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build CLI apps in dart - part 2
In part 2, let’s take some of those pieces and build an app or two.
Before we move forward just a couple of thoughts and some answers to some of the questions I received after part one.
When working on a code base it’s often easy to get bogged down in the details of fixing bugs and designing solutions. It’s at these times you need to remember to occasionally step back from the code face and look at the big picture.
Over the last couple of weeks I’ve actually started using DCli to write scripts. A key reason for doing this was to validate that the DCli library and tooling actually works.
When you’re building an app you should do the same thing. Every now and then stop and actually use your application like a user would. Look at how the parts fit together, how easy it is to move from one part of the app to another. Look for the common workflows and make certain the are designed to make the user's life easy.
With my hands on experience using DCli I’ve gone back into the code based and cleaned up a few of the workflows and have a few more planned.
I have to say that so far I’m rather pleased with how DCli is working in the real world. I’ve always ended up bashing my head against the wall whenever I need to write bash scripts, but DCli has been a real pleasure to work with which is largely to do with Darts simple syntax.
But enough navel gazing and ego inflation, let’s talk about you.
After my initial article the first question was; what magic is DCli doing and how does that magic affect what I can do with Dart?
The answer is very little.
DCli allows you to use any Dart feature that is supported in a console app
The second question is, why doesn’t DCli use futures.
The answer is that DCli does use futures and you too can use futures in a DCli script.
The longer answer is; in a cli script, futures are a pain and for most requirements don’t provide any advantage. DCli goes to considerable length to shield you from having to use futures so unless you are doing something tricky; stay away from them.
With that done let’s get back to building our first app.
Every Linux system ships with the ‘which’ command. The ‘which’ command is used to find which directory an application is run from.
For example if you type:
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which grep
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‘Which’ will report:
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/bin/grep
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‘Which’ searches each directory in your PATH until it finds the one containing the grep command.
Our version of ‘which’, called dwhich, searches for the command and validates each path as it goes. We are also going to include a verbose flag to print progress messages.
Start by creating a DCli script:
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dcli create dwhich.dart
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Now copy the below contents into your dwhich.dart script.
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#! /usr/bin/env dcli
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import 'dart:io';
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import 'package:dcli/dcli.dart';
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​
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/// dwhich appname - searches for 'appname' on the path
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void main(List<String> args) {
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var parser = ArgParser();
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parser.addFlag('verbose', abbr: 'v', defaultsTo: false, negatable: false);
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​
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var results = parser.parse(args);
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​
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var verbose = results['verbose'] as bool;
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​
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if (results.rest.length != 1) {
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print(red('You must pass the name of the executable to search for.'));
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print(green('Usage:'));
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print(green(' which ${parser.usage}<exe>'));
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exit(1);
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}
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​
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var command = results.rest[0];
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​
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for (var path in PATH) {
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if (verbose) {
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print('Searching: ${truepath(path)}');
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}
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if (!exists(path))
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{
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printerr(red('The path $path does not exist.'));
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continue;
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}
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if (exists(join(path, command))) {
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print(red('Found at: ${truepath(path, command)}'));
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}
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}
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}
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Given our requirements we need to pass two arguments to dwhich:
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dwhich [-v] <appname>
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The -v flag is optional and <appname> is the name of the application we are going to search the path for.
To make processing the command line arguments easy we are going to use the ‘args’ package that ships with DCli.
So let’s break things down.
Line 7 we create an instance of the ArgParser
Line 8 we tell the ArgParser that we can accept an optional flag on the command line. The user can either type ‘ —verbose’ or ‘-v’ to cause ‘dwhich’ to output verbose details.
Line 10 parses the command args and gives us the results.
Line 12 extracts the ‘verbose’ flag from the results map and converts it to a bool.
Line 14 checks ‘results.rest’ to see if the expected appname argument was passed. The appname is stored in ‘results.rest’ which is a simple String list containing all of the arguments passed to main(), after the verbose switch was removed.
Line 22 takes the first argument from ‘result.rest’ which contains the name of the application that the user wants to search for and stores it into the variable ‘command’.
Line 23 uses DCli's ‘PATH’ variable to loop through all of the paths on the OS’ PATH environment variable. DCli conveniently converts the PATH environment variable to a List<String> containing each of the paths.
Line 27 validates that each path included in PATH is valid and prints an error if it isn’t. Here we use ‘printerr’ rather than ‘print’. ‘printerr’ writes to stderr whilst ‘print’ writes to stdout.
You should always use printerr to print error messages.
Line 32 uses the ‘join’ function from the ‘path’ package to create a path by joining current ‘path’ and the ‘command’ and then tests if the command exists at that path.
Line 33: print a message when we find the command. Note the use of the function ‘truepath’. Truepath is a convenience function provided by DCli. Truepath combines three operations into one. The following two lines give the same result:
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pwd = ‘/home/me’;
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truepath(‘apps’, bin’, ‘..’, ‘dart); == ‘/home/me/apps/dart’
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canonicalize(absolute(join(‘apps’, bin’, ‘..’, ‘dart));
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So why do we need to do all of that?
Line 33 prints the commands path for the user. To make the users life easier you should always print an absolute path. It is no end of frustration for a user to read an error message that mentions a path but only gives a relative path. Whilst sometimes it will be obvious what path the file is relative to, often it’s not. So good practice is to always print an absolute path.
The canonicalize call is for safety. Hackers have often used THIS ONE TRICK (sorry) to break out of sandboxes.
Where does the following path point to?
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/home/me/../../usr
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We canonicalize the above path it reduces to:
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/usr
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So by canonicalizing the path we make it easier to read and if your code is checking for a prefix of /home (thinking that’s safe) then the call to canonicalize will show your code that the path isn’t actually safe.
So use truepath whenever you show a user a path or when you need to validate a path.
So we now have a ‘dwhich’ command and it was surprisingly simple to implement.
Let’s try and run it. If you created the script using ‘dcli create’ the script is ready to run:
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./dwhich.dart grep
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or
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./dwhich.dart -v grep
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If you created the script by hand then you must first mark it as executable:
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chmod +x dwhich.dart
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./dwhich.dart grep
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Remember that the first time you run your script, dshell needs to do some housekeeping!
Make it go faster
Let’s make our dwhich command go faster by compiling it.
Run
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dcli compile dwhich.dart
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This will output an executable called ‘dwhich’.
Let’s run our new executable:
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./dwhich
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Notice how much faster the compiled ‘dwhich’ starts.
Install and copy to other servers
We can also install our dwhich command into the OS PATH by passing the ‘— install’ flag to the compile command.
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dcli compile — install dwhich.dart
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Dshell will copy the resulting executable to the ~/.dshell/bin directory which was added to your path when you installed Dshell.
You can now run dwhich as:
dwhich grep
But wait there’s more;
Now that we have compiled our ‘dwhich’ command we have a single file executable.
When dshell compiles a script it includes all of the scripts dependencies and a minimal Dart runtime. A compiled Dshell script is typically about 8MB in size.
But wait there’s even more….
Now we have a compiled executable we can copy just the executable to any binary compatible machine and run it. There is no need to install Dart nor DCli.
Hopefully you are now starting to get a feel for how the pieces of Dshell fit together.
Let’s build one more app before we finish up.
duntar.dart
I don’t know about you, but I often have to untar a file or a .tar.gz file and I can never remember the correct set of switches to get the correct result.
So I built a little script that remembers for me; after all isn’t that what scripting is all about.
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#! /usr/bin/env dcli
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import 'dart:io';
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import 'package:dcli/dcli.dart';
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​
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/// duntar <tarfile>
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/// untars a file
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void main(List<String> args) {
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var parser = ArgParser();
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var results = parser.parse(args);
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​
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if (results.rest.length != 1)
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{
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print('untars a .tar or .tar.gz file');
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print('');
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printerr(red('You must provide the name of the file to untar'));
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print('The file will be untared in the current working directory');
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exit(1);
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}
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​
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var tarFile = results.rest[0];
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​
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if (tarFile.endsWith('.tar.gz'))
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{
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'tar -zxvf $tarFile'.run;
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}
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else if (tarFile.endsWith('.tar'))
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{
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'tar -xvf $tarFile'.run;
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}
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else
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{
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print("The tar file $tarFile does not have a know extension of '.tar.gz' or '.tar'");
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}
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}
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So duntar takes the name or path of a tar file or tar.gz file, check the extension and run untar with the correct switches.
So once again let’s pull the command apart.
Line 9 we have seen before. Declare an ArgParser and ask it to parse the command line arguments.
Line 12 checks that the tar name was passed by checking the size of ‘results.rest’. I should note at this point that we really didn’t need ArgParser as we could have simply accessed ‘args’ directly.
Line 14 and friends print a message telling the user to try again.
Line 21 extracts the name of the tar file from the results.rest array.
Line 23 checks if the tarFile ends with .tar.gz and if so runs the correct tar command.
Line 27 checks again for a .tar file and again runs tar.
Line 31 is a catch all incase they passed in a file that isn’t supported.
Homework
So for some homework why don’t you try to make duntar support some additional file types such as .zip or 7z. You could end up with a single app which will uncompressed any file type.
Wrap up
So that’s it for this part.
To me at least Dshell is beginning to feel like a really useful tool. I love Dart and it’s really nice being able to script in a modern language.
I’ve dropped a few more examples in a git repo.
Feel free to issue a pull request and contribute your own little tool. They don’t need to be high quality, just useful.
Finally if you’re new to Dart, Dshell is a great way to get started as it lets you work directly with the language rather than through the prism of a large framework like Flutter.
Links as promised:
The above example code and more little tools.
A completely unrelated package that I wrote that lets you parse, format and do maths with Money and Currencies.