Create and Make Executable Script via Java Program
Let us see how we can create a script and make it executable via a Java Program.
Creating a Script File
Creating a file and storing it in the FileSystem (writing) is a trivial task in any programming language and Java has no exception to this.
For the betterment, we will see the new/improved API classes introduced in Java 11 (JDK11), than the typical/old school FileWriter, (or its wrapper BufferedWriter) classes.
The sample programs use the language constructs introduced in Java 10, 11 and 13. So any version of JDK13 or higher is recommended to try out the programs listed in this article. Also, I use zsh as a preferred shell and hence the prompt displayed in the terminal is prefixed with the text zsh.
Note: It is recommended to always try out the latest, or close-to-the-latest version of the JDK.
import java.nio.file.Files;
import java.nio.file.Path;
import java.io.IOException;
public class WriteAShellScript
{
public static void main(String... args)
throws IOException
{
//First, let us keep the content of the script file ready
//We use var construct introduced in Java 10,
// and the text blocks which was introduced in Java 13.
var script = """
#!/bin/bash
echo "hello, world - via Java Program"
""";
//Let's write the content into a file, and store it
// in the FileSystem by specifying the file name.
// As no directory is mentioned, it would be stored in
// the current directory where this Program is executed.
var path = Files.writeString(Path.of("hello.sh"), script);
// A confirmation message to the user
System.out.println("File [hello.sh] has been written");
}
}
If we compile and execute this program, we get the following output.
➜ io git:(master) ✗ exa -l
.rw-r--r-- 908 raghavan.muthu 20 Jun 08:23 WriteAShellScript.class
.rw-r--r-- 389 raghavan.muthu 19 Jun 23:58 WriteAShellScript.java
➜ io git:(master) ✗ java WriteAShellScript
File [hello.sh] has been written
➜ io git:(master) ✗ exa -l
.rw-r--r-- 53 raghavan.muthu 20 Jun 08:24 hello.sh
.rw-r--r-- 908 raghavan.muthu 20 Jun 08:23 WriteAShellScript.class
.rw-r--r-- 389 raghavan.muthu 19 Jun 23:58 WriteAShellScript.java
➜ io git:(master) ✗ ./hello.sh
zsh: permission denied: ./hello.sh
➜ io git:(master) ✗
If you observe, the file hello.sh has been created successfully in the same directory, but watch the file attributes (access markers) which is rw-r--r-- meaning the owner can only read and write and NOT execute. It is the standard/default file permission 644 (User-Read,Write, Group-Read, Others-Read).
Hence, an attempt to execute the file has failed with the message stating that permission denied: ./hello.sh.
Making it executable
We can make it executable in two different ways. First, the usual old-school way of setting permissions on the terminal/ command line via chmod command where we add the +x to the user as follows.
chmod u+x <fileName>
However, we want to manage this as well via Java Program via the API. Let us see how we can achieve the same by writing a different version of the program.
import java.nio.file.Files;
import java.nio.file.Path;
import java.io.IOException;
import java.util.Set;
import java.nio.file.attribute.PosixFilePermission;
public class WriteAndMakeExecutableShellScript
{
public static void main(String... args)
throws IOException
{
var script = """
#!/bin/bash
echo "hello, world - via Java Program"
""";
var path = Files.writeString(Path.of("hello.sh"), script);
System.out.println("File [hello.sh] has been written");
/* Set the File permission with the meaningful methods.
setPosixFilePermission method takes two arguments:
1. The path to the target file
2. A `java.util.Set` that takes the list of permissions,
each denoted by a predefinex constant defined
in the `PosixFilePermission` class.
`OWNER_XXX` for Owners,
`GROUP_XXX` for the Group,
and `OTHERS_XXX` for Others,
where the XXX can be of any `READ`, `WRITE`, `EXECUTE`.
*/
Files.setPosixFilePermissions(path,
Set.of(PosixFilePermission.OWNER_READ, PosixFilePermission.OWNER_EXECUTE));
System.out.println("The file permissions are set to be executed by the User!");
}
}
Now, let us run the revised program and verify the changes.
➜ io git:(master) ✗ javac WriteAndMakeExecutableShellScript.java
➜ io git:(master) ✗ exa -l
.rw-r--r-- 1.4k raghavan.muthu 20 Jun 08:42 WriteAndMakeExecutableShellScript.class
.rw-r--r-- 683 raghavan.muthu 20 Jun 08:42 WriteAndMakeExecutableShellScript.java
.rw-r--r-- 389 raghavan.muthu 19 Jun 23:58 WriteAShellScript.java
.rw-r--r-- 389 raghavan.muthu 19 Jun 23:58 WriteAShellScript.class
.rw-r--r-- 53 raghavan.muthu 20 Jun 08:24 hello.sh
➜ io git:(master) ✗ java WriteAndMakeExecutableShellScript
File [hello.sh] has been written
The file permissions are set to be executed by the User!
➜ io git:(master) ✗ exa -l
.r-x------ 53 raghavan.muthu 20 Jun 08:43 hello.sh
.rw-r--r-- 1.4k raghavan.muthu 20 Jun 08:42 WriteAndMakeExecutableShellScript.class
.rw-r--r-- 683 raghavan.muthu 20 Jun 08:42 WriteAndMakeExecutableShellScript.java
.rw-r--r-- 389 raghavan.muthu 19 Jun 23:58 WriteAShellScript.java
Note: If you observe the file permissions of the
hello.shcarefully, the permission bits are actually overwritten by the Java Program.
Earlier, the permission bits were set to 644, but now we have explicitly overwritten with the setPosixPermission() method by passing in the required attributes to be applied - which was an instance of java.util.Set with PosixFilePermission.OWNER_READ, and PosixFilePermission.OWNER_EXECUTE. This is indeed the appropriate one as we are taking control over the file permission bits via the API.
➜ io git:(master) ✗ ./hello.sh
hello, world - via Java Program
➜ io git:(master) ✗
No surprise, we can run / execute the program now without any hassles.
Source Code
The Source codes for this article are available in the Github repository → TechNotes - javaNotes - WriteMakeExecutableShellScript
Cheers,
RM…
Raghavan alias Saravanan Muthu
20 Jun 2023 | Tue | 08:09:30 AM IST