Create a File Using Runtime.Exec Using Echo in Linux

Unable using Runtime.exec() to execute shell command echo in Android Java code

@Adi Tiwari, I've found the cause.
Runtime.getRuntime.exec() doesn't execute a shell command directly, it executes an executable with arguments.
"echo" is a builtin shell command. It is actually a part of the argument of the executable sh with the option -c.
Commands like ls are actual executables.
You can use type echo and type ls command in adb shell to see the difference.

So final code is:

String[] cmdline = { "sh", "-c", "echo $BOOTCLASSPATH" }; 
Runtime.getRuntime().exec(cmdline);

Java Runtime.exec() works for some command but not others

Because of shell parsing.

These are all concepts that the OS just does not have:

The concept of 'every space separates one argument from another (and the command from the list of arguments'). The concept that a single string can possibly run anything, in fact; at the OS level that's just not what the 'run this thing' API looks like. When you type commands on the command prompt, your shell app is 'interpreting' these strings into a command execution request.
The concept of figuring out that bash means /bin/bash, i.e. $PATH resolution.
The concept that *.txt is supposed to refer to all files that ends in .txt.
The concept that $FOO should be replaced with the value of the environment variable 'FOO'
The concept that ; separates 2 commands, and it's supposed to run both.
The concept that single and double quotes escape things. "Escape things" implies that things can cause interpretation to happen. The OS interprets nothing, therefore there's nothing to escape. Obvious, then, that the OS doesn't know what ' is, or ".
That >foo means: Please set the standard output of the spawned process such that it sends it all to file 'foo'.
In windows shells, that @ in front of the command means 'do not echo the command itself'. This and many other things are shellisms: /bin/bash does that, maybe /bin/zsh does something else. Windows's built in shell thing definitely is quite different from bash!

Instead, an OS simply wants you to provide it a full path to an executable along with a list of strings, and pick targets for standard in, standard out, and standard err. It does no processing on any of that, just starts the process you named, and passes it the strings verbatim.

You're sort of half there, as you already figured out that e.g. ls >foo cannot work if you execute it on its own, but it can work if you tell bash to execute it. As ALL of that stuff in the above list? Your shell does that.

It gets more complicated: Turning *.txt into foo.txt bar.txt is a task of bash and friends, e.g. if you attempted to run: ls '*.txt' it does not work. But on windows, it's not the shell's job; the shell just passes it verbatim to dir, and it is the command's job to undo it. What a mess, right? Executing things is hard!

So, what's wrong here? Two things:

Space splitting isn't working out.
Quote application isn't being done.

When you write:

bash -c 'ls >foo'

in a bash shell, bash has to first split this up, into a command, and a list of arguments. Bash does so as follows:

Command: bash
arg1: -c
arg2: ls >foo

It knows that ls >foo isn't 2 arguments because, effectively, "space" is the bash operator for: "... and here is the next argument", and with quotes (either single or double), the space becomes a literal space instead of the '... move to next argument ...' operator.

In your code, you ask bash to run java, and then java to run bash. So, bash first does:

cmd: java
arg1: bash -c 'ls >foo'

With the same logic at work. Your java app then takes that entire string (that's args[0]: "bash -c 'ls >foo'"), and you then feed it to a method you should never use: Runtime.exec(). Always use ProcessBuilder, and always use the list-based form of it.

Given that you're using the bad method, you're now asking java to do this splitting thing. After all, if you just tell the OS verbatim, please run "bash -c 'ls >foo'", the OS dutifully reports: "I'm sorry, but file ./bash -c ;ls >foo' does not exist", because it does not processing at all". This is unwieldy so java's exec() method is a disaster you should never use: Because people are confused about this, it tries to do some extremely basic processing, except every OS and shell is different, java does not know this, so it does a really bad job at it.

Hence, do not use it.

In this case, java doesn't realize that those quotes mean it shouldn't split, so java tells the OS:

Please run:

cmd: /bin/bash (java DOES do path lookup; but you should avoid this, do not use relative path names, you should always write them out in full)
arg1: -c
arg2: 'ls
arg3: >foo'

and now you understand why this is just going completely wrong.

Instead, you want java to tell the OS:

cmd: /bin/bash
arg1: -c
arg2: ls >foo

Note: ls >foo needs to be one argument, and NOTHING in the argument should be containing quotes, anywhere. The reason you write:

/bin/bash -c 'ls >foo'

In bash, is because you [A] want bash not to treat that space between ls and >foo as an arg separator (you want ls >foo to travel to /bin/bash as one argument), and [B] that you want >foo to just be sent to the bash you're spawning and not to be treated as 'please redirect the output to file foo' at the current shell level.

Runtime.exec isn't a shell, so the quotes stuff? Runtime.exec has no idea.

This means more generally your plan of "I shall write an app where the entire argument you pass to it is just run" is oversimplified and can never work unless you write an entire quotes and escaper analyser for it.

An easy way out is to take the input, write it out to a shell script on disk, set the exec flag on it, and always run /bin/bash -c /path/to/script-you-just-wrote, sidestepping any need to attempt to parse anything in java: Let bash do it.

The ONE weird bizarro thing I cannot explain, is that literally passing 'ls' to /bin/bash -c, with quotes intact, DOES work and runs ls as normal, but 'ls *' does not, perhaps because now bash thinks you want executable file /bin/ls * which obviously does not exist (a star cannot be in a file name, or at least, that's not the ls executable, and it's not an alias for the ls built-in). At any rate, you want to pass ls without the quotes.

Let's try it!

import java.util.*;
import java.io.*;

public class runtime {
    public static void main(String args[]) throws Exception{
        ProcessBuilder pb = new ProcessBuilder();
        pb.command("/bin/bash", "-c", "echo 1234");
//        pb.command("/bin/bash", "-c", "'echo 1234'");
        Process p = pb.start();
        OutputStream os = p.getOutputStream();
        InputStream in = p.getInputStream();
        DataInputStream dis = new DataInputStream(in);
        String disr = dis.readLine();
        while ( disr != null ) {
                System.out.println("Out: " + disr);
                disr = dis.readLine();
        }
        int errCode = p.waitFor();
        System.out.println("Process exit code: " + errCode);
    }
}

The above works fine. Replace the .command line with the commented out variant and you'll notice it does not work at all, and you get an error. On my mac I get a '127' error; perhaps this is bash reporting back: I could not find the command you were attempting to execute. 0 is what you're looking for when you invoke waitFor: That's the code for 'no errors'.

Using Java's getRuntime.exec() to Run a Linux Shell Command: How?

The extra quotes around echo ... should be removed:

Process p = Runtime.getRuntime().exec(new String[]{
        "bash", "-c",
        "echo Hello World > ./output"
});

The python version needs extra quotes to tell the underlying system that echo Hello World > ./output is a single argument. The java version explicitly specifies arguments as separate strings, so it doesn't need those quotes.

Also, your version doesn't "run without complaint", you just don't see the complaints, because you don't read the error stream of the created process.

Redirecs with /bin/bash from Java Runtime exec()

The cause is that exec uses simple StringTokenizer with any white space as a delimiter to parse the actual command. Therefore it's portable as it does work nowhere when you pass something complex :-)

The workaround you chose is correct way, portable and most of all safest asyou ddon't need to escape if the command contained for example quotes etc.

How to use linux commands involving through runTime.exec()

Pipes and redirection are features provided by the shell. The easy (and dirty) solution is to spawn the command inside a shell: "/bin/sh -c 'ps -e > /home/root/workspace/MyProject/ProcessList.txt'".

Edit: I had forgotten that the default StringTokenizer does not work with quoted strings. Provide arguments as an array of strings.

String[] args = {
    "/bin/sh",
    "-c",
    "ps -e > /home/root/workspace/MyProject/ProcessList.txt"
};
java.lang.Runtime.getRuntime(args);