How to check if a process id PID exists

Question

In a bash script  I want to do the following  in pseudo-code    if   a process exists with  PID    then      kill  PID   fi   What s the appropriate expression for the conditional statement

User · Answer

if   -n   PID  -a -e  proc  PID    then     echo  process exists  fi   or  if   -n    ps -p  PID -o pid        In the latter form  -o pid  is an output format to display only the process ID column with no header  The quotes are necessary for non-empty string operator -n to give valid result

User · Answer

You have two ways   Lets start by looking for a specific application in my laptop    root pinky     ps fax   grep mozilla  3358          S      0 00      bin sh  usr lib firefox-3 5 run-mozilla sh  usr lib firefox-3 5 firefox 16198 pts 2    S      0 00                 grep mozilla   All examples now will look for PID 3358   First way  Run  ps aux  and grep for the PID in the second column  In this example I look for firefox  and then for it s PID    root pinky     ps aux   awk   print  2      grep 3358 3358   So your code will be   if   ps aux   awk   print  2      grep -q  PID 2 gt   dev null    then     kill  PID  fi   Second way  Just look for something in the  proc  PID directory  I am using  exe  in this example  but you can use anything else    root pinky     ls -l  proc 3358 exe  lrwxrwxrwx  1 elcuco elcuco 0 2010-06-15 12 33  proc 3358 exe - gt   bin bash   So your code will be   if   -f  proc  PID exe    then     kill  PID  fi   BTW  whats wrong with kill -9  PID    true      EDIT   After thinking about it for a few months    about 24     the original idea I gave here is a nice hack  but highly unportable  While it teaches a few implementation details of Linux  it will fail to work on Mac  Solaris or  BSD  It may even fail on future Linux kernels  Please - use  ps  as described in other responses

User · Answer

For example in GNU Linux you can use   Pid   pidof  process name    if    Pid  gt  0    then     do something else     do something fi    Or something like  Pin   ps -A   grep name   awk  print  4    echo  PIN   and that shows you the name of the app  just the name without ID

User · Answer

To check for the existence of a process  use  kill -0  pid   But just as  unwind said  if you re going to kill it anyway  just  kill  pid   or you will have a race condition   If you want to ignore the text output of kill and do something based on the exit code  you can  if   kill  pid  gt   dev null 2 gt  amp 1  then     echo  Could not send SIGTERM to process  pid   gt  amp 2 fi

User · Answer

here i store the PID in a file called  pid  which is kind of like  run      and only execute the script if not already being executed      bin bash if   -f  pid    then   read pid  lt   pid   echo  pid   ps -p  pid  gt   dev null   r      if    r -eq 0    then     echo   pid is currently running  not executing  0 twice  exiting now         exit 1   fi fi  echo     gt   pid    do things here  rm  pid   note  there is a race condition as it does not check how that pid is called  if the system is rebooted and  pid exists but is used by a different application this might lead  unforeseen consequences

User · Answer

It seems like you want  wait  PID   which will return when  pid finishes   Otherwise you can use  ps -p  PID   to check if the process is still alive  this is more effective than kill -0  pid because it will work even if you don t own the pid

User · Answer

ps command with -p  PID can do this     ps -p 3531   PID TTY          TIME CMD  3531          00 03 07 emacs

User · Answer

By pid  pgrep  pid   gt  dev null  By name  pgrep -u  user  -x  name   gt  dev null   quot -x quot  means  quot exact match quot

User · Answer

The best way is   if ps -p  PID  gt   dev null then    echo   PID is running       Do something knowing the pid exists  i e  the process with  PID is running fi   The problem with   kill -0  PID   is the exit code will be non-zero even if the pid is running and you dont have permission to kill it  For example   kill -0 1   and   kill -0  non-running-pid   have an indistinguishable  non-zero  exit code for a normal user  but the init process  PID 1  is certainly running   DISCUSSION  The answers discussing kill and race conditions are exactly right if the body of the test is a  kill   I came looking for the general  how do you test for a PID existence in bash     The  proc method is interesting  but in some sense breaks the spirit of the  ps  command abstraction  i e  you dont need to go looking in  proc because what if Linus decides to call the  exe  file something else

User · Answer

I think that is a bad solution  that opens up for race conditions  What if the process dies between your test and your call to kill  Then kill will fail  So why not just try the kill in all cases  and check its return value to find out how it went

[bash] How to check if a process id (PID) exists

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