Deth

Here's a new script for the new year.

It sometimes happens when I need to, say generate a dot file from a SableCC grammar to have a nice little diagram showing me how the damn thing is supposed to work. When you do one of those, not only does it tent to facilitate using your laptop as a simple toaster, or perhaps a very expensive heating system, but also usually eats up a lot of time. Something like 7 hours, for instance, and you don't know at all when it's going to end... or if it's going to generate anything, or maybe die out of lack of memory.

So it's not something that you want to wait up for until 2 AM - just let it run on it's own, and go to sleep instead. Good plan, yeah?

On the other hand, maybe it's wise to take those electricity bill into account? How about turning it off right after the process finishes (or dies)?

Ah, and did you have the foresight to actually write the shabang which shuts the computer down when the process is done? I never do...

That's where this script comes in. You just tell it to watch the process, and do stuff if it happens to die. It's as simple as that.

So, in the particular case that I described, I'd use it like this:

sudo ./deth sablecc "shutdown -h now";

And it won't work. The script is not prepared to handle the sudo-ing out of the box. Why? Well... you know, because I suck. But it's not a problem, all you need to do is take a peek at line 40 and change `whoami` to just your username, for instance: joe (with no spaces, no inverted commas, no nothing... well, double quotes, if you have to). Problem solved.

There's another variable that you can handle as well, in line 37. The sleep time between checks is here, in seconds. If 5 minutes doesn't cut it for you, modify it right there; no worries, she'll be right.

Yeah, for some reason I didn't feel like doing the entire getopt thing, and make these things settable... that's laziness, that is.

Here's the code.

 
  1 #!/bin/bash 
  2 # 
  3 # Deth 
  4 # 
  5 # A watchdog for the death of processes. After specifying  
  6 # the list of processes to watch (either by name of PID) 
  7 # the script waits for them to turn off, in which case the  
  8 # specified command is run (and sent into the background). 
  9 # When all watched processes die, the script exits. 
 10 #  
 11 # The commands are translated into PIDs at the beginning  
 12 # only, so if you want to watch gcalctool, and, while this 
 13 # script is already running, turn on another instance of 
 14 # gcalctool, the new instance will be completely ignored. 
 15 # 
 16 # The script checks for the death of the specified processes 
 17 # and then goes to sleep for a specified period of time.  
 18 # Since this does not need to be a very precise script, it's  
 19 # currently set to checking every 5 minutes - this can be 
 20 # changed by modyfing the value of SLEEP_TIME. 
 21 # 
 22 # The script only watches processes which belong to the 
 23 # current user. To modify this set the value of USERNAME. 
 24 #  
 25 # Potential issues: 
 26 #   When selecting by command name, all fitting PIDs 
 27 #   are watched, without any further discrimination. 
 28 # Parameters: 
 29 #   List of running commands and/or PIDs. 
 30 #   The last argument is always the command to run when 
 31 #       each of them turns off. 
 32 # Author: 
 33 #   Konrad Siek 
 34 # 
 35  
 36 # How long to wait between pings. In seconds. 
 37 SLEEP_TIME=300 
 38  
 39 # Limit the watched processes to the current user. 
 40 USERNAME=`whoami` 
 41  
 42 # Check if enough arguments to even try getting the PIDs. 
 43 if [ $# -lt 2 ] 
 44 then 
 45     echo -e "Usage: \n\t$0 [pids] [commands] action\n" 
 46     exit -1 
 47 fi 
 48  
 49 # Convert all arguments to a PID array 
 50 declare -a array 
 51 while [ "$2" != "" ] 
 52 do 
 53     if [ `expr "$1" : "[0-9][0-9]*"` != 0 ] 
 54     then 
 55         # Add PIDs to the array as they are. 
 56         if [ $(\ 
 57             ps U $USERNAME -u $USERNAME -o pid \ 
 58                         | grep "^ *$1 *\$" | wc -l) != 0 ] 
 59         then 
 60             array=( ${array[@]} $1 ) 
 61         else 
 62             echo "PID $1 is not valid, ignoring." >& 2 
 63         fi 
 64     else 
 65         # Extract PIDs from name and add them to the array. 
 66         array=( ${array[@]} $(\ 
 67             ps U $USERNAME -u $USERNAME -o pid,comm \ 
 68             | awk -v f=$1 '$2==f {printf($1" ")}' ) ) 
 69     fi 
 70     shift 
 71 done 
 72  
 73 # The last parameter is the action done on death of processes. 
 74 ACTION=$1 
 75  
 76 # No array can be generated - no PIDs were discerned. 
 77 if [ ${#array[@]} = 0 ] 
 78 then 
 79     echo "No PIDs were supplied." >& 2 
 80     exit -2 
 81 fi 
 82  
 83 # Work the magic.  
 84 while [ 1 ] 
 85 do 
 86     # Check if anything changed, and act. 
 87     i=0; len=${#array[@]} 
 88     while [ $len -gt 0 ] 
 89     do 
 90         # Ignore empty elements. 
 91         if [ "${array[$i]}" = '' ] 
 92         then 
 93             # Point to next element 
 94             i=$(($i + 1)) 
 95             continue 
 96         fi 
 97  
 98         # Fewer elements to visit left. 
 99         len=$((len - 1)) 
100  
101         # Check if process dies. 
102         if [ $(\ 
103                         ps U $USERNAME -u $USERNAME -o pid \ 
104                         | grep "^ *${array[$i]} *\$" | wc -l) = 0 ] 
105         then 
106             # Process is dead, execute action. 
107             $ACTION & 
108             unset array[$i] 
109         fi 
110  
111         # Point to next element 
112         i=$(($i + 1)) 
113     done 
114  
115     # If nothing left to do, quit. 
116     if [ ${#array[@]} = 0 ] 
117     then 
118         exit 0; 
119     fi 
120  
121     # Wait a bit before continuing. 
122     sleep $SLEEP_TIME; 
123 done 

The code is also available at GitHub as bash/deth.

P.S. I hate those gorram, depressing date changes.