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Java Forum / General / October 2006exec() and sending to STDIO and reading from STDIO
I have an external program I need to execute which takes an argument. So I have the code: String command = "..."; String argument = "..."; process = Runtime.getRuntime().exec(command); And then for sending arguments: java.io.OutputStream out = process.getOutputStream(); out.write(argument.getBytes()); out.close(); And the for reading output from the external command: java.io.InputStream in = encryptProcess.getInputStream(); int c; while ((c = in.read()) != -1) { System.out.print("OUT: " + (char)c); } in.close(); = = = = = = = = = = = = = = = = = = But the problem is that the output is not actually read. It's still being outputted when I close the InputStream (in.close();) Is there not a way to but execute an external program, send arguments to it and get it's output? I have tried to insert the command "echo argument|command" e.g. "echo test|ls" in the command, but this merely echos out the whole line ("test|ls") rather than sending the argument "test" to "ls". Thanks /Rune > I have tried to insert the command "echo argument|command" e.g. > "echo test|ls" in the command, but this merely echos out the whole > line ("test|ls") rather than sending the argument "test" to > "ls". It's hardly meaningful to send input to ls that way, but it fails because you are attempting to use shell features where there is no shell. You need to do it like this: String[] cmd = { "/bin/sh", "-c", "echo foo | gurka" }; Process p = System.getRuntime().exec(cmd); /gordon  Signature[ don't email me support questions or followups ] g o r d o n + n e w s @ b a l d e r 1 3 . s e > > I have tried to insert the command "echo argument|command" e.g. > > "echo test|ls" in the command, but this merely echos out the whole [quoted text clipped - 7 lines] > String[] cmd = { "/bin/sh", "-c", "echo foo | gurka" }; > Process p = System.getRuntime().exec(cmd); Hello thanks for the feedback, but this merely echos out "foo | gurka". > Hello thanks for the feedback, but this merely echos out "foo | gurka". No, it doesn't. Post your *actual* code. You are not running a shell as I suggested. This is what happens when I run it with real commands: bash$ cat Test.java import java.io.*; public class Test { public static void main(String[] args) throws Exception { String[] cmd = { "/bin/sh", "-c", "echo this is a test | wc" }; Process p = Runtime.getRuntime().exec(cmd); BufferedReader br = new BufferedReader(new InputStreamReader (p.getInputStream())); String line; while ((line = br.readLine()) != null) { System.out.println(line); } br.close(); } } base$ javac Test.java bash$ java Test 1 4 15 bash$ The example is complete and compilable. Try it. /gordon Signature[ don't email me support questions or followups ] g o r d o n + n e w s @ b a l d e r 1 3 . s e > > Hello thanks for the feedback, but this merely echos out "foo | gurka". > [quoted text clipped - 27 lines] > > The example is complete and compilable. Try it. - your example works. But when I substitude the program gpg for wc, I stop getting the output. Can it be that it sends on STDERR or something? import java.io.*; public class Test1 { public static void main(String[] args) throws Exception { String[] cmd = { "/bin/sh", "-c", "echo mypasswd | gpg --passphrase-fd 0 -o tobe.zip -q --decrypt tobe.zip.gpg" }; Process p = Runtime.getRuntime().exec(cmd); BufferedReader br = new BufferedReader(new InputStreamReader (p.getInputStream())); String line; while ((line = br.readLine()) != null) { System.out.println("XXX: " + line); } br.close(); } } - it's the same. I just changed the "String[] cmd =" line > But when I substitude the program gpg for wc, I stop getting the > output. Can it be that it sends on STDERR or something? It's quite possible that gpg is sending output to stderr, but you should be reading from both streams anyway (which my simple example doesn't do). To do this correctly you need to read from the error stream from a separate thread. An alternative is to to use ProcessBuilder, which can combine both streams into one and you don't need any extra thread. But since you're using a shell anyway, you can make a simple change to the command and it will combine the output streams for you: String[] cmd = { "/bin/sh", "-c", "echo mypasswd | gpg --passphrase-fd 0 -o tobe.zip -q --decrypt tobe.zip.gpg 2>&1" }; /gordon Signature[ don't email me support questions or followups ] g o r d o n + n e w s @ b a l d e r 1 3 . s e how can i reading from stdio use this command: type mytest.txt (win32) use String[] cmd = {"cmd","/c","type mytest.txt"} ,it dosn't work. > how can i reading from stdio use this command: > type mytest.txt (win32) > > use String[] cmd = {"cmd","/c","type mytest.txt"} ,it dosn't work. You don't need cmd.exe for this, other than that I have no idea. (in fact you don't need Runtime.exec() for this at all). /gordon Signature[ don't email me support questions or followups ] g o r d o n + n e w s @ b a l d e r 1 3 . s e >> how can i reading from stdio use this command: >> type mytest.txt (win32) >> >> use String[] cmd = {"cmd","/c","type mytest.txt"} ,it dosn't work. > > You don't need cmd.exe for this, other than that I have no idea. Since type is an internal command in cmd and not a type.exe, then I would think cmd is needed. Arne > Since type is an internal command in cmd and not a type.exe, then > I would think cmd is needed. I was assuming that type was a "real" program, but if that's not the case then of course you're right. /gordon (not a windows user) Signature[ don't email me support questions or followups ] g o r d o n + n e w s @ b a l d e r 1 3 . s e > how can i reading from stdio use this command: > type mytest.txt (win32) > > use String[] cmd = {"cmd","/c","type mytest.txt"} ,it dosn't work. Try: String[] cmd = {"cmd","/c", "type", "mytest.txt"}; Arne incase you get error paste your result here........... > I have an external program I need to execute which takes an argument. > So I have the code: [quoted text clipped - 34 lines] > Thanks > /Rune I think you are doing a mistake here........If i am right you are supposed to wait for the process to end . you can try out this sniplet. try{ Process p=Runtime.getRuntime().exec(yourCommand); int result=p.waitFor(); if(result==0){ /*this means that the program ran successfully. *now use your buffered stream and read from the process using the *process.getInputStream() call*/ }else{ /*program did not run successfully! *use your stream and read from the process using the * process.getErrorStream()*/ } }catch(Exception e){ /*you reach here incase the process did not execute properly or wrong arguements */ } > I think you are doing a mistake here........If i am right you are > supposed to wait for the process to end . > Process p=Runtime.getRuntime().exec(yourCommand); > int result=p.waitFor(); [quoted text clipped - 7 lines] > * process.getErrorStream()*/ > } This is a poor suggestion. You must read from both output and error streams while the process is still running, or it will very likely deadlock. Your method can only work for programs with very little output (a few lines at most), and it certainly doesn't work for programs that should run continuously. /gordon Signature[ don't email me support questions or followups ] g o r d o n + n e w s @ b a l d e r 1 3 . s e > > I think you are doing a mistake here........If i am right you are > > supposed to wait for the process to end . [quoted text clipped - 22 lines] > [ don't email me support questions or followups ] > g o r d o n + n e w s @ b a l d e r 1 3 . s e Yes my suggestion was for a code which will end quick enough . hence there is no point of the suggestion being poor since it is well understood from the sniplet that the code is expecting the process to finish running quickly/or after a time interval. Incase you want the process to finish its execution within a time limit , you could always spawn a thread which will count the time since the process's execution start and it can be used to kill it in case it exceeds the time limit .(used by programming contests i suppose) . So dont use it if your code is going to run continuously! > Yes my suggestion was for a code which will end quick enough . hence > there is no point of the suggestion being poor since it is well > understood from the sniplet that the code is expecting the process to > finish running quickly/or after a time interval. My comment had really nothing to do with the length of time a process runs. It has everything to do with the amount of output it produces. Any program that produces more than a few lines of output will *hang* if you run it using your example. Yes it was necessary to point this out since many posters to this group make that mistake anyway, even without you telling them that this is a good way to use Runtime.exec() (it isn't). /gordon Signature[ don't email me support questions or followups ] g o r d o n + n e w s @ b a l d e r 1 3 . s e > > Yes my suggestion was for a code which will end quick enough . hence > > there is no point of the suggestion being poor since it is well [quoted text clipped - 15 lines] > [ don't email me support questions or followups ] > g o r d o n + n e w s @ b a l d e r 1 3 . s e you must have more experience to say so.........anyway i have never tried out with programs giving larger output......... probably it would.......but i wonder why ? > > Any program that produces more than a few lines of output will *hang* > > if you run it using your example. Yes it was necessary to point this > > out since many posters to this group make that mistake anyway, even > > without you telling them that this is a good way to use Runtime.exec() > > (it isn't). [...] > you must have more experience to say so.........anyway i > have never tried out with programs giving larger output......... > probably it would.......but i wonder why ? No need for experience -- just a little knowledge will do ;-) If the program attempts to write more to its stdout, or stderr, than the OS is willing to buffer-up on its behalf (i.e. data that is sitting in the pipes between that process and the parent Java process); then the OS will block the writer until the reader has consumed some of the data. Since the reader (Java) is waiting for the writer to exit before it will read /anything/ from the pipe, and since the writer is blocked waiting for the reader to read something before it can exit, they both end up waiting for each other. Deadlock. Note that you have no control over how much data the OS will allow the writer to put in the pipe before blocking it. It is certainly OS-dependent, and might be installation-dependent. Don't ever use code like that in a production environment, and be very cautious about using it even in throwaway code. -- chris > > > Any program that produces more than a few lines of output will *hang* > > > if you run it using your example. Yes it was necessary to point this [quoted text clipped - 26 lines] > > -- chris Excellent explanation!!! Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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