Shebang4J

Shebang4J is a bash script to facilitate specifying jshell (Java 9 REPL) in the shebang of shell scripts

JShell is a Java REPL (Read-Eval-Print-Loop) that ships with JDK 9. Like other REPLs (e.g. node in NodeJS, irb in Ruby) it can be run interactively or given a script file and it will execute the code in the script.

Unfortunately, both invoking ${JAVA_HOME}/bin/jshell myScript.jsh and putting the path to the jshell binary in the shebang line of myScript.jsh and invoking ./myScript.jsh have some drawbacks:

• jshell tries to parse the shebang line and throws errors at line 1: illegal character: '#' and illegal start of expression
• jshell can't receive arguments and make them available to the script
• after running the script, jshell stays open and must manually be exited by the user

This utility gets around those limitations by providing a script to be used as the interpreter directive in Java shell scripts in place of jshell. It does the following:

• prevents parsing errors caused by the shebang line
• makes command-line arguments available to the script in a variable called args, type: String[]
• automatically exits at the end of the script, like other types of shell scripts (jshell might still stay open under certain conditions, e.g. OutOfMemoryError)

Installation

Put the Shebang4J script file shebang4j from this repo in a directory on the system PATH, such as /usr/bin, and make sure that it's executable (chmod +x).

Usage:

Create a shell script with some Java code in it, and add a shebang line to the top with the path to the JShell Shebang script (#!/usr/bin/shebang4j if you followed the example in the previous step).

You can now execute Java shell scripts as you would other types of shell scripts.

Complete example

# clone repo and cd into it
$ sudo cp shebang4j /usr/bin/shebang4j
$ sudo chmod +x /usr/bin/shebang4j
# create a script in a file called test.jsh
$ cat test.jsh
#!/usr/bin/shebang4j

System.out.println("I'm a Java shell script");
if (args.length == 0) {
    System.out.println("I didn't get any arguments");
} else {
    System.out.printf("I got %d arguments: %s\n", args.length, Arrays.toString(args));
}
System.out.println("exiting now");
$
$
$ chmod +x test.jsh
$ ./test.jsh
I'm a Java shell script
I didn't get any arguments
exiting now
$
$
$ jshell test.jsh
I'm a Java shell script
I didn't get any arguments
exiting now
$
$
$ ./test.jsh first second "a \"third\" arg"
I'm a Java shell script
I got 3 arguments: [first, second, a "third" arg]
exiting now
$

Options to JShell

The jshell binary can take options when started interactively or with a script (run jshell --help for info). To pass options to the jshell binary, you have to invoke Shebang4J, passing it options and a script (shebang4j --no-startup myScript). You can't pass options to jshell by directly invoking an executable script (./myScript).

Examples

$ ls
commons-math3-3.6.1.jar  withoutCp.jsh  withCp.jsh
$ cat withoutCp.jsh
#!/usr/bin/shebang4j

# jar needs to be included in the classpath on the command line
import org.apache.commons.math3.analysis.function.Add;

Add a = new Add();
System.out.println(a.value(Double.parseDouble(args[0]), Double.parseDouble(args[1])));
$
$ shebang4j --class-path commons-math3-3.6.1.jar withoutCp.jsh 3 4
7.0
$
$
$
# the same script, with the classpath in the script itself
$ cat withCp.jsh
#!/usr/bin/shebang4j

/env -class-path commons-math3-3.6.1.jar

import org.apache.commons.math3.analysis.function.Add;

Add a = new Add();
System.out.println(a.value(Double.parseDouble(args[0]), Double.parseDouble(args[1])));
$
$ ./withCp.jsh 5 6
11.0
$

Notes

• so far, it's only been tested on Ubuntu
• The script tries to find the jshell binary by looking in the same directory as java. If you have JDK 9 installed, but a different version set as the default, it will try to find the binary in any nearby directory named like "java-9" or "java-1.9".
• with the jshell binary, you can pass multiple arguments, each being a script that gets evaluated. Shebang4J doesn't allow that behavior since it treats arguments after the script as arguments to the script
• calling System.exit(int) in a script doesn't exit jshell and /exit doesn't take arguments, so there isn't a way to use exit codes in scripts