Why Does Executing 'screenrecord' From Java Fail to Create a Video File?

Executing Shell Commands from Android: Solving the Execution Issue

When attempting to execute the command "screenrecord --time-limit 10 /sdcard/MyVideo.mp4" from a Java application using Runtime.getRuntime().exec(), the resulting video file fails to get created. This occurs despite the command working successfully when run from the application emulator terminal.

The root cause of this issue lies in the fact that executing the command as is from Java assigns the current user ID (UID) to the command, rather than using the elevated privileges granted by the su command. As a consequence, the file is not created.

Solution: Sub-Process I/O Redirection

To resolve this issue, it is necessary to redirect the standard input of the su process to the command to be executed. This ensures that the command runs with the intended elevated privileges.

Here is a revised code snippet that implements this 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);
}

By redirecting the standard input stream, the "screenrecord" command is effectively executed under the elevated privileges granted by the su process, resulting in successful file creation.

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