How to merge one file to another \ Linux

Question

I am trying to append one text file to another by using linux commands from my Java program. I am completely new to Linux. I tried sorting and it works just fine, so I have no idea what I am doing wrong with using 'cat'. Could you please review my code and help me figure out what I am doing wrong.

public static void mergeRecords(String fileName, String overflowFileName)
{
    String command = "cat " + overflowFileName + " >> " + fileName;
    try {
        Process r = Runtime.getRuntime().exec(command);
    } catch (Exception e) {
        throw new RuntimeException(e);
    }
}

Why you are calling a Linux command from java code. Why you do not use tha java classes? — Jens, Sep 11 '16 at 15:13
Because I was specifically asked by my professor to use Linux commands :) @Jens — useruser123, Sep 11 '16 at 15:14
Nothing happens, the files are not being merged together at all. @Jens — useruser123, Sep 11 '16 at 15:14
The content from the overflow file doesn't appear in the "file"? — cohenjo, Sep 11 '16 at 15:16
So file is unchanged? Try to append ` 2>&1` at the end of your command and all errors will be written to file, and you may be able to find the problem — Jens, Sep 11 '16 at 15:16
@cohenjo just added getErrorStream() and this is the error that shows up: cat: >>: No such file or directory — useruser123, Sep 11 '16 at 15:17
Looks like your arguments are empty... Could you print them before... — cohenjo, Sep 11 '16 at 15:18
@Jezor it's not, I printed it out to see if that might be the case. The error is here: cat: >>: No such file or directory — useruser123, Sep 11 '16 at 15:19
@cohenjo they're not empty. I even tried hard coding file names to test it but it still doesn't work — useruser123, Sep 11 '16 at 15:20
@useruser123 i refactored your code a little bit. If you print "command" string, what do you get? — Jezor, Sep 11 '16 at 15:24
@useruser123 wait a minute... are you executing your code on Linux? — Jezor, Sep 11 '16 at 15:25
@useruser123 are you sure your files exist? Your code works fine for me. — Jezor, Sep 11 '16 at 15:43
Possible duplicate of [Want to invoke a linux shell command from Java](http://stackoverflow.com/questions/1410741/want-to-invoke-a-linux-shell-command-from-java) — Jezor, Sep 11 '16 at 15:46

score 4 · Accepted Answer · answered Sep 11 '16 at 15:25

4

Runtime#exec is not a shell.

This is a very common misconception. What you need to do is:

create a Process with the command cat file1 file2,
take the output of that process,
dump that output into a file.

Hint: use a ProcessBuilder, this will make your job much easier.

answered Sep 11 '16 at 15:25

fge

119,121
33
254
329

Or just add [bash -c](http://stackoverflow.com/a/1410778/5922757) (if he's using `bash` of course). – Jezor Sep 11 '16 at 15:46
Yep, this was the problem. As I said I'm completely new to Linux and on an assignment deadline so I didn't really do a lot of research. I used ProcessBuilder and now it works :) Thank you – useruser123 Sep 11 '16 at 16:07

VGR · Answer 2 · 2016-09-11T21:42:31.370

1

As others have pointed out, you should not use external commands to do something Java can easily do:

try (OutputStream existingFile = Files.newOutputStream(
    Paths.get(fileName),
    StandardOpenOption.WRITE,
    StandardOpenOption.APPEND)) {

    Files.copy(Paths.get(overflowFileName), existingFile);
}

edited Sep 11 '16 at 21:42

answered Sep 11 '16 at 16:13

VGR

40,506
4
48
63

Why the use of a `BufferedOutputStream`? To be honest, I have never seen any difference in speed between this and a "classical" `OutputStream`, at least as far as file I/O is concerned... – fge Sep 11 '16 at 18:42
FileOutputStream is analogous to C’s write(), while BufferedOutputStream is analogous to C’s fwrite(), so it’s good practice, usually. But you made me curious, so I looked at the source of the Files class… and it is indeed doing its own buffering for both Files.copy(Path, OutputStream, options) and Files.copy(InputStream, Path, options). Which means, for those two methods at least, BufferedOutputStream and BufferedInputStream are not needed. I’ve removed it. – VGR Sep 11 '16 at 21:42

How to merge one file to another \ Linux

2 Answers2