Cannot execute a Linux command from Java

Question

I have a Linux command that I cannot execute from Java:

[ ! -d "/tmp/flow" ] && echo "The directory DOES NOT exist."

It works fine in command line but when I use the following code:

    String command = "[ ! -d \"/tmp/flow\" ] && echo \"The directory DOES NOT exist.\"";
    Process proc = Runtime.getRuntime().exec(command);
    Boolean successful = proc.waitFor()==0 && proc.exitValue()==0;
    System.out.println("successful:"+successful);
    BufferedReader stdInput = new BufferedReader(new InputStreamReader(proc.getInputStream()));
    BufferedReader stdError = new BufferedReader(new InputStreamReader(proc.getErrorStream()));

    String line = null;
    System.out.println("************** input");
    while ((line = stdInput.readLine()) != null) {
        System.out.println(line);
    }
    System.out.println("************** error");
    while ((line = stdError.readLine()) != null) {
        System.out.println(line);
    }

it says that

successful:false
************** input
************** error
[: missing ']'

Any idea? Is it possible that these IF bash operators cannot be called from Java...?

Thanks!

**************** UDATE ****************

vikhor@adnlt653-vm1:~> which '['
[ is a shell builtin
[ is /usr/bin/[
[ is /bin/[

are you in same box where you are trying to execute mention command. if out side try to use jsch or any other libaray — Sandeep Pandey
– Sandeep Pandey, Commented Feb 6, 2018 at 10:47
I dont think there is an executable named "[" on Linux which does what you want. And do you really want to check for the presence of some directory using an external command? — Gyro Gearless
– Gyro Gearless, Commented Feb 6, 2018 at 10:50
Hi @GyroGearless, basically it is an IF operator, it tries to check if the folder exists. It works fine in command line. Usually it is used in bash scripts. — Viktor
– Viktor, Commented Feb 6, 2018 at 10:54
Boolean successful = proc.waitFor()==0 && proc.exitValue()==0; there's no need to check exitValue() if you've just checked the return of waitFor() as they will be the same. Print the actual int value instead of creating a Boolean object. — Klitos Kyriacou
– Klitos Kyriacou, Commented Feb 6, 2018 at 11:06

DodgyCodeException · Accepted Answer · 2018-02-06 16:20:28Z

1

The [ command is a shell built-in. The which command shows that it's also an executable file. It is this executable file that gets executed when you pass the command line to exec(). So then the whole command line is passed to the [ command. However, the && operator is a shell operator. The [ executable doesn't like it.

So you need to run the bash executable and pass the command as an argument:

String command = "bash -c '[ ! -d \"/tmp/flow\" ] && echo \"The directory DOES NOT exist.\"'";

edited Feb 6, 2018 at 16:20

answered Feb 6, 2018 at 11:31

DodgyCodeException

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5 Comments

Viktor Over a year ago

Hi, thanks, you must be right! However I cannot prove it as I get the following error when executing the command above: !: -c: line 0: unexpected EOF while looking for matching `'' !: -c: line 1: syntax error: unexpected end of file

DodgyCodeException Over a year ago

Your error message says "unexpected EOF while looking for matching `". That's a backquote character. Make sure you're using a straight quote.

Viktor Over a year ago

Well, the strange thing is that there is no backquote in the command but only aposthrophe (I copied the command you provided). Anyway your answer is the correct one so I will accept it.

Viktor Over a year ago

eventually the bash -c was the solution but your answer was the key, thanks again!

DodgyCodeException Over a year ago

Glad it helped. And thanks for pointing out what the correct command should have been. I've edited the answer for the benefit of any future readers.

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