A 10 min tutorial to create the shellfire application 'overdrive'

If you'd rather not follow along, or if you'd prefer to see a complete application, take a look at the files in overdrive. (On a Mac with TextMate, mate tutorial/overdrive). To run the completed tutorial, clone this repository, then run in the project root:

git submodule update --init
cd overdrive
./overdrive gearbox.json
cat gearbox.xml

'overdrive' is intended to be a simple application that converts 'GearBox' JSON files to XML. It shows how to quickly parse command lines, validate arguments and use the JSON and XML libraries.

Create a skeleton folder structure

Create a new repository on GitHub. For this example, we'll assume you called it overdrive and you are normanville. The shellfire application is called overdrive. Now, let's create the following folder structure:-

overdrive\
	.git\
	overdrive           # your shellfire application script
	etc\
		shellfire\
			paths.d\    # git submodule add https://github.com/shellfire-dev/paths.d
	lib\
		shellfire\
			core\       # git submodule add https://github.com/shellfire-dev/core
			jsonreader\ # git submodule add https://github.com/shellfire-dev/jsonreader
			unicode\    # git submodule add https://github.com/shellfire-dev/unicode
			xmlwriter\  # git submodule add https://github.com/shellfire-dev/xmlwriter
			overdrive\  # any code for your application broken out into namespaces
output\

So, let's do it:-

repository=overdrive
user=normanville
git clone "[email protected]:$user/$repository.git"
cd "$repository"

mkdir -p etc/shellfire
cd etc/shellfire
git submodule add "https://github.com/shellfire-dev/paths.d"
cd -

mkdir -p lib/shellfire
cd lib/shellfire
git submodule add "https://github.com/shellfire-dev/core"
git submodule add "https://github.com/shellfire-dev/jsonreader"
git submodule add "https://github.com/shellfire-dev/unicode"
git submodule add "https://github.com/shellfire-dev/xmlwriter"
mkdir overdrive
cd -

git submodule update --init

touch overdrive
chmod +x overdrive

cd ..

Creating Hello World

Now all that's left is to add some boilerplate to the overdrive executable. This is unfortunate, but there's nothing to be done about it. We need something so shellfire can bootstrap. Open overdrive in a text editor, and paste in this boilerplate to create a 'Hello World':-

#!/usr/bin/env sh

_program()
{	
	overdrive()
	{
		echo "Hello World"
	}
}

# boilerplate code
# starts here --> 

_program_name='overdrive'
_program_version='unversioned'
_program_package_or_build=''
_program_path="$([ "${_program_fattening_program_path+set}" = 'set' ] && printf '%s\n' "$_program_fattening_program_path" || ([ "${0%/*}" = "${0}" ] && printf '%s\n' '.' || printf '%s\n' "${0%/*}"))"
_program_libPath="${_program_path}/lib"
_program_etcPath="${_program_path}/etc"
_program_varPath="${_program_path}/var"
_program_entrypoint='overdrive'

# < -- ends here

# Assumes pwd, and so requires this code to be running from this folder
. "$_program_libPath"/shellfire/core/init.functions "$@"

Boilerplate code will enable program to load libraries and other dependency code , which makes this script standalone

Now run it with ./overdrive - you should see Hello World. Try ./overdrive --help and ./overdrive --version. Of course, this isn't a very useful program. Let's at least give it a purpose.

Parsing the command line

In many programs, parsing the command line is probably a large proportion of the logic. Its complex, brittle and frequently just hard work. shellfire aims to make it a little easier.

Let's start by taking some arguments using core's command line parser. We're going to modify our hello world program to one that reads JSON gear box files and writes them as XML gear box files. So it'd be useful to be able to do something like ./overdrive /path/to/gearbox.json.

Let's add the function _program_commandLine_handleNonOptions(). This is called back by the parser to let us handle non-options. We could use this take a list of files to work on. Let's use shellfire arrays:-

_program_commandLine_handleNonOptions()
{
	core_variable_array_initialise overdrive_jsonGearBoxFiles
	
	local jsonGearBoxFile
	for jsonGearBoxFile in "$@"
	do
		core_variable_array_append overdrive_jsonGearBoxFiles "$jsonGearBoxFile"
	done
}

# Assumes pwd, and so requires this code to be running from this folder
. "$_program_libPath"/shellfire/core/init.functions "$@"

Note, it's very important that the very last line of your program is always . "$_program_libPath"/shellfire/core/init.functions "$@". It's magic. Sorry. Actually, when fattened, this line disappears - but you do want to run your code first, don't you?

Is it an error to not have any files? Well it, certainly isn't useful. Let's issue a warning.

# Replace _program_commandLine_handleNonOptions() with
_program_commandLine_handleNonOptions()
{
	core_variable_array_initialise overdrive_jsonGearBoxFiles
	
	local jsonGearBoxFile
	for jsonGearBoxFile in "$@"
	do
		core_variable_array_append overdrive_jsonGearBoxFiles "$jsonGearBoxFile"
	done
	
	if core_variable_array_isEmpty overdrive_jsonGearBoxFiles; then
		core_message WARN "You haven't specified any JSON gear box files - are you sure this is what you wanted?"
	fi
}

Actually, let's make that an error after all:-

# Replace _program_commandLine_handleNonOptions() with
_program_commandLine_handleNonOptions()
{
	core_variable_array_initialise overdrive_jsonGearBoxFiles
	
	local jsonGearBoxFile
	for jsonGearBoxFile in "$@"
	do
		core_variable_array_append overdrive_jsonGearBoxFiles "$jsonGearBoxFile"
	done
	
	if core_variable_array_isEmpty overdrive_jsonGearBoxFiles; then
		core_exitError $core_commandLine_exitCode_USAGE "You haven't specified any JSON gear box files - are you sure this is what you wanted?"
	fi
}

We need somewhere to store out output. How about an option --output-path? Let's tell the parser what to do:-

# Place this code above _program_commandLine_handleNonOptions()
_program_commandLine_optionExists()
{
	case "$optionName" in
	
		output-path)
			echo 'yes-argumented'
		;;
		
		*)
			echo 'no'
		;;
	
	esac
}

Of course, we want to actually get the value of that option! In this case, the parser will call _program_commandLine_processOptionWithArgument():-

# Place this code below _program_commandLine_optionExists()
_program_commandLine_processOptionWithArgument()
{
	case "$optionName" in
	
		output-path)
			overdrive_outputPath="$optionValue"
		;;
	
	esac
}

By convention, we name variables set through command line options as ${_program_name}_lowerTitle. Of course, it'd be nice to have a short option, -o, too, so let's do that:-

# Replace _program_commandLine_optionExists() and _program_commandLine_processOptionWithArgument() with
_program_commandLine_optionExists()
{
	case "$optionName" in
	
		o|output-path)
			echo 'yes-argumented'
		;;
		
		*)
			echo 'no'
		;;
	
	esac
}

_program_commandLine_processOptionWithArgument()
{
	case "$optionName" in
	
		o|output-path)
			overdrive_outputPath="$optionValue"
		;;
	
	esac
}

Now, we really ought to validate that output path. Do we need to create it? Possibly. Let's use one of the convenience functions in core_validate:-

# Replace _program_commandLine_processOptionWithArgument() with
_program_commandLine_processOptionWithArgument()
{
	case "$optionName" in
	
		o|output-path)
			core_validate_folderPathIsReadableAndSearchableAndWritableOrCanBeCreated $core_commandLine_exitCode_USAGE 'option' "$optionNameIncludingHyphens" "$optionValue"
			overdrive_outputPath="$optionValue"
		;;
	
	esac
}

Now, we always need an output path. We can't know for sure until all the options have been parsed. Of course, the parser let's us manage that in _program_commandLine_validate():-

# Place this below _program_commandLine_handleNonOptions()
_program_commandLine_validate()
{
	if core_variable_isUnset overdrive_outputPath; then
		core_exitError $core_commandLine_exitCode_USAGE "Please specify --output-path"
	fi
}

That's a bit tough, though. Why don't we let an administrator set a value in configuration? Configuration is automatically parsed and loaded immediately prior to command line parsing. Of course, if that's the case, we'll need to validate what they've chosen. And, in this case, just because it makes sense, we could default the output path to the current working directory, but let the user know.

# Replace _program_commandLine_handleNonOptions() with
_program_commandLine_validate()
{
	if core_variable_isSet overdrive_outputPath; then
		core_validate_folderPathIsReadableAndSearchableAndWritableOrCanBeCreated $core_commandLine_exitCode_CONFIG 'configuration setting' 'overdrive_outputPath' "$overdrive_outputPath"
	else
		core_message INFO "Defaulting --output-path to current working directory"
		overdrive_outputPath="$(pwd)"
	fi
}

Of course, we ought to write an useful help message after all of this. Let's do that with _program_commandLine_helpMessage():-

# Place this above _program_commandLine_optionExists()
_program_commandLine_helpMessage()
{
	_program_commandLine_helpMessage_usage="[OPTION]... -- [JSON GEAR BOX FILE]..."
	_program_commandLine_helpMessage_description="Turns JSON into XML."
	_program_commandLine_helpMessage_options="
  -s, --output-path PATH      PATH to output to.
                              Defaults to current working directory:-
                              $(pwd)"
    _program_commandLine_helpMessage_optionsSpacing='     '
	_program_commandLine_helpMessage_configurationKeys="
  swaddle_outputPath     Equivalent to --output-path
"
	_program_commandLine_helpMessage_examples="
  ${_program_name} -o /some/path -- some-json-gear-box-file.json
"
}

Let's check out our new help: ./overdrive --help.

Now, we're repeating our self with the default value for the output path - once in _program_commandLine_helpMessage(), once in _program_commandLine_validate(). It's also a dynamic value. In a normal shell script, we might put that in a global value. But because of the way shellfire works, that's a bad idea (as it is in most normal programs). It'll be lost when the program's fattened, as all expression outside of functions aren't preserved ordinarily. And even if it wasn't, it'd be the value on the development machine. Instead, let's use an initialisation function:-

# Place this above _program_commandLine_helpMessage()
_program_commandLine_parseInitialise()
{
	overdrive_outputPath_default="$(pwd)"
}

# Replace _program_commandLine_helpMessage() with
_program_commandLine_helpMessage()
{
	_program_commandLine_helpMessage_usage="[OPTION]... -- [JSON GEAR BOX FILE]..."
	_program_commandLine_helpMessage_description="Turns JSON into XML."
	_program_commandLine_helpMessage_options="
  -s, --output-path PATH      PATH to output to.
                              Defaults to current working directory:-
                              $overdrive_outputPath_default"
    _program_commandLine_helpMessage_optionsSpacing='     '
	_program_commandLine_helpMessage_configurationKeys="
  swaddle_outputPath     Equivalent to --output-path
"
	_program_commandLine_helpMessage_examples="
  ${_program_name} -o /some/path -- some-json-gear-box-file.json
"
}

# Replace _program_commandLine_validate() with
_program_commandLine_validate()
{
	if core_variable_isSet overdrive_outputPath; then
		core_validate_folderPathIsReadableAndSearchableAndWritableOrCanBeCreated $core_commandLine_exitCode_CONFIG 'configuration setting' 'overdrive_outputPath' "$overdrive_outputPath"
	else
		core_message INFO "Defaulting --output-path to current working directory"
		overdrive_outputPath="$overdrive_outputPath_default"
	fi
}

Let's check out our new help: ./overdrive --help. To make use of the configuration, you could create a file at, say, $HOME/.overdrive/rc:-

overdrive_outputPath="~/overdrive-output"

Now we might want to be able to force the output to overwrite files. Let's add a --force long option, with -f for short hand, with the last function the parser uses, core_commandLine_processOptionWithoutArgument:-

# Replace _program_commandLine_optionExists() with
_program_commandLine_optionExists()
{
	case "$optionName" in
	
		o|output-path)
			echo 'yes-argumented'
		;;
	
		f|force)
			echo 'yes-argumentless'
		;;
		
		*)
			echo 'no'
		;;
	
	esac
}

# Place this below _program_commandLine_optionExists()
_program_commandLine_processOptionWithoutArgument()
{
	case "$optionName" in
		
		f|force)
			overdrive_force='yes'
		;;
		
	esac
}

# Replace _program_commandLine_validate() with
_program_commandLine_validate()
{
	if core_variable_isSet overdrive_outputPath; then
		core_validate_folderPathIsReadableAndSearchableAndWritableOrCanBeCreated $core_commandLine_exitCode_CONFIG 'configuration setting' 'overdrive_outputPath' "$overdrive_outputPath"
	else
		core_message INFO "Defaulting --output-path to current working directory"
		overdrive_outputPath="$overdrive_outputPath_default"
	fi

	if core_variable_isSet overdrive_force; then
		core_validate_isBoolean $core_commandLine_exitCode_CONFIG 'configuration setting' 'overdrive_force' "$overdrive_force"
	else
		overdrive_force='no'
	fi
}

Of course, there's more we could do. We could validate that the JSON files in _program_commandLine_handleNonOptions() are extant, readable and not empty. We should document --force. We leave that as an exercise for you.

Doing something useful

Recall in our boilerplate we had the following at the top of our program:-

_program()
{	
	overdrive()
	{
		echo "Hello World"
	}
}

Let's replace that with something more useful. Let's start by importing the namespaces we need:-

# Replace _program() with
_program()
{
	core_usesIn jsonreader
	core_usesIn xmlwriter
	
	overdrive()
	{
		echo "Hello World"
	}
}

We don't import unicode, even though jsonreader depends on it - it has a core_usesIn line in its logic.

Now, what's our program going to do? It's going to loop over each JSON file, and write each to a XML file. We need to create the output path, check if the XML files exist, and only overwrite if --force is specified. Let's write a loop in overdrive():-

# Replace _program() with
_program()
{
	core_usesIn jsonreader
	core_usesIn xmlwriter
	
	# document dependency
	core_dependency_requires '*' mkdir
	overdrive()
	{
		mkdir -m 0755 -p "$overdrive_outputPath"
		core_variable_array_iterate overdrive_jsonGearBoxFiles overdrive_convertJsonFileToXml
	}
}

overdrive_convertJsonFileToXml is a callback that'll be passed each JSON file path. It's the name of a function we'll define (very few people seem to know that callbacks are both easy and powerful in shell script). Now, we could write this in our shellfire application:-

# Replace _program() with
_program()
{
	core_dependency_requires '*' mkdir
	overdrive()
	{
		mkdir -m 0755 -p "$overdrive_outputPath"
		core_variable_array_iterate overdrive_jsonGearBoxFiles overdrive_convertJsonFileToXml
	}
	
	overdrive_convertJsonFileToXml()
	{
		:
	}
}

But it's getting to get large, quickly. We should use a module. Let's create a private one for ourselves. Create the file overdrive/lib/shellfire/overdrive/overdrive.functions, and put the logic in there:-

core_usesIn jsonreader
core_usesIn xmlwriter
overdrive_convertJsonFileToXml()
{
	:
}

Now, let's import the module like any other:-

# Replace _program() with
_program()
{
	core_usesIn overdrive
	core_dependency_requires '*' mkdir
	overdrive()
	{
		mkdir -m 0755 -p "$overdrive_outputPath"
		core_variable_array_iterate overdrive_jsonGearBoxFiles overdrive_convertJsonFileToXml
	}
}

Right, let's add some logic to overdrive_convertJsonFileToXml() in overdrive/lib/shellfire/overdrive/overdrive.functions:-

# Replace overdrive_convertJsonFileToXml() with
overdrive_convertJsonFileToXml()
{
	# core_variable_array_element is set by core_variable_array_iterate
	local jsonGearBoxFilePath="$core_variable_array_element"
	
	local jsonGearBoxFileName="$(core_compatibility_basename "$jsonGearBoxFilePath")"
	# Of course, you could use the file program
	local extension='.json'
	if ! core_variable_endsWith "$jsonGearBoxFileName" "$extension"; then
		core_exitError $core_commandLine_exitCode_DATAERR "The JSON gear box file '$jsonGearBoxFilePath' doesn't end in '.json'"
	fi
	
	# Strip .json
	local gearBoxFileNameWithoutExtension="$(core_variable_allButLastN "$jsonGearBoxFileName" ${#extension})"
	
	local xmlOutputFilePath="$overdrive_outputPath"/"$gearBoxFileNameWithoutExtension".xml
	
	# Don't overwrite
	if [ -e "$xmlOutputFilePath" ]; then
		if core_variable_isFalse "$overdrive_force"; then
			core_message WARN "Skipping conversion of '$jsonGearBoxFileName' to XML as output file already exists"
			return 0
		fi
	fi
	
	{
		xmlwriter_declaration '1.0' 'UTF-8' 'no'
		xmlwriter_open JsonGearBox creator overdrive
			jsonreader_parse "$jsonGearBoxFilePath" overdrive_convertJsonFileToXml_callback
		xmlwriter_close JsonGearBox
	} >"$xmlOutputFilePath"
}

Let's write that conversion code:-

# Place below overdrive_convertJsonFileToXml()
overdrive_convertJsonFileToXml_callback()
{
	case "$eventKind" in
	
		root)
			xmlwriter_leaf value type "$eventVariant" "$eventValue"
		;;
		
		object)
			
			case "$eventVariant" in
				
				start)
					if [ "$eventValue" = 'object' ]; then
						xmlwriter_open object key "$eventKey" index "$eventIndex"
					else
						xmlwriter_open object index "$eventIndex"
					fi
				;;
				
				boolean|number|string)
					# eg <value key="hello" type="boolean">true</value>
					xmlwriter_leaf value key "$eventKey" index "$eventIndex" type "$eventVariant" "$eventValue"
				;;
				
				end)
					xmlwriter_close object
				;;
				
			esac
			
		;;
		
		array)
			
			case "$eventVariant" in
				
				start)
					if [ "$eventValue" = 'object' ]; then
						xmlwriter_open array key "$eventKey" index "$eventIndex"
					else
						xmlwriter_open array index "$eventIndex"
					fi
				;;
				
				boolean|number|string)
					# eg <value type="boolean">true</value>
					xmlwriter_leaf value index "$eventIndex" type "$eventVariant" "$eventValue"
				;;
				
				end)
					xmlwriter_close array
				;;
				
			esac
			
		;;
		
	esac
}

Now, let's try it out. Copy this JSON to `overdrive/gearbox.json:-

{
	"hello": "world",
	"array":
	[
		-0.5e+6,
		true,
		null,
		false,
		"something",
		{
			"nested": "value"
		}
	],
	"number": 50,
	"boolean": true
}

Let's convert the data: ./overdrive --output-path ~/output-path -- ./gearbox.json. Take a look at ~/output-path/gearbox.xml. Right, now, let's try again: ./overdrive --output-path ~/output-path -- ./gearbox.json. Good, our logic stops an overwrite. Specify -f and try again: ./overdrive --output-path ~/output-path -f -- ./gearbox.json.

fattening and swaddle using build

fattening is the process of turning our shellfire application into a standalone program. swaddle can then take this and create packages, tarballs, Apt repositories and Yum repos, release notes on GitHub, etc. shellfire has a build framework that you can use to fatten, swaddle and more: build scripts are just regular shellfire code, so you can incorporate whatever you want. To see how to add build to your project, see the Quick Tutorial. To incorporate swaddle, you can then follow the Build with swaddle Tutorial.