Java Runtime.exec fail with space in linux

Question

I searched a lot but did not find the solution. My goal is using java to call commands and get output in windows and linux. I found Runtime.exec method and did some experiments. Everything went ok except when there's space in the command parameters. Test code as below, also in github.
The code works well on windows, but in linux, output is empty:

import java.io.BufferedReader;
import java.io.InputStreamReader;

public class Main {
public static void main(String[] args) {
    try {
        Runtime rt = Runtime.getRuntime();
        String[] commandArray;
        if (isWindows()) {
            commandArray = new String[]{"cmd", "/c", "dir", "\"C:\\Program Files\""};
        } else {
            commandArray = new String[]{"ls", "\"/root/a directory with space\""};
        }
        String cmd = String.join(" ",commandArray);
        System.out.println(cmd);

        Process process = rt.exec(commandArray);
        BufferedReader input = new BufferedReader(
                new InputStreamReader(process.getInputStream()));
        String result = "";
        String line = null;
        while ((line = input.readLine()) != null) {
            result += line;
        }
        process.waitFor();
        System.out.println(result);

    } catch (Exception e) {
        System.out.println(e.getMessage());
    }
}

public static boolean isWindows() {
    String OS = System.getProperty("os.name").toLowerCase();
    return (OS.indexOf("win") >= 0);
    }
}

if I execute the printed command in bash directly, then the output is as expected.

[root@localhost javatest]# javac Main.java 
[root@localhost javatest]# java Main
ls "/root/a directory with space"

[root@localhost javatest]# ls "/root/a directory with space"
a.txt  b.txt
[root@localhost javatest]#

Can anyone explain why and give ways to solve?

Andreas · Accepted Answer · 2018-07-04 12:54:14Z

2

There are two versions of exec.

exec(String command)

Here you specify a command in a similar way to how you would do it on the command-line, i.e. you need to quote arguments with spaces.
```
cmd /c dir "C:\Program Files"
```
exec(String[] cmdarray)

Here you specify the arguments separately, so the arguments are given as-is, i.e. without quotes. The exec method will take care of any spaces and quote-characters in the argument, correctly quoting and escaping the argument as needed to execute the command.
```
cmd
/c
dir
C:\Program Files
```

So, remove the extra quotes you added:

if (isWindows()) {
    commandArray = new String[] { "cmd", "/c", "dir", "C:\\Program Files"};
} else {
    commandArray = new String[] { "ls", "/root/a directory with space"};
}

answered Jul 4, 2018 at 12:54

Andreas

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3 Comments

Lei Yang Over a year ago

thanks. can you help me with my real senario? curl -H "user-agent:some version that is required by server that has space". I tried omitting quotes but still failed. why?

Andreas Over a year ago

@LeiYang Don't know. You should ask that as a new question, showing all the relevant information. --- Do note that -H and user-agent:blah blah are 2 arguments, not one.

Lei Yang Over a year ago

I forgot to say it is curl. which maybe you're familiar with.

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