Develop Reference

How to handle interactive commands using runtime.getRuntime?

I have to execute a command from Java program on Unix platform.
I am using Runtime.getRuntime() for it.
However, the problem is that my command is interactive and asks for certain parameters at runtime. For e.g., the command is createUser. It asks for userName as the runtime.
bash-4.1$ createUser
Enter the UserName:
How can I handle such scenario so that the user name is entered at runtime from Java program?
try {
Process proc;
proc = Runtime.getRuntime().exec(cmd, envp);
BufferedReader stdInput = new BufferedReader(new InputStreamReader(proc.getInputStream()));
BufferedReader stdError = new BufferedReader(new InputStreamReader(proc.getErrorStream()));
// read the output from the command
String s = null;
while ((s = stdInput.readLine()) != null) {
System.out.println(s);
sb.append(s);
}
// read any errors from the attempted command
while ((s = stdError.readLine()) != null) {
System.out.println(s);
sb.append(s);
}
} catch (Exception e) {
e.printStackTrace();
sb = null;
}
I heard that it can be done through expect. But How can I do it in Java?

Get also the standardOutput from proc. All you write in that standardOutput goes to the command
Send the username to standardOutput and don't forget to send the \n too.

You can check what was the last line of input steam and when you detect the prompt for user input input write to the output steam your value.
try {
Process proc;
proc = Runtime.getRuntime().exec(cmd, envp);
final BufferedReader stdInput = new BufferedReader(new InputStreamReader(proc.getInputStream()));
final PrintWriter stdOutput = new PrintWriter(proc.getOutputStream());
// read the output from the command
String s = null;
while ((s = stdInput.readLine()) != null) {
System.out.println(s);
if (s.equals("Enter your username")) {
stdOutput.println("MyUsername");
stdOutput.flush();
}
sb.append(s);
}
} catch (final Exception e) {
e.printStackTrace();
sb = null;
}
(removed the error stream for simplicity)
Note that this works only if the prompt ends with a new line.
If the prompt has no new line (eg. Username: <cursor here>) you can try just writing the value at the start:
...
final PrintWriter stdOutput = new PrintWriter(proc.getOutputStream());
stdOutput.println("MyUsername");
stdOutput.flush();
...
But if the the command clears the buffer this will no work, in that case (rare case) you have to change the way you read from the stream (eg. instead of lines, read bytes)

Method to execute Linux command fails

I created the below method to execute a Linux command:
public void executeGetLogs(){
try{
Runtime rt = Runtime.getRuntime();
String [] commands = {helper.getPythonPath(), helper.getLogsMainScript(), helper.getLogsManagementUrl(), helper.getLogsManagementUrlPrefix(), helper.getLogsManagementUsername(), helper.getLogsManagementPassword(), helper.getLogsNet(), helper.getLogsStorageUrl(), helper.getLogStorageUsername(), helper.getLogStoragePassword()};
Process proc = rt.exec(commands);
BufferedReader stdInput = new BufferedReader(new
InputStreamReader(proc.getInputStream()));
BufferedReader stdError = new BufferedReader(new
InputStreamReader(proc.getErrorStream()));
// read the output from the command
logger.debug("Standard output from execution of get_logs:\n");
String s = null;
while ((s = stdInput.readLine()) != null) {
logger.debug(s);
}
// read any errors from the attempted command
logger.debug("Standard error from execution of get_logs (if any):\n");
while ((s = stdError.readLine()) != null) {
logger.debug(s);
}
}
catch(IOException e){
logger.debug("Execution exception: " + e);
}
}
The method appears to start working correctly but then fails.
The debug shows the following output:
2017-04-21 12:27:42,391 DEBUG Standard output from execution of get_logs:
2017-04-21 12:27:44,360 DEBUG 411 new files found
2017-04-21 12:27:44,363 DEBUG Downloading files...
2017-04-21 12:27:44,446 DEBUG Standard error from execution of get_logs (if any):
What I would expect to see is
2017-04-21 12:27:44,360 DEBUG 411 new files found
2017-04-21 12:27:44,363 DEBUG Downloading files...
Downloaded 10 of 447
Downloaded 20 of 447
Downloaded 30 of 447
and so on until Downloaded 447 of 447.
I can also see that nothing gets downloaded.
My command runs when I run it in a terminal.
Is it possible something in the Java causes it to exit? One thing is that it can take a few seconds to process each block of 10. Is it possible the
while ((s = stdInput.readLine()) != null) {
logger.debug(s);
}
just sees a null because the stdInput hasn't appeared yet so it exits the loop? If so how can I fix this?

A much better way of executing code in java is ProcessBuilder. Example:
//Create the command.
ProcessBuilder command = new ProcessBuilder(helper.getPythonPath(), helper.getLogsMainScript(), helper.getLogsManagementUrl(), helper.getLogsManagementUrlPrefix(), helper.getLogsManagementUsername(), helper.getLogsManagementPassword(), helper.getLogsNet(), helper.getLogsStorageUrl(), helper.getLogStorageUsername(), helper.getLogStoragePassword());
//Will show console output in java console.
command .inheritIO();
//Launches de command.
Process process= command .start();
Hope it helps.

Got to the bottom of it. I need to wait for the process to end. This can be done with the below code:
proc.waitFor();
My method now looks like this:
public void executeGetLogs(){
try{
Runtime rt = Runtime.getRuntime();
String [] commands = {helper.getPythonPath(), helper.getLogsMainScript(), helper.getLogsManagementUrl(), helper.getLogsManagementUrlPrefix(), helper.getLogsManagementUsername(), helper.getLogsManagementPassword(), helper.getLogsNet(), helper.getLogsStorageUrl(), helper.getLogStorageUsername(), helper.getLogStoragePassword()};
Process proc = rt.exec(commands);
BufferedReader stdInput = new BufferedReader(new
InputStreamReader(proc.getInputStream()));
try{
proc.waitFor();
}
catch(InterruptedException e){
logger.debug("Interrupted Exception : " + e);
}
BufferedReader stdError = new BufferedReader(new
InputStreamReader(proc.getErrorStream()));
// read the output from the command
logger.debug("Standard output from execution of get_logs:\n");
String s = null;
while ((s = stdInput.readLine()) != null) {
logger.debug(s);
}
// read any errors from the attempted command
logger.debug("Standard error from execution of get_logs (if any):\n");
while ((s = stdError.readLine()) != null) {
logger.debug(s);
}
}
catch(IOException e){
logger.debug("Execution exception: " + e);
}
}

Shell command not executed

I have a weird problem when trying to execute a shell command from within a java program. Since there exist thousands of websites that explain how to do it I used the following recommended code:
public String executeShellCommand (String command)
{
try
{
StringBuffer sb = new StringBuffer();
String line = "";
Process p = Runtime.getRuntime().exec(command);
BufferedReader reader = new BufferedReader(new InputStreamReader(
p.getInputStream()));
while ((line = reader.readLine()) != null)
sb.append(line + "\n");
p.waitFor();
return sb.toString();
}
catch (Exception e)
{
e.printStackTrace();
}
return null;
}
Acutally, when I try to execute for instance ls -aF is works fine and I get some output as a result. Therefore I'm pretty sure that the above code is, in principal, correct. However, I got another program I'd like to run and that produces a file as an output. I would like to execute it the above way but it never is executed and no output file is generated. Also I do not get any error, warnings or whatsoever in java. When copy and pasting the actual command argument string into the console the execution of the programm/command directly in the shell works fine and the output file is generated. So the command I pass to the method is also correct.
Are there additional things I need to pay attention to when trying to execute a shell command from within java?
UPDATE: I modified my code according to the suggestions. However, it is still hanging:
public String executeShellCommand(List<String> command, String logfile, boolean waitForProcess) { try {
ProcessBuilder pb = new ProcessBuilder(command);
System.out.println("pb.toString() = " + pb.toString());
Process p = pb.start();
System.out.println("2");
BufferedReader err = new BufferedReader(new InputStreamReader(p.getErrorStream()));
BufferedReader out = new BufferedReader(new InputStreamReader(p.getInputStream()));
System.out.println("3");
StringBuilder errSb = new StringBuilder();
StringBuilder outSb = new StringBuilder();
String line;
System.out.println("4");
while ((line = err.readLine()) != null) { // <--- code hangs here
errSb.append(line + "\n");
System.out.println("errSb = " + errSb.toString());
}
System.out.println("4a");
while ((line = out.readLine()) != null) {
outSb.append(line + "\n");
System.out.println("outSb = " + outSb.toString());
}
System.out.println("5");
if(waitForProcess) {
System.out.println("Wait for process");
p.waitFor();
} else {
System.out.println("Sleep 5000");
Thread.sleep(5000);
}
System.out.println("6");
//Log result to file
if(logfile != null) {
OutputStreamWriter outWriter = new OutputStreamWriter(new FileOutputStream(logfile));
outWriter.write(errSb.toString());
outWriter.close();
}
return errSb.toString();
} catch(Exception e) { e.printStackTrace(); } return null; }

This will block if your command writes too many characters to stderr. Like for sdtout, Java redirect stderr through a pipe, and if you do not read the pipe, it can fill up and block (size of the pipe is probably less than 256 bytes). To avoid that, you need to read from the Process.getErrorStream(), preferable from another thread as the main thread is busy reading from the Process.getInputStream().
A simpler way to avoid that is to use the ProcessBuilder class instead of Runtime.exec() and ProcessBuilder.redirectErrorStream(true) so that both stdout and stderr are merged into the Process.getInputStream()

As per Process javadoc :
Because some native platforms only provide limited buffer size for standard input and output streams, failure to promptly write the input stream or read the output stream of the subprocess may cause the subprocess to block, or even deadlock.
You are calling p.waitFor(). If we carefully read the waitFor() documentation:
Causes the current thread to wait, if necessary, until the process represented by this Process object has terminated.
You are waiting for a process which hangs, because its error stream and output stream are never read.
What you should do, is to read these streams:
p.start();
BufferedReader err= new BufferedReader(new InputStreamReader(p.getErrorStream()));
BufferedReader out = new BufferedReader(new InputStreamReader(p.getOutputStream()));
StringBuilder errSb = new StringBuilder();
StringBuilder outSb = new Stringbuilder();
String line;
while ((line = err.readLine()) != null) {
errSb.append(line);
}
while ((line = out.readLine()) != null) {
outSB.append(line);
}
int retCode = p.waitFor(); //0 for success
System.out.println(retCode);
System.err.println(errSB.toString());
You should always read the error stream when calling external programs via the Process class, else you may find yourself in this odd situation where a process hangs forever. (well until someone else -the operating system, another application, etc- kills it, more exactly).
I've also noticed that you use the Runtime.getRuntime() which is not the recommended way to run external programs, starting with java 1.5, as per javadoc:
As of 1.5, ProcessBuilder.start() is the preferred way to create a Process.
ProcessBuilder pb = new ProcessBuilder("ls" , "-aF");
Process p = pb.start();

Java function to toggle socket state via raspberry pi

I have a function in java which is being executet on my raspberry pi and should send a signal to toggle the targeted sockets state to on / off.
Thats my current function:
public static void rcswitch(int housecode,int unitcode, int onoff) throws InterruptedException, IOException {
String housestring = Integer.toString(housecode);
String unitstring = Integer.toString(unitcode);
String onoffstring = Integer.toString(onoff);
ProcessBuilder builder = new ProcessBuilder("/bin/bash", "-c", "sudo /home/pi/rcswitch-pi/send", housestring, unitstring, onoffstring);
Process proc = builder.start();
BufferedReader reader =
new BufferedReader(new InputStreamReader(proc.getInputStream()));
String line = "";
while((line = reader.readLine()) != null) {
System.out.print(line + "\n");
}
}
However, it doesn't seem like the terminal is receiving the command as it does not output anything. It should show something like "command received" and execute it then. When I normally execute the /send command in the terminal it works just fine. In eclipse it just works fine and throws the expected error.
Thanks for your answers :)

It is most likely that an error has occured while executing the command. Keep in mind that Process#getInputStream() does not include standard error stream of the process. You should use Process#getErrorStream(). Something like:
BufferedReader reader = new BufferedReader(new InputStreamReader(proc.getErrorStream()));
String line = null;
while((line = reader.readLine()) != null) {
System.out.print(line + "\n");
}

How to Execute Windows Commands Using Java - Change Network Settings

In Java, I want to be able to execute a Windows command.
The command in question is netsh. This will enable me to set/reset my IP address.
Note that I do not want to execute a batch file.
Instead of using a batch file, I want to execute such commands directly. Is this possible?
Here is my implemented Solution for Future Reference:
public class JavaRunCommand {
private static final String CMD =
"netsh int ip set address name = \"Local Area Connection\" source = static addr = 192.168.222.3 mask = 255.255.255.0";
public static void main(String args[]) {
try {
// Run "netsh" Windows command
Process process = Runtime.getRuntime().exec(CMD);
// Get input streams
BufferedReader stdInput = new BufferedReader(new InputStreamReader(process.getInputStream()));
BufferedReader stdError = new BufferedReader(new InputStreamReader(process.getErrorStream()));
// Read command standard output
String s;
System.out.println("Standard output: ");
while ((s = stdInput.readLine()) != null) {
System.out.println(s);
}
// Read command errors
System.out.println("Standard error: ");
while ((s = stdError.readLine()) != null) {
System.out.println(s);
}
} catch (Exception e) {
e.printStackTrace(System.err);
}
}
}

Runtime.getRuntime().exec("netsh");
See Runtime Javadoc.
EDIT: A later answer by leet suggests that this process is now deprecated. However, as per the comment by DJViking, this appears not to be the case: Java 8 documentation. The method is not deprecated.

Use ProcessBuilder
ProcessBuilder pb=new ProcessBuilder(command);
pb.redirectErrorStream(true);
Process process=pb.start();
BufferedReader inStreamReader = new BufferedReader(
new InputStreamReader(process.getInputStream()));
while(inStreamReader.readLine() != null){
//do something with commandline output.
}

You can run the command with Runtime.getRuntime().exec("<command>") (eg. Runtime.getRuntime().exec("tree")). But, this will only run executables found in path, not commands like echo, del, ... But only stuff like tree.com, netstat.com, ... To run regular commands, you will have to put cmd /c before the command (eg Runtime.getRuntime().exec("cmd /c echo echo"))

public static void main(String[] args) {
String command="netstat";
try {
Process process = Runtime.getRuntime().exec(command);
System.out.println("the output stream is "+process.getOutputStream());
BufferedReader reader=new BufferedReader( new InputStreamReader(process.getInputStream()));
String s;
while ((s = reader.readLine()) != null){
System.out.println("The inout stream is " + s);
}
} catch (IOException e) {
e.printStackTrace();
}
}
This works.

Runtime#exec().