Why Does My Shell Command Work in Terminal but Fail in Java?

Executing Shell Commands from Android: A Journey from Terminal to Java

In the quest to automate device functionalities, Android developers often encounter the need to execute shell commands programmatically. While accomplishing this from the terminal emulator is straightforward, executing the same commands from Java can present challenges. This question-and-answer article delves into why the same command that works in a terminal emulator may not succeed in Java and provides a solution to overcome this hurdle.

Issue: Shell Command Execution Fails in Java

A developer attempting to record their screen using the newly introduced screenrecord command in Android KitKat noticed that the command worked flawlessly in the terminal emulator but failed when executed from within a Java application using Process.exec(). They sought guidance on where the issue lay and how to resolve it.

Answer: Granting Permissions to the Command

The crux of the problem stemmed from the fact that the screenrecord command requires elevated permissions, typically granted with the use of 'su' (super-user). When executing the command from the terminal emulator, the user provides 'su' manually, granting the necessary permissions. However, when executing the command from Java, the application lacked the required elevation.

Solution: Prompting for Root Access via Process Standard Input

The solution lay in prompting for root access within the Java code. The 'su' command is typically followed by a new line and the command that needs to be executed. By grabbing the standard input stream of the 'su' process and writing the command to it, the developer could effectively mimic the behavior of the terminal emulator.

The following code snippet illustrates the solution:

try{
    Process su = Runtime.getRuntime().exec("su");
    DataOutputStream outputStream = new DataOutputStream(su.getOutputStream());
    
    outputStream.writeBytes("screenrecord --time-limit 10 /sdcard/MyVideo.mp4\n");
    outputStream.flush();
    
    outputStream.writeBytes("exit\n");
    outputStream.flush();
    su.waitFor();
}catch(IOException e){
    throw new Exception(e);
}catch(InterruptedException e){
    throw new Exception(e);
}

Conclusion

By employing the provided solution, developers can successfully execute shell commands from Java applications, even those requiring elevated permissions. This technique unlocks the ability to automate various device functions and enhance application capabilities.

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