Develop Reference

How can I write to a file in linux using java code which runs on my windows machine?

My application server is hosted in Linux having Tomcat Server. I want to change some files through Java code that is running on my windows machine. How can I do it? I know how to connect to Linux through Java, but don't know about the command that used to write, append or clear the files.
Many thanks!

You can do that with external library JSch.
The below should do the job.
JSch jsch = new JSch();
Session session = jsch.getSession("remote_user_name", "remote_host_or_ip", 22); // 22 for SFTP
session.setPassword("remote_password");
java.util.Properties config = new java.util.Properties();
config.put("StrictHostKeyChecking", "no");
session.setConfig(config);
session.connect(10000);
Channel channel = session.openChannel("sftp");
channel.connect();
System.out.println("Connection Opened");
ChannelSftp channelSftp = (ChannelSftp) channel;
InputStream inputStream = new FileInputStream("text_file.txt");
channelSftp.put(inputStream, "/remote/folder/file_to_be_rewritten.txt");
System.out.println("File should be uploaded");
channelSftp.disconnect();
session.disconnect();

Your server should provide a REST API to allow the file to be modified via a HTTP request. This way, you can manage all updates to the file, and prevent the file being corrupted by attempts to make multiple concurrent updates, using synchronized blocks, locks or actors.
However, you should also consider storing the contents of the file in a database (SQL or NoSQL) instead of a file. This would handle concurrency control in an easier-to-manage way, especially if the update was atomic (one row or one document).

If you are trying to perform file operations using Java, check out this tutorial and documentation on reading, writing, creating, and opening files.
Here is sample code that reads from a file, and writes to a file.
import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.IOException;
import java.nio.charset.Charset;
import java.nio.file.FileSystems;
import java.nio.file.Files;
public class FileOps {
public static void main(String[] args) {
readFile();
writeFile();
}
private static void readFile() {
Charset charset = Charset.forName("US-ASCII");
try (BufferedReader reader = Files.newBufferedReader(FileSystems.getDefault().getPath("/path/on/disk/file1.txt"), charset)) {
String line = null;
while ((line = reader.readLine()) != null) {
System.out.println(line);
}
} catch (IOException x) {
System.err.format("IOException: %s%n", x);
}
}
private static void writeFile() {
Charset charset = Charset.forName("US-ASCII");
String s = "Sample Java Code";
try (BufferedWriter writer = Files.newBufferedWriter(FileSystems.getDefault().getPath("/path/on/disk/file2.txt"), charset)) {
writer.write(s, 0, s.length());
} catch (IOException x) {
System.err.format("IOException: %s%n", x);
}
}
}

Shell commands are not running using jsch and expcetit library in java

I am following below steps for running commands on ssh server.
Connecting to ssh server
Using devpush command again logging to server (using expectit library for user input prompt).
Finally running remote commands using jsch library.
The issue I am facing is that, my code is going in infinite loop it is able to login to ssh server but not able to run the commands.
Can any one please help me on this as I am using both the libraries for first time.
package com.dmotorworks.cdk;
import java.io.BufferedReader;
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import net.sf.expectit.*;
import net.sf.expectit.matcher.Matchers;
import com.jcraft.jsch.Channel;
import com.jcraft.jsch.JSch;
import com.jcraft.jsch.JSchException;
import com.jcraft.jsch.Session;
public class ShellClient {
public void loginToPabloServer() throws IOException, JSchException{
String hostname="pablo.dmotorworks.com";
String username="gaikwasu";
String password="Nexus#80900";
final String endLineStr=" # ";
JSch jsch = new JSch();
Session session = jsch.getSession(username, hostname, 22);
session.setPassword(password);
jsch.setKnownHosts("C://Users//gaikwasu//.ssh//known_hosts");
session.connect();
System.out.println("Connected");
Channel channel=session.openChannel("shell");
channel.connect();
Expect expect=new ExpectBuilder()
.withOutput(channel.getOutputStream())
.withInputs(channel.getInputStream(), channel.getExtInputStream())
.withEchoOutput(System.out)
.withEchoInput(System.err)
.withExceptionOnFailure()
.build();
expect.expect(Matchers.contains("-bash-4.1$"));
expect.send("devpush\n");
expect.expect(Matchers.contains("[sudo] password for"));
expect.send(password+"\n");
DataInputStream dataIn = new DataInputStream(channel.getInputStream());
DataOutputStream dataOut = new DataOutputStream(channel.getOutputStream());
BufferedReader bufferReader= new BufferedReader(new InputStreamReader(dataIn));
dataOut.writeBytes("cd custom\n");
dataOut.writeBytes("ls -lrt\n");
dataOut.flush();
String line=bufferReader.readLine();
while(!line.endsWith(endLineStr)) {
System.out.println(line);
}
channel.disconnect();
session.disconnect();
expect.close();
}
public static void main(String[] args) {
ShellClient shellClient=new ShellClient();
try {
shellClient.loginToPabloServer();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (JSchException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}

Most probably the cause of the infinitive loop is because the Expect instance reads the input data in a separate thread at the same time while you are trying to same data in the main thread.
Try to close the Expect instance before reading the data. It may help.
However, I strongly recommend to keep using the Expect instance after the sending password. ExpectIt can help with extracting the information from the remote command output. For example:
expect.sendLine(password);
expect.expect(Matchers.contains("-bash-4.1$"));
expect.sendLine("cd custom");
expect.expect(Matchers.contains("-bash-4.1$"));
expect.sendLine("ls -lrt");
// the lsResult variable should contain the result of the 'ls -l' command
String lsResult = expect.expect(Matchers.contains("-bash-4.1$")).getBefore();
System.out.println(lsResult);
Please take a look at this example.

Connection refused in sftp server

I am trying to use localhost as SFTP server. I'm using Jsch library of Java to implement SFTP with SSH2. The following code upload a text file to a directory on local machine with sftp.
But I cannot connect to localhost.
import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.OutputStream;
import com.jcraft.jsch.Channel;
import com.jcraft.jsch.ChannelSftp;
import com.jcraft.jsch.JSch;
import com.jcraft.jsch.Session;
public class Server{
public Server() {
}
public static void main(String[] args) {
String SFTPHOST = "localhost";
int SFTPPORT = 22;
String SFTPUSER = "root";
String SFTPPASS ="";
String SFTPWORKINGDIR = "D:\\Upload";
Session session = null;
Channel channel = null;
ChannelSftp channelSftp = null;
try{
JSch jsch = new JSch();
session = jsch.getSession(SFTPUSER,SFTPHOST,SFTPPORT);
session.setPassword(SFTPPASS);
java.util.Properties config = new java.util.Properties();
config.put("StrictHostKeyChecking", "no");
session.setConfig(config);
session.connect();
channel = session.openChannel("sftp");
channel.connect();
channelSftp = (ChannelSftp)channel;
channelSftp.cd(SFTPWORKINGDIR);
File f = new File("trial.txt");
channelSftp.put(new FileInputStream(f), f.getName());
}catch(Exception ex){
ex.printStackTrace();
}
}
}

"Connection refused" means that there is no process listening for connections at the IP address and port which you're trying to connect to. In this case, the most likely explanation is that your SSH server process isn't running, or perhaps it has been configured to listen on some other port than port 22.
It looks like this is a Windows system? If that's the case, you could check the task manager to see if there is a copy of the sshd program running. To see if something is listening on port 22, open a command-line window and run netstat -an. Then look for a line like this:
TCP 0.0.0.0:22 0.0.0.0:0 LISTENING
^^-- Listening on port 22
If sshd isn't running, it needs to be started. If it is running, but nothing is listening on port 22, then you should examine sshd's configuration file.
If you need further help starting sshd, you should ask on https://superuser.com/.

multiple shell script execution using jsch

I am using Jsch library to execute about 1000 different of shell scripts in few linux machines and update the status into a table.
I used jsch exec channel [ChannelExec], it was working only for the single script, if the shell script calls another script, ChannelExec was not giving correct result.
now I am using shell channnel of jsch. it is working well to get output from any kind of shell script.
the problem is, if I execute many shell scripts at once , I am getting all result in one bulk.
there is no way to get one Shell script executed and its result received.
if i want to get individual scripts execution result, I need to login to the machine for each script execution , this is taking very long time.
can somebody post a solution, suggestion on how to go about , login into machine once and execute multiple scripts and receive each script result individually.
import com.jcraft.jsch.Channel;
import com.jcraft.jsch.JSch;
import com.jcraft.jsch.JSchException;
import com.jcraft.jsch.Session;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.io.PrintStream;
public class JschShellExample {
public static void main(String[] args) {
try {
JSch jsch = new JSch();
Session session = jsch.getSession("user", "10.32.248.158", 22);
session.setPassword("password");
java.util.Properties config = new java.util.Properties();
config.put("StrictHostKeyChecking", "no");
config.put("PreferredAuthentications","publickey,keyboard-interactive,password");
session.setConfig(config);
session.connect(100);
Channel channel = session.openChannel("shell");
OutputStream inputstream_for_the_channel = channel.getOutputStream();
PrintStream commander = new PrintStream(inputstream_for_the_channel, true);
channel.setOutputStream(null);
channel.connect(100);
//shell script
commander.println("cd /user/home/work ; ./checkstaus.sh ; exit");
commander.flush();
System.out.println(channel.getExitStatus());
InputStream outputstream_from_the_channel = channel.getInputStream();
BufferedReader br = new BufferedReader(new InputStreamReader(outputstream_from_the_channel));
String line = null;
StringBuilder sb = new StringBuilder();
boolean isloginStringPassed = false ;
while ((line = br.readLine()) != null) {
sb.append(line.trim());
}
System.out.println("Result ="+sb.toString());
channel.disconnect();
session.disconnect();
System.out.println("completed .. ");
} catch (JSchException ex) {
ex.printStackTrace();
} catch (IOException ex) {
ex.printStackTrace();
} catch (Exception ex) {
ex.printStackTrace();
}
}
}

Usually when you connect to a remote machine for the first time and after executing a command you'll get a shell command line prompt printed to the output stream. You can use it as a marker between output of different shell commands.
You may consider using an Expect-like third party library which simplify working with a remote services and capturing output. There a good set of options you can try:
Expect4J
ExpectJ
Expect-for-Java
However, when I was about to solve similar problem I found these libraries are rather old and hard to use in a commercial software. So I created my own and made it available for others. It it called ExpectIt. The advantages of my library it are stated on the project home page.
Here is an example of interacting with a public remote SSH service:
JSch jSch = new JSch();
Session session = jSch.getSession("new", "sdf.org");
Properties config = new Properties();
config.put("StrictHostKeyChecking", "no");
session.setConfig(config);
session.connect();
Channel channel = session.openChannel("shell");
Expect expect = new ExpectBuilder()
.withOutput(channel.getOutputStream())
.withInputs(channel.getInputStream(), channel.getExtInputStream())
.withEchoOutput(adapt(System.out))
// .withInputFilters(removeColors(), removeNonPrintable())
.withErrorOnTimeout(true)
.build();
// try-with-resources is omitted for simplicity
channel.connect();
expect.expect(contains("[RETURN]"));
expect.sendLine();
String ipAddress = expect.expect(regexp("Trying (.*)\\.\\.\\.")).group(1);
System.out.println("Captured IP: " + ipAddress);
expect.expect(contains("login:"));
expect.sendLine("new");
expect.expect(contains("(Y/N)"));
expect.send("N");
expect.expect(regexp(": $"));
// finally is omitted
channel.disconnect();
session.disconnect();
expect.close();
You can also take a look a at this example interacting with a Karaf shell which captures individual command output.

Retrieving data from an SFTP server using JSch

I am using JSch for retrieving a file from a remote machine by SFTP. Here is the code
public class TestSFTPinJava {
public static void main(String args[]) {
JSch jsch = new JSch();
Session session = null;
try {
session = jsch.getSession("username", "sftp.abc.com", 22);
session.setConfig("StrictHostKeyChecking", "no");
session.setPassword("password");
session.connect();
Channel channel = session.openChannel("sftp");
channel.connect();
ChannelSftp sftpChannel = (ChannelSftp) channel;
System.out.println("Directory:" + sftpChannel.pwd());
sftpChannel.cd("remoteDirectory/");
System.out.println("Directory after cd:" + sftpChannel.pwd());
sftpChannel.get("remote-data.txt");
sftpChannel.put("C:\\Users\\mona\\Documents\\local-copy.txt");
sftpChannel.exit();
session.disconnect();
} catch (JSchException e) {
e.printStackTrace();
} catch (SftpException e) {
e.printStackTrace();
}
}
}
Now, I have two questions:
sftpChannel.get("remote-data.txt"); throws an exception:
no such file
at com.jcraft.jsch.ChannelSftp.throwStatusError(ChannelSftp.java:2297)
at com.jcraft.jsch.ChannelSftp._stat(ChannelSftp.java:1750)
at com.jcraft.jsch.ChannelSftp.get(ChannelSftp.java:1020)
at com.jcraft.jsch.ChannelSftp.get(ChannelSftp.java:995)
at TestSFTPinJava.main(TestSFTPinJava.java:29)
I am not sure how to specify the location in my local system where the file will be saved.
sftpChannel.put("C:\\Users\\mona\\Documents\\localCopy.txt"); does not look right to me.
Please help with suggestions, Thanks!

Concerning your point 1, I suspect that the default directory after connecting is not what you expect. Try using an absolute remote path. Does sftpChannel.pwd() return the directory the file remote-data.txt is in on the remote machine ?
Concerning your point 2, looking at http://grepcode.com/file/repo1.maven.org/maven2/com.jcraft/jsch/0.1.42/com/jcraft/jsch/ChannelSftp.java#290 one sees that there is the following method in ChannelSftp:
public void put(String src, String dst)
which indeed has a source and destination file name argument.
I guess you had already a look the Jsch sftp example at http://www.jcraft.com/jsch/examples/Sftp.java ?

Simple example of app.
I get file from remote server (from /tmp/qtmp) and save it in local machine in the current path
package connector;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import com.jcraft.jsch.ChannelSftp;
import com.jcraft.jsch.JSch;
import com.jcraft.jsch.JSchException;
import com.jcraft.jsch.Session;
import com.jcraft.jsch.SftpException;
public class Fetcher {
public void fetchFile(String username, String host, String passwd) throws JSchException, SftpException, IOException {
JSch conn = new JSch();
Session session = null;
session = conn.getSession(username, host, 22);
session.setPassword(passwd);
session.setConfig("StrictHostKeyChecking", "no");
session.connect();
ChannelSftp channel = (ChannelSftp)session.openChannel("sftp");
channel.connect();
//change folder on the remote server
channel.cd("/tmp/qtmp");
InputStream in = channel.get("testScp");
// set local file
String lf = "OBJECT_FILE";
FileOutputStream tergetFile = new FileOutputStream(lf);
// read containts of remote file to local
int c;
while ( (c= in.read()) != -1 ) {
tergetFile.write(c);
}
in.close();
tergetFile.close();
channel.disconnect();
session.disconnect();
}
}

i had a similar issue, i was trying to get some files from a server where everything was fine, but i was getting always this error:
sftpChannel.get("fil1.txt","file1.txt")
error message:
2: No such file
at com.jcraft.jsch.ChannelSftp.throwStatusError(ChannelSftp.java:2846)
at com.jcraft.jsch.ChannelSftp._stat(ChannelSftp.java:2198)
at com.jcraft.jsch.ChannelSftp._stat(ChannelSftp.java:2215)
at com.jcraft.jsch.ChannelSftp.get(ChannelSftp.java:913)
at com.jcraft.jsch.ChannelSftp.get(ChannelSftp.java:873)
...
I was listing properly all the elements of the directory by using
java.util.Vector v1 = sftpChannel.ls(dir);
I think that was that confused me, that i was able to read the content of the directory by using the ls command, when you want to get / put files make sure you move first by using "cd".
The solution was to use the next command to move to the directory that contains my files using a simple cd command:
sftpChannel.cd(dir);
Hope this helps, i took my sometime to figure it out. jojo.
Lessons learned:
1.- ls can read any directory no matter if you are not inside of it.
2.- To get and put files always make sure you are in the directory that contains the files by using cd.