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calling system commands within Java

Posted on 2011-03-16
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Last Modified: 2012-06-22
Hi,
Can we directly call a system command within java code?

Here is an example:

grep '^(SdD)' args[2]; 

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args[2] is the path to a text file.

Thanks,

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Question by:Tolgar
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21 Comments
 
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Expert Comment

by:for_yan
ID: 35151445
Yes we can - you should use

Runtime.getRuntime().exec("command arguments");
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Expert Comment

by:for_yan
ID: 35151470
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Expert Comment

by:for_yan
ID: 35151480

And this is how you read the ouput from the second example (importnat part of the process):
http://www.exampledepot.com/egs/java.lang/ReadFromCommand.html
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Expert Comment

by:CEHJ
ID: 35151504
You'd be better off calling a shell script. But see my comments here http:Q__26889988.html
Process p = Runtime.getRuntime().exec(new String[] { "sh mygrep.sh", args[2] });

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Author Comment

by:Tolgar
ID: 35151736
So as a result is this correct?

java.lang.String myFiles = Runtime.getRuntime().exec(grep '^(SdD)' args[3]);

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Thanks,
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Expert Comment

by:CEHJ
ID: 35151787
Sorry that should have been
Process p = Runtime.getRuntime().exec(new String[] { "sh", "mygrep.sh", args[2] });

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Expert Comment

by:CEHJ
ID: 35151813
With a shell script consisting of

grep '^(SdD)' "${@}"

I've just tested the following code and it works fine.

You can grep in files in pure Java too
public class GrepTest {
    public static void main(String[] args) throws Exception {
	Process p = Runtime.getRuntime().exec(new String[] { "sh",  "mygrep.sh", args[2] });

	StringBuilder stderr = new StringBuilder();
	StringBuilder stdout = new StringBuilder();
	net.proteanit.io.IOUtils.collectProcessStreams(p, stderr, stdout);
	System.out.println(stdout);
    }
}

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Author Comment

by:Tolgar
ID: 35151893
So is this gonna work?

java.lang.String myFiles = Runtime.getRuntime().exec(grep '^(SdD)' args[3]);

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Thanks,
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Expert Comment

by:for_yan
ID: 35152037
No, one thing
you need to havwe arguments in quotes:

Process p = Runtime.getRuntime().exec("grep '^(SdD)'  " + args[3]);
and then you need to collect the output as CEHJ showed:

      StringBuilder stderr = new StringBuilder();
      StringBuilder stdout = new StringBuilder();
      net.proteanit.io.IOUtils.collectProcessStreams(p, stderr, stdout);
      System.out.println(stdout);

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Expert Comment

by:CEHJ
ID: 35152045
Let's take a closer look there: i don't know what version of grep you're using, but the effect of that command with the greps i know would effectively either return args[3] or nothing. What is your intention there?
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Author Comment

by:Tolgar
ID: 35152134
So args[3] is a path to a file. What I would like to do is:

grep the lines in this file that starts with s or d or D.

Thanks,
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Expert Comment

by:CEHJ
ID: 35152151
That would be
grep '^[sdD]' foo

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Expert Comment

by:for_yan
ID: 35152176
Maybe it is still easier to do it in Java, rather than going to the system ?
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Author Comment

by:Tolgar
ID: 35152251
You may be right. So how can I make it in Java only without going to the system?

Thanks,
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Expert Comment

by:CEHJ
ID: 35152263
Just read into a List and add if

s.matches("^[sDd].*");
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Expert Comment

by:for_yan
ID: 35152272


DataInputStream in = new DataInputStream(new FileInputStream(args[3]))
String buff = null;
while((buff=in.readLine() )!= null){
buff = in.readLine();
if(buff.startsWith("s") || buff.startsWith("S") || buff.startsWith("D")){
System.out.println(buff);
}
in.close();
}
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Accepted Solution

by:
for_yan earned 200 total points
ID: 35152294

Or you can collect it to the String like that:

String s = "";
try{
DataInputStream in = new DataInputStream(new FileInputStream(args[3]))
String buff = null;
while((buff=in.readLine() )!= null){
buff = in.readLine();
if(buff.startsWith("s") || buff.startsWith("S") || buff.startsWith("D")){
System.out.println(buff);
s += buff + System.getProperty("line.separator");
}

}
in.close();
} catch(Exception ex){
ex.printStackTrace();
}
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Expert Comment

by:for_yan
ID: 35152303
I guess in the first case I missed one closing brace;
in the second case should be OK
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Assisted Solution

by:CEHJ
CEHJ earned 50 total points
ID: 35152321
Try the following (the superclass is at http://technojeeves.com/joomla/index.php/free/74-string-list ) and try it out with a file called 'x.txt'
public class SelectiveStringList extends StringList {

    @Override
    public boolean add(String s) {
	boolean result = s.matches("^[sDd].*");
	if (result) {
	    super.add(s);
	}
	return result;
    }
    public static void main(String[] args) {
	SelectiveStringList matches = new SelectiveStringList();
	matches.read("x.txt");
	for(String s :  matches) {
	    System.out.println(s);
	}
    }
}

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Expert Comment

by:for_yan
ID: 35152360
Sorry one readline() too much in my code
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Expert Comment

by:for_yan
ID: 35152365
Just remobve first line inside the while loop
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