/tokio/latest/tokio/sync/struct.Semaphore.html) is used carefully to limit the number of commands that run concurrently.  Do not spawn tasks for all input lines immediately to limit memory usage.
* Support running commands on local machine only, not on remote machines.

# Tech Stack:
* [anyhow](https://github.com/dtolnay/anyhow) used for application error handling to propogate and format fatal errors.
* [clap](https://docs.rs/clap/latest/clap/) command line argument parser.
* [tokio](https://tokio.rs/) asynchronous runtime for rust.  From tokio this app uses:
  * `async` / `await` functions (aka coroutines)
  * Singleton `CommandLineArgs` instance using [`tokio::sync::OnceCell`](https://docs.rs/tokio/latest/tokio/sync/struct.OnceCell.html).
  * Asynchronous command execution using [`tokio::process::Command`](https://docs.rs/tokio/latest/tokio/process/struct.Command.html)
  * [`tokio::sync::Semaphore`](https://docs.rs/tokio/latest/tokio/sync/struct.Semaphore.html) used to limit number of commands that run concurrently.
     * Life would be a bit easier if `acquire_many` took a `usize` parameter: https://github.com/tokio-rs/tokio/issues/4446
  * [`tokio::sync::Mutex`](https://docs.rs/tokio/latest/tokio/sync/struct.Mutex.html) used to protect access to stdout/stderr to prevent interleaved command output.
* [tracing](https://docs.rs/tracing/latest/tracing/) used for debug and warning logs.