pyargs 0.6.0

Python script to run commands in parallel (think xargs and GNU parallel) without intermixing the output. The name `pyargs` is for python_workgang.

# pyargs, Python Args Utility


`pyargs` is a python cli program to run a series of scripts or commands using multiple set or arguments provided from a file or stdin and to use more than one process to achieve some parallel execution.

The name is deliberately a play on `xargs` as pyargs is a simplified alternative to `xargs`.

The motivation for this project was to get parallel execution of commands where the output from each command invocation is kept in a contigous block rather than being intermixed.

To make for easy replacement of `xargs` in existing scripts I used the same general usage pattern as `xargs` and kept of few of the option short names the same.

I should offer an apology to the __python__ community. This is my first python effort so there is probably a lot in this small project the is no very __*pythonic*__.

## Usage

```
usage: wg-runner.py [-h] [-v] [--in INFILE_PATH] [--out OUTFILE_PATH]
[-n NARGS] [-P NPROCS] [--stream] [--debug] [--mark]
[--lines]
cmd [cmd_args [cmd_args ...]]

Runs multiple instances of a command in parallel with different arguments.
Think xargs.

positional arguments:
cmd Command to execute
cmd_args Arguments for command to be used for every execution.
If any of these are options like -c you might have to
enclose them in quotes.

optional arguments:
-h, --help show this help message and exit
-v, --version Prints the version number.
--in INFILE_PATH Path to input file, each line has arguments for
command. If not provided uses stdin.
--out OUTFILE_PATH Path to output from all commands go to this file path.
If not provided stdout.
-n NARGS, --nargs NARGS
Number of args to be found on each line of infile,
default = 1.
-P NPROCS, --nprocs NPROCS
Number of parallel process, default = 1.
--stream Treat input as a single string rather than a series of
line for the purposes of tokenizing into arguments
--debug Prints out the command to be executed rather than
execute the command, to help problem solve
--mark, -m Put markers in the output to make visible the start
and output of each command.
--lines, -L Send the output line by line rather than keep output
frm each execution together.


```

## Install


Using `pip`

pip install pyargs

Alternatively download or clone the [github repo](https://github.com/robertblackwell/pyargs) and from the project directory

python setup.py install

## Testing

Pythonic testing is via

python setup.py test

There are only two test cases each of which reside in the `tests/test_pyargs.py`. These tests
cases execute shell scripts

./tests/end_to_end_test.sh

and

./tests/writer_to_reader_test.sh

### ./tests/end\_to\_end_test.sh

This script executes multiple instances of `./tests/writer_cmd.py` using `pyargs` with arguments provided from `tests/test_args`. The output is piped into, and inspected by `tests/reader_cmd.py` which parses that output and verifies that it is as expected.

### ./tests/writer\_to\_reader\_test.sh

Pipes the output form `writer_cmd.py` into `reader_cmd.py` to ensure that the previous test
is using valid test data.

## Command options - a bit more detail

The options `--debug`, `--mark`, `--lines` maybe require a little more explanation.

The script `./tests/ping_example.sh` demonstrates the `--line` and `--mark` options, and `./tests/ping_example_nl_m.sh` demonstrates the `--mark` option without `--lines`.

- --debug

When set `pyargs` does not execute the commands but rather outputs the full command that would have been executed. This enables a user to see how `pyargs` has interpreted its options and input. This option can be helpful in debugging commands that fail.

- --lines

The original motivation for `pyargs` was to keep all the output from a single command invocation in a single contiguous block. However this may not always be necessary so this option allows or requires that `pyargs` will print each line of output from command invocations as soon as possible without waiting for the command to complete. This means that lines from different command invocations can be intermixed. Though __note__ that concurrent output is still coordinated to ensure that lines do not corrupt each other.

In order that each line of output can be attributed to the command that created it, in this mode, each output line is prefixed with the command string of the command that caused the output.

- --mark

Sometimes it is difficult to be sure that the output from different command invocations have not intermixed (this is when --list is NOT set), particularly when many commands are being executed and each command generates a lot of output.

To assist users examine such a situation the ``--mark` options is provided.

When `--mark` is set `pyargs` will modify the output in the following manner:

- just before the execution of a command instance starts `pyargs` will output a string like

```
MARK: <the command string to be executed> ===================
```
- the output from each command invocation will be bracketed (that is have a additional marker line before and after the actual command output). This lines will look like this:

```
START OUTPUT[<command string>]

...... output lines in here

END OUTPUT[<command string>]
```
These lines (between and including START and END) should be contiguous and should be the output from only one command and that command should be the one identified in the START and END lines (which or course should be the same command). If any of this is not the case you have found a bug in `pyargs`.

## Examples

The two scripts `tests/ping_example.sh` and `tests/curl_example.sh` demonstrate the usage of `pyargs`.

Note that both these examples attempt to contact hosts/urls that do not exist and will hence timeout. Hence the output include error messages.