Shell script to batch bulk convert *.flac files to *.mp3

I have many music files in flac format and I wanted to convert them to a more compressed mp3 format with ffmpeg on Ubuntu so I can upload them to a storage limited portable music player. To ease the conversion task, I decided to write this simple shell script to do the job. In brief, the script will do the following:

• find all the files with the extension .flac in the current directory
• replace the file name extension .flac with the .mp3 extension
• create a temporary script that calls the ffmpeg command to convert
  
• run the temporary script

The listing of the shell script is shown below.  

# Define the internal field separator as a newline
IFS=$'\n'

# Find all the *.flac files in the current directory and perform the conversion
for f in `find . -name "*.flac" `;
do
	# Use the input flac file name prefix and replace the .flac extension with a .mp3 extension
	f=$(echo $f | cut -c 3-)
	outfile=$(basename $f .flac)
	outfile=$outfile.mp3
	
	echo "Convert $f->$outfile..."
	
	# Form the ffmpeg command to convert the input flac file to mp3
	cmd="ffmpeg -hide_banner -i \"$f\" -ab 320k -map_metadata 0 -id3v2_version 3 \"$outfile\" "
	
	# Create a temporary shell script for running the conversion
	echo $cmd > /tmp/tmp.sh
	
	# Run the conversion to mp3
	bash /tmp/tmp.sh
	
	# Clean up
	rm /tmp/tmp.sh
done

To use this shell script, you can do the following:

1. Save the code listing above to a file e.g. run.sh in a directory, e.g. /path/to
   /directory/
   
   
2. Open up a Linux Terminal.
   
   
3. In the Terminal, type in the command to change directory to the location of the flac files, e.g. /path/to/music/
   
   $ cd /path/to/music
   
   
   
   
   
   
4. At the prompt, type in the command to run the shell script.
   
   $ bash /path/to/run.sh
   
   The flac files are converted to mp3 files.
   
   

 

Using rostopic to simulate publishing odometry topic messages

While developing (Robotic OS) ROS1 node callbacks, and you don't have any inertial motion devices on hand, you can use the rostopic utility command with pub option to simulate the publishing of odometry messages. Basically, before being able to use the command, you have to find out how the odometry message is structured. Once you have the fields - names and format, simply create a shell script and type in the rostopic command with the correct argument structure. More information about the rostopic command is available on http://wiki.ros.org/rostopic.

Identify the Odometry message fields

This can be done using the rosmsg command. In a Terminal, type in the following command:

$ rosmsg info nav_msgs/Odometry

The odometry fields are displayed.

Create a shell script

Using your favorite text editor, create a shell script e.g. test_rostopic.sh to publish odom topic messages of type nav_msgs/Odometry. Type in the following with the fields and corresponding values in yaml format. Make sure no tabs are used and be careful of the spaces.

rostopic pub /odom nav_msgs/Odometry '
{
header: {seq: 1, stamp: now},
pose: 
  { 
  pose: { 
    position: { x: 10, y: 20, z: 30}, 
    orientation: { x: 0.2, y: 0.1, z: 0}
  }, 
  covariance: [0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0]},
}'
-r 0.1

Note: the last line "-r 0.1" simply says to repeat the rostopic pub command at 0.1 hz.

Run the shell script

 In a Terminal, type in the following to run the rostopic pub command.

$ bash /path/to/test_rostopic.sh

Optional. To see whether the odometry messages published by the script, open up another Terminal and run the following command:

$ rostopic echo odom

The odom topic is displayed.

How to setup a C++ WebAssembly component to run in a create-react-app React project

I had a hard time trying to get a C/C++ WebAssembly component to work in a React application generated from the create-react-app utility. Some blogs suggested using the react-app-rewired package to override the configuration and use the wasm-loader; but I kept getting magic header not detected errors when loading the wasm file or some ES6 syntax errors. Finally I found a StackOverflow post that gave me some ideas on getting it to work. 

In this post, I outline the steps I did to create, use a C/C++ WebAssembly in a ReactJS application generated by the create-react-app tool, on a Linux Ubuntu machine. This example will call the WebAssembly's function to add in two numbers (1 and 2) and return the resultant value. The resultant value would be displayed at the top of the web page as shown in the screen shot below.

Create a ReactJS application with create-react-app

1. Open up a Terminal. Change to your project directory and type in the following commands to create the ReactJS application cra-wasm-adder.
   
   $ cd /path/to/reactprj/
   $ npx create-react-app cra-wasm-adder
   
   The cra-wasm-adder ReactJS application is created.
   
   
2. Underneath the root of the newly created project, create a source directory, e.g. src_cpp for the C/C++ WebAssembly code.
   
   $ cd /path/to/reactprj/cra-wasm-adder/
   $ mkdir src_cpp
   
   The src_cpp directory is created.
   
   

Create the C/C++ source code files

1. In the newly created src_cpp directory, use a favorite source code editor to create a C/C++ file, e.g. adder.cpp. Enter the following code.
   
   
   
   Note: the adder function simply sums up two integer numbers and returns the value.
   
   
2. Create a Shell script file, e.g. make.sh to call the Emscripten compiler emcc to compile the C/C++ source code.
   
   Note: the emcc compiles the C/C++ source code into a Javascript plumbing helper file and a WebAssembly (*.wasm) file.
   
   
3. In the make.sh file, define the module name and output file names.
   
   
   
   
4. Then type in the emcc command to generate a Javascript plumbing helper file and the WebAssembly (*.wasm) file.
   
   
   Note: the Javascript generated by the emcc compiler (my version is 2.0.5) is not fully compatible with the ReactJS transpiler. The file has to be edited for compatibility purposes. This modifications may differ for different versions of the compiler. 
   
   
5. Type in the sed command to insert a line /* eslint-disable */  at the top of the Javascript plumbing file to disable the Ecmascript linting.
   
   
   
   
6. Type in the sed command to replace the import.meta.url string to the WebAssembly file relative to the root of the public directory of the ReactJS project, e.g. /path/to/reactprj/public/.
   
   
   Note: in this example, the *.wasm file is placed directly underneath the root of the /path/to/reactprj/public/; so the resultant change looks like the code below.
   
   
   
7. Type in the sed command to add in the window object to the self.location.href string.
   
   
   The result change Javascript looks like the snippet below.
   
   
   
8. Type in the sed commands to comment out the dataURIPrefix declaration  and isDataURI function block.
   
   
   
   The resultant Javascript code is shown below.
   
   
   
9. Next, type in the sed commands to modify the wasmBinaryFile variable to point to the actual relative location in the ReactJS' public directory and to remove and replace the getBinary and getBinaryPromise function blocks.
   
   
   After running the commands, the Javascript should like the code below:
   
   
   
   
10.  Finally, type in the sed command to remove the call to the isDataURI function in the instantiateAsync function.
    
    
    The resultant Javascript:
    
    
    
11. The last thing the make.sh script needs to do is to move the generated WebAssembly file to the /path/to/reactprj/public/ directory and the modified Javascript file to the /path/to/reactprj/src/ directory.
    
    
    mv $OUTPUT_JS ../src/

    mv $OUTPUT_WASM ../public/

    
    
    
    
    The directory structure should look like the screen shot below:
    

Run the Shell script

1. In a Terminal, change directory to the location of the make.sh shell script.
   
   $ cd /path/to/reactprj/src_cpp/
   
   
2. Type in the following to run the script.
   
   $ bash make.sh
   
   If the compilation is successful, the resultant Javascript and WebAssembly should be placed in the directories as shown below.
   
   
   

Create WebAssembly React component

1. In a text editor, create the React component under the /path/to/reactprj/src/ directory to use the WebAssembly component, e.g. AddNumbers.js .
   
   
   
2. Type in the following code to import in the generated Javascript plumbing file and call the WebAssembly adder function.
   
   
   

Running the ReactJS application

1. In a Terminal, run the React JS application.
   
   $ npm run start
   
   The browser opens up the development site.
   
   
   
2. Note the message at the top which prints out the result of calling the WebAssembly's adder function: Hello 1+2=3.
   

The source code for this example is freely available on https://gitlab.com/dominoc925/cra-wasm-adder. 

How to run a shell script from a ROS launch file

I wanted to execute a bash shell script from the ROS launch file but the ROS Wiki were not very clear. After some trial and error, I figured out how to do it. The following steps illustrate the procedure I used:

Create a shell script

1. In the ROS workspace package, e.g. /path/to/workspace/package/script/ folder, create a shell script e.g. run_script.sh.
2. Type in the script commands, e.g. see the code listing below.
   
   Note 1: ensure the shebang statement is at the top i.e. #!/bin/bash and a exit status code (0 for success or other values) is returned from the script.
   
   Note 2: Use the chmod command to make the script executable, e.g. $ chmod a+x run_script.sh

#!/bin/bash

# just print this out
echo "Hello ROS world"

# exit gracefully by returning a status 
exit 0

Create a launch file

1. In the ROS workspace package launch folder, create a launch file e.g. /path/to/workspace/package/launch/hello_script.launch.
2. Using a text editor, type in the following:
   
   Note: fill in the package name, e.g. beginner_tutorials, and the type, which should be the shell script name; name is any label you want to associate with the script node.

<launch>
        <node pkg="beginner_tutorials"
                type="run_script.sh" name="run_script"
                output="screen"
        />
</launch>

Run the launch file

1. In a terminal, type in the ros launch command:
   
   $ roslaunch beginner_tutorials hello_script.launch
   
   Note: change beginner_tutorials to your package name and hello_script.launch to the launch file created previously.
   
   The script is executed as shown in the print out of "Hello ROS world" below.