I am trying create a basic file system to imitate the terminal. I am currently stuck on getting the names after the command. My first thought was to use regex to parse the commands.
Examples of commands would be:
mkdir hello
ls
cd hello
However to account for many whitespaces an input could be mkdir hello. I was wondering if there is another way without using regex? Also I was curious to which method is faster? With or without regex?
You could try splitting the lines like
String[] tokens = line.split(" ");
And for basic commands, most likely your command will be at tokens[0] followed by arguments.
for(String current: line.split("\\s+"){
//do something.
}
Usually, regex is faster because you can compile it.
see java.util.regex - importance of Pattern.compile()?
(Internally, I think the JVM always compile the regex at some point, but if you do it explicitly, you can reuse it. I am not sure if the JVM is smart enough to reuse a compiled regex locally, maybe it is)
Related
I want some code in my program to run only if the user has input the character '*' at the command-line as a command-line argument. This is the code I've used:-
//myfile.java
import java.io.*;
public class myfile {
public static void main(String[] args) {
if(args[0].equals("*")){
//do stuff
System.out.println(args[0]);//added this line to see what exactly was being passed
}
}
}
When this program is executed at the command-line by entering:-
java myfile *
the output I'm expecting to see on the screen is the asterisk character, instead the output displayed is 'myfile.class'. Where am I going wrong? Why does Java change the asterisk to the .class file?
Also, note that the program worked perfectly the first four times I executed it and then started doing this!
Thanks in advance for your help.
Where am I going wrong?
The star character needs to be quoted or escaped. Run your java program like this:
java myfile "*"
or
java myfile \*
Why does Java change the asterisk to the .class file?
It doesn't. It is your shell that is doing it. It is shell file expansion ... or "globbing" as it is also called.
Run "ls *" or "echo *" and you will see that the same thing happens.
The command terminal already replaces the asterisk and java already gets the value that you see. I'd use any other character, that has no special meaning to the command terminal or otherwise you must escape the asterisk in your command.
Actually escaping arguments on Windows and especially in cmd.exe is non-trivial. This nice article explains it in detail: Everyone quotes command line arguments the wrong way :
the takaway for your case is: surround the asterisk with quotes.
Answer to your question in the comment:
Using the escape character worked! But I still don't get why it worked without the escape character the first few times
I am not sure, but maybe you run into this behavior: It makes a difference if the pattern can be expanded or not. For example, when I pass Test* as argument, then there are 2 cases to consider:
in the current folder there is a file called Test1.txt: then your java program will get Test1.txt as argument
when there are no matching files, your program will get Test* as argument
However, I am not sure, how this would apply to your case, since you only pass *: that should only work in an empty directory.
I need to use Java to rsync several files using one command
the following command works fine in shell
rsync -avrz --timeout=100 rsync://10.149.21.211:8730/'logflow/click/file1 logflow/click/file2' /home/kerrycai/puller"
but when i use the following Java code , it does not work
String cmd = "rsync -avrz --timeout=100 rsync://10.149.21.211:8730/'logflow/click/file1 logflow/click/file2' /home/kerrycai/puller";
Process p = Runtime.getRuntime().exec(cmd);
int ret = p.waitFor();
the ret value is not equal to 0 (5 in my test), and the command is not executed succeed, after some debugging , it seem the problem is caused by the single quote
So, my questions is
Can I using java to execute a shell command which has single quote in it (Pls note, the single quote is in the middle of a parameter, not start/end) ?
Can I have a shell command to rsync several files in one command , and the command does not have single(double) quotes in it ?
Note to #Chris: this combination of multiple filenames (really modified-partly-like-filenames) in one argument is indeed very unusual and even 'suspicious' for Unix in general, but is (or at least was) correct for rsync in particular.
Preface: Java Runtime.exec does NOT 'execute a shell command' (unless you explicitly run a shell and give it a command); it runs a program, with arguments. These different things are often confused because most of the shell commands used by normal users are commands to run programs, but this is one case where the difference matters. In particular quoting a space to shell causes the shell to pass a single argument to the program containing the space instead of splitting into two (or more) arguments, but the quote itself is NOT included in the argument.
First you should look at the man page (on your system or online at https://download.samba.org/pub/rsync/rsync.html) under ADVANCED USAGE. Current (and IME even moderately old) versions of rsync have a more convenient syntax with separate arguments to get multiple files, which the simple parsing used by Runtime.exec(String) can handle like this:
rsync -avrz --timeout=100 --port=8730 10.149.21.211::logflow/click/file1 ::logflow/click/file2 /home/kerrycai/puller
But if you need (or really want) to use the quoted-space form then you need to do the tokenization yourself and use the String[] overload as suggested by #EJP -- although you can still use Runtime, you don't need ProcessBuilder for this. Specifically do something along the lines of:
String[] cmdarray = {"rsync",
"-avrz",
"--timeout=100",
"rsync://10.149.21.211:8730/logflow/click/file1 logfile/click/file2",
// separated argument contains space but not single (or other) quote
"/home/kerrycai/puller" };
... Runtime.getRuntime.exec(cmdarray); ...
You're calling a somewhat large command from Java. Why not just use a shell script? Put your gnarly command in myScript.sh and then have Java invoke /bin/bash myScript.sh. Makes all the weirdness to do with string handling in Java go away.
To capture the output of process in Groovy I use the following:
"command".execute().text
I want to do the same in Java, but all responses I found contain a lot of boilerplate code involving loops, BufferedReader, Scanner, etc.:
Get Command Prompt Output to String In Java
How to run Windows commands in JAVA and return the result text as a string
java runtime.getruntime() getting output from executing a command line program
Can I do the same thing within 1-2 lines of code? Maybe Guava or Apache have something to make life simpler?
I've managed to come up with the following two-liner:
Process process = Runtime.getRuntime().exec("command")'
String output = IOUtils.toString(process.getInputStream());
I run egrep using Java Runtime.exec()
String command = "egrep \'(Success|Loading\\.\\.\\.|Loaded : READY|Found a running instance)\' "+ instance.getPath() + "/log";
Runtime.getRuntime().exec(command);
The stdout is always null and stderr shows "egrep: Unmatched ( or (". but when I copy the command to shell and run, it returns the correct value.
The solution is pretty simple: (Success|Loading\\.\\.\\.|Loaded is not a valid regex.
You can't protect white space with quotes when using Process.exec(String). Always use the versions of exec() that take an array or, even better, use ProcessBuilder.
That way, you can pass each argument as a single Java String and spaces and other special characters won't create any problems.
The single quotes should not be escaped. You don't escape them on the command line, either, do you?
I've been working on a small Java problem set and have come across some trouble. I'm not very experienced writing regular expressions and could really use two for verifying line entries in /etc/group and /etc/passwd in Java.
I found Regex Verification of Line in /etc/passwd earlier and have yet to test it, but it looks adaptable for what I need. Could anyone else help in providing a regex string for either file?
I'm looking to verify user-entered passwd and group lines, in java, before writing them out to disk. If not, I'll likely end up tokenizing each piece and running various expensive operations.
Rather than writing a regex you should probably just read the files with Scanner and parse each line with String.split(":"). Then you can check that each part is valid without dealing with a complex expression to handle all cases. It'll probably be easier to write the code and easier to read it later.
Why do you want to use regular expressions? Just split the line on the colons and inspect the pieces.