Get exit code of a background process

Question

I have a command CMD called from my main bourne shell script that takes forever  I want to modify the script as follows   Run the command CMD in parallel as a background process  CMD   amp    In the main script  have a loop to monitor the spawned command every few seconds  The loop also echoes some messages to stdout indicating progress of the script  Exit the loop when the spawned command terminates  Capture and report the exit code of the spawned process   Can someone give me pointers to accomplish this

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My solution was to use an anonymous pipe to pass the status to a monitoring loop   There are no temporary files used to exchange status so nothing to cleanup   If you were uncertain about the number of background jobs the break condition could be   -z    jobs -p          bin bash  exec 3 lt  gt   lt        sleep 15   echo  sleep exit      gt  amp 3      amp   while read -u 3 -t 1 -r STAT CODE    STAT  timeout    do     echo  stat    STAT   code    CODE       if      STAT      sleep exit      then         break     fi done

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The pid of a backgrounded child process is stored in     You can store all child processes  pids into an array  e g  PIDS     wait  -n   jobspec or pid        Wait until the child process specified by each process ID pid or job specification jobspec exits and return the exit status of the last command waited for  If a job spec is given  all processes in the job are waited for  If no arguments are given  all currently active child processes are waited for  and the return status is zero  If the -n option is supplied  wait waits for any job to terminate and returns its exit status  If neither jobspec nor pid specifies an active child process of the shell  the return status is 127   Use wait command you can wait for all child processes finish  meanwhile you can get exit status of each child processes via    and store status into STATUS    Then you can do something depending by status   I have tried the following 2 solutions and they run well  solution01 is   more concise  while solution02 is a little complicated   solution01     bin bash    start 3 child processes concurrently  and store each pid into array PIDS    process  a sh b sh c sh  for app in   process      do       app   amp    PIDS       done    wait for all processes to finish  and store each process s exit code into array STATUS    for pid in   PIDS      do   echo  pid   pid     wait   pid    STATUS       done    after all processed finish  check their exit codes in STATUS    i 0 for st in   STATUS      do   if      st  -ne 0     then     echo   i failed    else     echo   i finish    fi     i  1   done   solution02     bin bash    start 3 child processes concurrently  and store each pid into array PIDS    i 0 process  a sh b sh c sh  for app in   process      do       app   amp    pid      PIDS  i    pid      i  1   done    wait for all processes to finish  and store each process s exit code into array STATUS    i 0 for pid in   PIDS      do   echo  pid   pid     wait   pid    STATUS  i         i  1   done    after all processed finish  check their exit codes in STATUS    i 0 for st in   STATUS      do   if      st  -ne 0     then     echo   i failed    else     echo   i finish    fi     i  1   done

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A simple example  similar to the solutions above   This doesn t require monitoring any process output   The next example uses tail to follow output     echo     bin bash   gt  tmp sh   echo  sleep 30  exit 5   gt  gt  tmp sh   chmod  x tmp sh     tmp sh  amp   1  7454   pid      wait  pid  1    Exit 5                    tmp sh   echo    5   Use tail to follow process output and quit when the process is complete     echo     bin bash   gt  tmp sh   echo  i 0  while let   i  lt  10   do sleep 5  echo   i   let i  i 1  done  exit 5    gt  gt  tmp sh   chmod  x tmp sh     tmp sh 0 1 2  C     tmp sh  gt   tmp tmp log 2 gt  amp 1  amp   1  7673   pid      tail -f --pid  pid  tmp tmp log 0 1 2 3 4 5 6 7 8 9  1    Exit 5                    tmp sh  gt   tmp tmp log 2 gt  amp 1   wait  pid   echo    5

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This may be extending beyond your question  however if you re concerned about the length of time processes are running for  you may be interested in checking the status of running background processes after an interval of time   It s easy enough to check which child PIDs are still running using pgrep -P     however I came up with the following solution to check the exit status of those PIDs that have already expired   cmd1     sleep 5  exit 24    cmd2     sleep 10  exit 0     pids    cmd1  amp  pids         cmd2  amp  pids          lasttimeout 0 for timeout in 2 7 11  do   echo -n  interval- timeout      sleep    timeout-lasttimeout        you can only wait on a pid once   remainingpids      for pid in   pids      do      if   ps -p  pid  gt  dev null   then         wait  pid         echo -n  pid- pid exited             else         echo -n  pid- pid running            remainingpids     pid        fi   done   pids     remainingpids          lasttimeout  timeout   echo done   which outputs   interval-2  pid-28083 running  pid-28084 running   interval-7  pid-28083 exited 24   pid-28084 running   interval-11  pid-28084 exited 0      Note  You could change  pids to a string variable rather than array to simplify things if you like

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Our team had the same need with a remote SSH-executed script which was timing out after 25 minutes of inactivity  Here is a solution with the monitoring loop checking the background process every second  but printing only every 10 minutes to suppress an inactivity timeout   long running sh  amp   pid       Wait on a background job completion  Query status every 10 minutes  declare -i elapsed 0    ps -p   pid   works on macOS and CentOS  On both OSes  ps   pid   works as well  while ps -p   pid   gt  dev null  do   sleep 1   if     elapsed   600    0    then     echo  Waiting for the completion of the main script     elapsed   60  m and counting        fi done    Return the exit code of the terminated background process  This works in Bash 4 4 despite what Bash docs say     If neither jobspec nor pid specifies an active child process of the shell  the return status is 127   wait   pid

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I would change your approach slightly   Rather than checking every few seconds if the command is still alive and reporting a message  have another process that reports every few seconds that the command is still running and then kill that process when the command finishes   For example      35   bin sh  cmd     sleep 5  exit 24     cmd       35  Run the long running process pid       35  Record the pid    35  Spawn a process that coninually reports that the command is still running while echo    date    pid is still running   do sleep 1  done   echoer       35  Set a trap to kill the reporter when the process finishes trap  kill  echoer  0    35  Wait for the process to finish if wait  pid  then     echo  cmd succeeded  else     echo  cmd FAILED    returned      fi

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This is how I solved it when I had a similar need     Some function that takes a long time to process longprocess               Sleep up to 14 seconds         sleep    RANDOM   15             Randomly exit with 0 or 1         exit    RANDOM   2      pids      Run five concurrent processes for i in  1  5   do           longprocess    amp            store PID of process         pids        done    Wait for all processes to finish  will take max 14s   as it waits in order of launch  not order of finishing for p in  pids  do         if wait  p  then                 echo  Process  p success          else                 echo  Process  p fail          fi done

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bin bash   pgm to monitor tail -f  var log messages  gt  gt   tmp log amp    background cmd pid pid      loop to monitor running background cmd while   do     ps ax   grep  pid   grep -v grep     ret        if test   ret      0      then         echo  Monitored pid ended          break     fi     sleep 5  done  wait  pid echo

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1   In bash     holds the PID of the last background process that was executed  That will tell you what process to monitor  anyway  4  wait  lt n gt  waits until the process with PID  lt n gt  is complete  it will block until the process completes  so you might not want to call this until you are sure the process is done   and then returns the exit code of the completed process  2  3  ps or ps   grep  quot      quot  can tell you whether the process is still running  It is up to you how to understand the output and decide how close it is to finishing   ps   grep isn t idiot-proof  If you have time you can come up with a more robust way to tell whether the process is still running   Here s a skeleton script    simulate a long process that will have an identifiable exit code  sleep 15    bin false   amp  my pid     while   ps   grep  quot   my pid  quot        might also need    grep -v grep  here do     echo  my pid is still in the ps output  Must still be running      sleep 3 done  echo Oh  it looks like the process is done  wait  my pid   The variable    always holds the exit code of the last command to finish    Here it holds the exit code of  my pid  since wait exits with that code   my status    echo The exit status of the process was  my status

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As I see almost all answers use external utilities  mostly ps  to poll the state of the background process  There is a more unixesh solution  catching the SIGCHLD signal  In the signal handler it has to be checked which child process was stopped  It can be done by kill -0  lt PID gt  built-in  universal  or checking the existence of  proc  lt PID gt  directory  Linux specific  or using the jobs built-in  bash specific  jobs -l also reports the pid  In this case the 3rd field of the output can be Stopped Running Done Exit       Here is my example   The launched process is called loop sh  It accepts -x or a number as an argument  For -x is exits with exit code 1  For a number it waits num 5 seconds  In every 5 seconds it prints its PID   The launcher process is called launch sh      bin bash  handle chld         local tmp        for  i 0 i lt    pids       i    do         if     -d  proc   pids i      then             wait   pids i               echo  Stopped   pids i    exit code              else tmp     pids i            fi     done     pids    tmp         set -o monitor trap  handle chld  CHLD    Start background processes   loop sh 3  amp  pids         loop sh 2  amp  pids         loop sh -x  amp  pids          Wait until all background processes are stopped while      pids     -gt 0    do echo  WAITING FOR    pids       sleep 2  done echo STOPPED   For more explanation see  Starting a process from bash script failed

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Another solution is to monitor processes via the proc filesystem  safer than ps grep combo   when you start a process it has a corresponding folder in  proc  pid  so the solution could be     bin bash      doSomething  amp  local pid    while   -d  proc  pid    do   While directory exists  the process is running     doSomethingElse          else   when directory is removed from  proc  process has ended     wait  pid     local exit status    done        Now you can use the  exit status variable however you like

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With this method  your script doesnt have to wait for the background process  you will only have to monitor a temporary file for the exit status   FUNCmyCmd     sleep 3 return 6      export retFile   mktemp    FUNCexecAndWait     FUNCmyCmd echo     gt  retFile      FUNCexecAndWait amp    now  your script can do anything else while you just have to keep monitoring the contents of retFile  it can also contain any other information you want like the exit time    PS   btw  I coded thinking in bash

[linux] Get exit code of a background process

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