The YUI Compressor needs Java to work and runs from the command line. The minimum version of Java required is 1.4. To install a Java runtime on Debian, for example, use the following command to find what runtimes are available:
aptitude search "?provides(java-runtime)"
and then e.g. to install openjdk-6-jre:
sudo apt-get install openjdk-6-jre
The YUI Compressor itself can be downloaded from the YUI Compressor page and clicking the "Download" link.
Using the YUI Compressor
It's simple to use, as shown in the following example. Replace /path/to with the actual path to the YUI Compressor, yuicompressor-2.4.2.jar with the actual version you are using, common.js with your input filename and common.min.js with the name you want the output file to be:
java -jar /path/to/yuicompressor-2.4.2.jar common.js -o common.min.js
And for CSS:
java -jar /path/to/yuicompressor-2.4.2.jar main.css -o main.min.css
To cut down on HTTP requests it's also a good idea to combine all your various CSS and JS files into a single file, and then compress them. On a Linux box it's as simple as using the "cat" command to concatenate them together into one file.
if [ -e combined.js ]; then rm combined.js; fi cat *.js > combined.js java -jar /path/to/yuicompressor-2.4.2.jar combined.js -o combined.js
and in the directory where the CSS files are:
if [ -e combined.css ]; then rm combined.css; fi cat *.css > combined.css java -jar /path/to/yuicompressor-2.4.2.jar combined.css -o combined.css
The if [ -e ... ] line tests to see if the combined file already exists and if it does it is deleted. This prevents it being included in the cat command. The *.js or *.css files are then concatenated into a single file which is then compressed with the YUI Compressor.
Note the input filename and output filename are the same for these commands; this is to avoid additional files being left over afterwards that need to be deleted.
Read the YUI Compressor page for more details, and additional command line flags etc.