Cannot empty linux cache via java

Question

ok here is my code:

String[] commands = {"sync && echo 3 > /proc/sys/vm/drop_caches"};

Process pr = Runtime.getRuntime().exec(commands);
pr.waitFor();

And I get:

Exception in thread "main" java.io.IOException: Cannot run program "sync && echo 3 > /proc/sys/vm/drop_caches": error=2, No such file or directory
    at java.lang.ProcessBuilder.start(ProcessBuilder.java:1029)
    at java.lang.Runtime.exec(Runtime.java:615)
    at java.lang.Runtime.exec(Runtime.java:483)
    at MyTest.main(MyTest.java:896)
Caused by: java.io.IOException: error=2, No such file or directory
    at java.lang.UNIXProcess.forkAndExec(Native Method)
    at java.lang.UNIXProcess.<init>(UNIXProcess.java:135)
    at java.lang.ProcessImpl.start(ProcessImpl.java:130)
    at java.lang.ProcessBuilder.start(ProcessBuilder.java:1021)

I copy paste the same string at the command line and I think works fine.

What is the problem? I am using Ubuntu 11.10. Thanks

Answer 1

Well, your command is using features of the shell (the && and the > redirection) , so you need to run that command in a shell. i.e. you need to run

/bin/sh -c "sync && echo 3 > /proc/sys/vm/drop_caches"

Which you can do by using this as a command

String[] commands = {"/bin/sh", "-c" , "sync && echo 3 > /proc/sys/vm/drop_caches"};