Using unix commands in java

[jagan raja]

Hi All,

I am able to retrieve and store file in the unix box using java.(by importing com.jcraft.jsch package).

Now I need to pass unix commands (cat, grep) in java. How can i do that?

I read the forum and used the below code to search for success string in filename.log.

 Process p = Runtime.getRuntime().exec("cat filename.log | grep success");
				InputStream in = p.getInputStream();
				
				int c;
		        while ((c = in.read()) != -1) {
		            
		        }
		         

And I get the following error

Cannot run program "cat": CreateProcess error=2, The system cannot find the file specified
at java.lang.ProcessBuilder.start(Unknown Source)
at java.lang.Runtime.exec(Unknown Source)
at java.lang.Runtime.exec(Unknown Source)
at java.lang.Runtime.exec(Unknown Source)

Can any one help me out?

[Ulf Dittmer]

The working directory is most likely not what you expected it to be - you should use absolute paths.

[Rob Spoor]

Also, "cat filename.log | grep success" is not one process. It's one process ("grep success") that is using the output of another process ("cat filename.log") as its input. This is handled by the shell (BASH, KSH, CSH etc) usually. When doing this in Java you need to do this yourself.

I see some options:
1) copy the contents of the input stream of the first process to the output stream of the second process, then read the input of the second process:

Process p1 = Runtime.getRuntime().exec("cat filename.log");
Process p2 = Runtime.getRuntime().exec("grep success");
InputStream in1 = p1.getInputStream();
OutputputStream out2 = p2.getOutputStream();
InputStream in2 = p2.getInputStream();
// copy everything from in1 to out2 in a different thread
// read from in2

2) replace the catting with a Java input stream:

Process p = Runtime.getRuntime().exec("grep success");
InputStream fileIn = new FileInputStream("filename.log");
OutputputStream out = p2.getOutputStream();
InputStream in = p2.getInputStream();
// copy everything from fileIn to out in a different thread
// read from in
// don't forget to close fileIn when you're done with it!

3) replace it all with Java code:

BufferedReader reader = new BufferedReader(new FileReader("filename.log"));
String line;
while ((line = reader.readLine()) != null)
{
    if (line.contains("success"))
    {
        // handle line
    }
}
reader.close();

If the regex becomes a little more complex you can use java.util.regex.Pattern instead of the contains method.

Personally, I would go for option three. If you don't, read this first.