Best way to make a shell script daemon

Question

I m wondering if there is a better way to make a daemon that waits for something using only sh than       bin sh trap processUserSig SIGUSR1 processUserSig       echo  doing stuff     while true  do   sleep 1000 done   In particular  I m wondering if there s any way to get rid of the loop and still have the thing listen for the signals

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If I had a script sh and i wanted to execute it from bash and leave it running even when I want to close my bash session then I would combine nohup and  amp  at the end   example  nohup   script sh  lt  inputFile txt  gt    logFile 2 gt  amp 1  amp   inputFile txt can be any file  If your file has no input then we usually use  dev null  So the command would be   nohup   script sh  lt   dev null  gt    logFile 2 gt  amp 1  amp   After that close your bash session open another terminal and execute  ps -aux   egrep  script sh  and you will see that your script is still running at the background  Of cource if you want to stop it then execute the same command  ps  and kill -9  lt PID-OF-YOUR-SCRIPT gt

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Have a look at the daemon tool from the libslack package   http   ingvar blog linpro no 2009 05 18 todays-sysadmin-tip-using-libslack-daemon-to-daemonize-a-script   On Mac OS X use a launchd script for shell daemon

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Here is the minimal change to the original proposal to create a valid daemon in Bourne shell  or Bash      bin sh if    quot  1 quot      quot   forked   quot     then     setsid  quot  0 quot    forked    quot    quot   amp      exit else     shift fi  trap  siguser1 true  SIGUSR1 trap  echo  quot Clean up and exit quot   kill  sleep pid  exit  SIGTERM exec  gt  outfile exec 2 gt  errfile exec 0 lt   dev null  while true  do      sleep 30000000  amp  gt  dev null   amp      sleep pid        wait     kill  sleep pid  amp  gt  dev null     if   -n  quot  siguser1 quot     then         siguser1            echo  quot Wait was interrupted by SIGUSR1  do things here  quot      fi done   Explanation   Line 2-7  A daemon must be forked so it doesn t have a parent  Using an artificial argument to prevent endless forking   quot setsid quot  detaches from starting process and terminal  Line 9  Our desired signal needs to be differentiated from other signals  Line 10  Cleanup is required to get rid of dangling  quot sleep quot  processes  Line 11-13  Redirect stdout  stderr and stdin of the script  Line 16  sleep in the background Line 18  wait waits for end of sleep  but gets interrupted by  some  signals  Line 19  Kill sleep process  because that is still running when signal is caught  Line 22  Do the work if SIGUSR1 has been caught   Guess it does not get any simpler than that

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See Bash Service Manager project  https   github com reduardo7 bash-service-manager  Implementation example     usr bin env bash  export PID FILE PATH   tmp my-service pid  export LOG FILE PATH   tmp my-service log  export LOG ERROR FILE PATH   tmp my-service error log       services sh  run-script       local action   1    Action    while true  do     echo      Running action    action        echo foo     echo bar  gt  amp 2          action     run     amp  amp  return 0     sleep 5         action     debug     amp  amp  exit 25   done    before-start       local action   1    Action    echo    Starting with  action     after-finish       local action   1    Action   local serviceExitCode  2   Service exit code    echo    Finish with  action  Exit code   serviceExitCode     action   1  serviceName  Example Service   serviceMenu   action    serviceName  run-script   workDir  before-start after-finish   Usage example      example-service   Actions   start stop restart status run debug tail - log error         example-service start   Starting Example Service service         example-service status   Serive Example Service is runnig with PID 5599      example-service stop   Stopping Example Service         example-service status   Service Example Service is not running

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Like many answers this one is not a  real  daemonization but rather an alternative to nohup approach   echo  script sh    at now   There are obviously differences from using nohup  For one there is no detaching from the parent in the first place  Also  script sh  doesn t inherit parent s environment   By no means this is a better alternative  It is simply a different  and somewhat lazy  way of launching processes in background   P S  I personally upvoted carlo s answer as it seems to be the most elegant and works both from terminal and inside scripts

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try executing using  amp  if you save this file as program sh  you can use     program sh  amp

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Just backgrounding your script    myscript  amp   will not daemonize it  See http   www faqs org faqs unix-faq programmer faq   section 1 7  which describes what s necessary to become a daemon  You must disconnect it from the terminal so that SIGHUP does not kill it  You can take a shortcut to make a script appear to act like a daemon   nohup   myscript 0 lt  amp -  amp  gt  dev null  amp    will do the job  Or  to capture both stderr and stdout to a file   nohup   myscript 0 lt  amp -  amp  gt  my admin log file  amp    However  there may be further important aspects that you need to consider  For example    You will still have a file descriptor open to the script  which means that the directory it s mounted in would be unmountable  To be a true daemon you should chdir       or cd   inside your script   and fork so that the parent exits  and thus the original descriptor is closed  Perhaps run umask 0  You may not want to depend on the umask of the caller of the daemon    For an example of a script that takes all of these aspects into account  see Mike S  answer

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double background your script to have it detach from the tty   cf  http   www linux-mag com id 5981     program sh  amp    amp

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cd    umask 0  setsid your script sh  lt  dev null  amp  gt  dev null  amp     amp

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Use your system s daemon facility  such as start-stop-daemon   Otherwise  yes  there has to be a loop somewhere

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It really depends on what is the binary itself going to do   For example I want to create some listener   The starting Daemon is simple task    lis deamon       bin bash    We will start the listener as Deamon process       LISTENER BIN  tmp deamon test listener test -x  LISTENER BIN    exit 5 PIDFILE  tmp deamon test listener pid  case   1  in       start              echo -n  Starting Listener Deamon                    startproc -f -p  PIDFILE  LISTENER BIN             echo  running                                          echo  Usage   0 start              exit 1                esac   this is how we start the daemon  common way for all  etc init d  staff   now as for the listener it self  It must be some kind of loop alert or else that will trigger the script  to do what u want  For example if u want your script to sleep 10 min and wake up and ask you how you are doing u will do this with the   while true   do sleep 600   echo  How are u       done   Here is the simple listener that u can do that will listen for your  commands from remote machine and execute them on local    listener       bin bash    Starting listener on some port   we will run it as deamon and we will send commands to it    IP   hostname --ip-address  PORT 1024 FILE  tmp backpipe count 0 while   -a  FILE     do  If file exis I assume that it used by other program   FILE  FILE  count   count     count   1   done    Now we know that such file do not exist    U can write down in deamon it self the remove for those files   or in different part of program  mknod  FILE p  while true   do    netcat -l -s  IP -p  PORT  lt   FILE   bin bash  gt   FILE done rm  FILE   So to start UP it    tmp deamon test listener start  and to send commands from shell  or wrap it to script     test host netcat 10 184 200 22 1024 uptime  20 01pm  up 21 days  5 10   44 users   load average  0 62  0 61  0 60 date Tue Jan 28 20 02 00 IST 2014  punt   Cntrl C    Hope this will help

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Some of the top-upvoted answers here are missing some important parts of what makes a daemon a daemon  as opposed to just a background process  or a background process detached from a shell  This http   www faqs org faqs unix-faq programmer faq  describes what is necessary to be a daemon  And this Run bash script as daemon implements the setsid  though it misses the chdir to root  The original poster s question was actually more specific than  quot How do I create a daemon process using bash  quot   but since the subject and answers discuss daemonizing shell scripts generally  I think it s important to point it out  for interlopers like me looking into the fine details of creating a daemon   Here s my rendition of a shell script that would behave according to the FAQ  Set DEBUG to true to see pretty output  but it also exits immediately rather than looping endlessly      bin bash DEBUG false    This part is for fun  if you consider shell scripts fun- and I do  trap process USR1 SIGUSR1  process USR1         echo  Got signal USR1      echo  Did you notice that the signal was acted upon only after the sleep was done      echo  in the while loop  Interesting  yes  Yes       exit 0     End of fun  Now on to the business end of things   print debug         whatiam  quot  1 quot   tty  quot  2 quot          quot  tty quot      quot not a tty quot      amp  amp            echo  quot  quot   gt  tty         echo  quot  whatiam  PID    quot   gt  tty         ps -o pid sess pgid -p     gt  tty         tty  gt  tty          me DIR  quot    cd  quot    dirname  quot   BASH SOURCE 0   quot    quot   amp  amp  pwd   quot  me FILE   basename  0  cd         CHILD HERE --------------------------------------------------------------------- gt  if    quot  1 quot     quot child quot      then     2  We are the child  We need to fork again      shift  tty  quot  1 quot   shift      DEBUG  amp  amp  print debug  quot     CHILD  NEW SESSION  NEW PGID quot   quot  tty quot      umask 0      me DIR  me FILE XXrefork daemonXX  quot  tty quot   quot    quot   lt  dev null  gt  dev null 2 gt  dev null  amp       DEBUG  amp  amp      quot  tty quot      quot not a tty quot      amp  amp  echo  quot CHILD OUT quot   gt  tty     exit 0 fi        ENTRY POINT HERE -------------------------------------------------------------- gt  if    quot  1 quot      quot XXrefork daemonXX quot      then   1  This is where the original call starts      tty   tty       DEBUG  amp  amp  print debug  quot     PARENT quot   quot  tty quot      setsid  me DIR  me FILE child  quot  tty quot   quot    quot   amp       DEBUG  amp  amp      quot  tty quot      quot not a tty quot      amp  amp  echo  quot PARENT OUT quot   gt  tty     exit 0 fi        RUNS AFTER CHILD FORKS  actually  on Linux  clone  s  See strace -------------- gt                                   3  We have been reforked  Go to work  exec  gt  tmp outfile exec 2 gt  tmp errfile exec 0 lt  dev null  shift  tty  quot  1 quot   shift   DEBUG  amp  amp  print debug  quot     DAEMON quot   quot  tty quot                                   The real stuff goes here  To exit  see fun  above   DEBUG  amp  amp      quot  tty quot      quot not a tty quot       amp  amp  echo NOT A REAL DAEMON  NOT RUNNING WHILE LOOP   gt  tty   DEBUG      while true  do     echo  quot Change this loop  so this silly no-op goes away  quot   gt  dev null     echo  quot Do something useful with your life  young padawan  quot   gt  dev null     sleep 10 done     DEBUG  amp  amp      quot  tty quot      quot not a tty quot      amp  amp  sleep 3  amp  amp  echo  quot DAEMON OUT quot   gt  tty  exit   This may never run  Why is it here then  It s pretty         Kind of like   quot The End quot  at the end of a movie that you        already know is over  It s always nice   Output looks like this when DEBUG is set to true  Notice how the session and process group ID  SESS  PGID  numbers change   lt shell prompt gt   bash blahd      PARENT  PID 5180   PID  SESS  PGID  5180  1708  5180  dev pts 6 PARENT OUT  lt shell prompt gt        CHILD  NEW SESSION  NEW PGID  PID 5188   PID  SESS  PGID  5188  5188  5188 not a tty CHILD OUT      DAEMON  PID 5198   PID  SESS  PGID  5198  5188  5188 not a tty NOT A REAL DAEMON  NOT RUNNING WHILE LOOP  DAEMON OUT

[bash] Best way to make a shell script daemon?

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