Java Process with Input Output Stream

Question

I have the following code example below  Whereby you can enter a command to the bash shell i e  echo test and have the result echo d back  However  after the first read  Other output streams don t work   Why is this or am I doing something wrong  My end goal is to created a Threaded scheduled task that executes a command periodically to  bash so the OutputStream and InputStream would have to work in tandem and not stop working  I have also been experiencing the error java io IOException  Broken pipe any ideas   Thanks   String line  Scanner scan   new Scanner System in    Process process   Runtime getRuntime    exec    bin bash    OutputStream stdin   process getOutputStream     InputStream stderr   process getErrorStream     InputStream stdout   process getInputStream      BufferedReader reader   new BufferedReader  new InputStreamReader stdout    BufferedWriter writer   new BufferedWriter new OutputStreamWriter stdin     String input   scan nextLine    input      n   writer write input   writer flush     input   scan nextLine    input      n   writer write input   writer flush     while   line   reader readLine        null    System out println   Stdout      line      input   scan nextLine    input      n   writer write input   writer close     while   line   reader readLine        null    System out println   Stdout      line

User · Answer

You have writer close    in your code  So bash receives EOF on its stdin and exits  Then you get Broken pipe when trying to read from the stdoutof the defunct bash

User · Answer

Firstly  I would recommend replacing the line  Process process   Runtime getRuntime    exec    bin bash      with the lines  ProcessBuilder builder   new ProcessBuilder   bin bash    builder redirectErrorStream true   Process process   builder start      ProcessBuilder is new in Java 5 and makes running external processes easier   In my opinion  its most significant improvement over Runtime getRuntime   exec   is that it allows you to redirect the standard error of the child process into its standard output   This means you only have one InputStream to read from   Before this  you needed to have two separate Threads  one reading from stdout and one reading from stderr  to avoid the standard error buffer filling while the standard output buffer was empty  causing the child process to hang   or vice versa   Next  the loops  of which you have two   while   line   reader readLine        null        System out println   Stdout      line       only exit when the reader  which reads from the process s standard output  returns end-of-file   This only happens when the bash process exits   It will not return end-of-file if there happens at present to be no more output from the process   Instead  it will wait for the next line of output from the process and not return until it has this next line   Since you re sending two lines of input to the process before reaching this loop  the first of these two loops will hang if the process hasn t exited after these two lines of input   It will sit there waiting for another line to be read  but there will never be another line for it to read   I compiled your source code  I m on Windows at the moment  so I replaced  bin bash with cmd exe  but the principles should be the same   and I found that    after typing in two lines  the output from the first two commands appears  but then the program hangs  if I type in  say  echo test  and then exit  the program makes it out of the first loop since the cmd exe process has exited   The program then asks for another line of input  which gets ignored   skips straight over the second loop since the child process has already exited  and then exits itself  if I type in exit and then echo test  I get an IOException complaining about a pipe being closed   This is to be expected - the first line of input caused the process to exit  and there s nowhere to send the second line    I have seen a trick that does something similar to what you seem to want  in a program I used to work on   This program kept around a number of shells  ran commands in them and read the output from these commands   The trick used was to always write out a  magic  line that marks the end of the shell command s output  and use that to determine when the output from the command sent to the shell had finished   I took your code and I replaced everything after the line that assigns to writer with the following loop   while  scan hasNext          String input   scan nextLine        if  input trim   equals  exit                 Putting  exit  amongst the echo --EOF--s below doesn t work          writer write  exit n          else           writer write        input       amp  amp  echo --EOF--     echo --EOF-- n              writer flush         line   reader readLine        while  line    null  amp  amp    line trim   equals  --EOF--              System out println   Stdout      line           line   reader readLine              if  line    null            break            After doing this  I could reliably run a few commands and have the output from each come back to me individually   The two echo --EOF-- commands in the line sent to the shell are there to ensure that output from the command is terminated with --EOF-- even in the result of an error from the command   Of course  this approach has its limitations   These limitations include    if I enter a command that waits for user input  e g  another shell   the program appears to hang  it assumes that each process run by the shell ends its output with a newline  it gets a bit confused if the command being run by the shell happens to write out a line --EOF--  bash reports a syntax error and exits if you enter some text with an unmatched      These points might not matter to you if whatever it is you re thinking of running as a scheduled task is going to be restricted to a command or a small set of commands which will never behave in such pathological ways   EDIT  improve exit handling and other minor changes following running this on Linux

User · Answer

I think you can use thread like demon-thread for reading your input and your output reader will already be in while loop in main thread so you can read and write at same time You can modify your program like this   Thread T new Thread new Runnable           Override     public void run             while true                        String input   scan nextLine                input      n               try                   writer write input                   writer flush                  catch  IOException e                       TODO Auto-generated catch block                 e printStackTrace                                         T start      and you can reader will be same as above i e   while   line   reader readLine        null        System out println   Stdout      line       make your writer as final otherwise it wont be able to accessible by inner class

[java] Java Process with Input/Output Stream

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