bash file contain executable/binary code

Question

I found this tool and when I downloaded it is a bash executable that contains binary code.
First of all I didn't know this is possible. Does anyone know who this can be done? Also I am not sure how safe generally this is since I can't really be sure what the bash file contains

Answer 1

This is a self-contained Java program which relies on a few “tricks”:

• Java programs can be packaged as JAR files, which are ZIP files containing a manifest (META-INF/MANIFEST.MF) which specifies the Java class to start;
• java -jar, given a JAR file, will look at the manifest and start the corresponding class, loading any other required resources from the JAR file and the Java runtime (one can also specify another class to run, but that’s not relevant here);
• ZIP files can start with arbitrary data; this is how self-extracting executables work (executable ZIP files which will extract themselves when run).

So svg2png is a JAR file, but with a prepended shell script:

#!/bin/sh exec java -Xmx1G -jar "$0" "$@" 

When run, this will run a shell, which will then replace itself with java -Xmx1G -jar and the name of the “script” (svg2png with whatever path was used), and any arguments. -Xmx1G sets the heap size, and -jar tells the JVM to “run” the given JAR file, svg2png in this case. The rest of svg2png is the JAR file; in particular, it contains the following manifest:

Manifest-Version: 1.0 Archiver-Version: Plexus Archiver Built-By: sterlp X-Compile-Target-JDK: 1.8 X-Compile-Source-JDK: 1.8 Created-By: Apache Maven 3.3.3 Build-Jdk: 1.8.0_51 Main-Class: org.sterl.svg2png.Main 

This instructs the JVM to run the org.sterl.svg2png.Main class, which starts the program.

You’ll find more details about the ZIP format on Wikipedia, in particular the structure description which shows how the format allows arbitrary data at the start of the file (and in other positions). ZIP files are identified by their “central directory”, which resides at the end of the file.