Java how to pass password from file to process

The underlying problem appears to be that you don't understand how terminal apps work.

When you type echo somePass | sudo whatever that's not 'a command your computer runs'. No, that's a string you type, and bash.exe picks that up, tears it into pieces, and treats it as a little programming script.

Bash is bash. ProcessBuilder is not.

Putting:

ProcessBuilder.command("echo", "somePass", "|", "sudo", "whatever");

in a java file is thus obviously not going to work; trying it means you don't understand this part of how it works. Which is somewhat sensible; this stuff is complicated!

ProcessBuilder interprets the first argument as a 'command' and does its own, extremely lightweight, 'smart processing', i.e. apply looking up the executable via PATH. You should probably opt out of that and always specify as first argument an absolute path (so, not 'echo', but /bin/echo).

The rest of the args are passed verbatim to that process. "password" is something that instructs bash, when tearing apart the command line, to not attempt to parse out spaces as 'separators'. ProcessBuilder is not bash! - so, PB just sends "password", quotes and all, straight to sudo or echo or whatever the first arg to pb.command() is. It will dutifully interpret "password" as the password. I don't think your password actually starts with a quote, so, it would be an incorrect password.

This command in bash:

echo "Hello World"

Is, in PB terms:

pb.command("/bin/echo", "Hello World");

there is no need for quotes. Because pb doesn't treat a command as a 'script' that needs to be executed. The quotes are for bash to tell it: Do not attempt to chop this up.

Similarly, < and > and such are bashisms - in pb terms you use getOutputStream, getInputStream and getErrorStream and read or write as needed.

TL;DR: If you put ", ', >, <, |, & or $ in any string you pass to pb.command, that's not going to do what you think it will; those are programming constructs that the bash programming language understands, it's not something the OS execution system understands, and that's what both bash and pb use to actually 'run' your command.

NB: If you know how to do something 'in bash' and can't figure out how to do it 'in pb', then you can ask pb to run bash and pass your bashisms to it: pb.command("/bin/bash", "-c", "the whole bashism thing here, quotes, dollars, pipes, you name it") usually works fine. You are now of course relying on /bin/bash existing.

NB: There are many shells; bash is just one. In that sense 'bashism' isn't exactly fair and oversimplifies a bit, but the answer is trying to cater to those quite new to the concept of shells, and trying to disentangle the mess that is '"the shell" is not necessarily "bash"' is not something I wanted to mess with in this answer.

I found a solution:

// Process Builder
ProcessBuilder pb = new ProcessBuilder();
String cmd = "cp";
String tableName = "password_entries.ibd";
String source = properties.getProperty("DB_PATH");
String target = ".";
String userPassword = secrets.getProperty("USER_PW");

try {
      // elevate privileges to root
      pb.command("sudo", "-S", "echo", "test");
      Process p = pb.start();
      p.getOutputStream().write((userPassword + "\n").getBytes());
      p.getOutputStream().flush();
      p.waitFor();

      // cp to host
      pb.command("sudo", "-S", cmd, source + tableName, target);
      pb.start().waitFor();
      
} catch (IOException | InterruptedException e) {
      throw new RuntimeException(e);
}