First step: create a utilities.sh file to add all your bash functions in it.

2nd step: add your bash function/s like this.

## utilities.sh
convertMP4toMP3(){
echo -n "Enter source mp4 file : "
read sourceFile

echo -n "Enter destination mp3 file : "
read destFile

avconv -i $sourceFile -vn -acodec libmp3lame -ac 2 -ab 160k -ar 48000 $destFile
}

I created a bash function to convert mp4 video file to mp3 audio file from command line.

3rd step: source the utilities.sh Bash file

source utilities.sh

Finally, you can call the function from the command line (terminal) and use it like this.

$ convertMP4toMP3
Enter source mp4 file : video.mp4
Enter destination mp3 file : audio.mp3