3

Here is a shell script. Please extrapolate the controlled-environment random number generation subshell to the use of this in an asynchronous logging situation (I intend to use it for inotifywait output).

group=0
(
  for val in {1..10}; do 
    echo "$RANDOM/20000" | bc | xargs sleep # this waits 0, 1, or 2 seconds before each number is printed
    echo $val 
  done 
) | while true; do 
      while read -t 1 line; do 
        echo "read time=$group read=$line" 
      done
      ((group++))
    done

This produces output like this: 

read time=1 read=1
read time=2 read=2
read time=2 read=3
read time=3 read=4
read time=3 read=5
read time=3 read=6
read time=4 read=7
read time=5 read=8
read time=5 read=9
read time=5 read=10

But it then hangs and does not exit. It is stuck in the outer loop, continually incrementing group: 

$ echo $group 
1336794

Clearly the inner loop has exited due to input having finished, and the incrementation of the variable went into hyperdrive.

How to exit the loop? Is there like a else clause of some sort that I can hack on to the inner for loop in order to break out of the outer loop, once the input has finished coming in?

Surely there must be a more robust method than doing timing in the outer loop to see if it went through too fast.

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4
  • did you try break n .... n you can specify the nesting/number of loops you wanna get out of .... (if check for the break 2 stmt.) Commented Jun 6, 2013 at 4:09
  • Notice how i never wrote a break. Where would it go? The inner loop times itself out every second. Commented Jun 6, 2013 at 4:09
  • its a intresting question , +1 for that. Commented Jun 6, 2013 at 4:55
  • A controversial question, it seems Commented Jun 6, 2013 at 16:28

3 Answers 3

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3 
#!/bin/bash
mkfifo outputs
for val in {1..10}; do
    echo "$RANDOM/20000" | bc | xargs sleep
    echo $val 
done > outputs &
group=0
time=$(date +%s%N)
while read line; do
    ctime=$(date +%s%N)
    [ $(( $ctime - $time )) -gt 1000000000 ] && let group++
    echo "read time=$group read=$line"
    time=$ctime
done < outputs
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15 Comments

Where do i put it? The only thing I can think of is make the inner loop also infinite loop, and then manually break out of both loops when I notice the right condition from read.
Okay, that's reasonable to solve this meaningless example question, but in real life I don't know my log's gonna end right there, now am I?
according to manpage, read -t returns false when it times out. Doesn't it also return false when the input ends? Therefore i wouldnt be able to tell when input ends. As to the second point, I concede that, will edit
where does the input come from?
inotifywait -m. Trying to "collect" output so it wont run a script 50 times if I change 50 files simultaneously
|
1

Here is another solution based on read return code. According to bash documentation when read -t exit for timeout it has a return code greater than 128, so you can distinguish EOF from timeout.

function random_timed_output
{
  for val in {1..10}; do
    sleep $(( $RANDOM/20000 ))
    echo $val 
  done
}

group=0
random_timed_output | while true; do
  read -t 1 line
  rc=$?
  if [ $rc -gt 128 ]; then
    #read exit for timeout
    ((group++))
  elif [ $rc -gt 0 ]; then
    #read exit for error or EOF
    break
  else
    #read exit OK
    echo "read time=$group read=$line" 
  fi
done
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3 Comments

Not on CentOS. Returns 1 both cases. Where'd you test this?
@StevenLu: Ubuntu 13.04, bash 4.2, see also documentation for `read' builtin here: gnu.org/software/bash/manual/bashref.html#Bash-Builtins
Well then. % bash --version GNU bash, version 3.2.25(1)-release (x86_64-redhat-linux-gnu) I think working around it with a manual timer makes for more portability. But definitely +1.
0 
 Kaizen ~/so_test $ cat zcntr.sh
 ilimit=10; ## the length of list in for loop ... 

grp=0
(   for val in {1..10}; do
     #echo "$RANDOM/10000" | bc | xargs sleep
     sleep $(($RANDOM/10000));
     echo $val
     done
 ) | while true; do

  while read -t 1 line; do
    echo "read time=$grp read=$line"
  done

  ((grp++));

  if [ $grp -eq $ilimit ]
     then
       break ;
  fi

done

output is not satisfactory , but i will have a look at it later :

Kaizen ~/so_test $ ./zcntr.sh
  read time=0 read=1
  read time=0 read=2
  read time=1 read=
  read time=1 read=4
  read time=1 read=5
  ./zcntr.sh: line 13: read: read error: 0: Permission denied
  read time=4 read=7
  read time=6 read=
  read time=7 read=
  read time=7 read=10

does this help anyhow ?

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5 Comments

@Steven Lu , okay ..... srry i didnt look at that , was trying to figure out why the read's failing ??
Maybe your $(($RANDOM/10000)) isn't as good as my original and writes zero bytes sometimes or does something else that is weird?
now i got even more confused ..... @ Steve could you provide a bit more info on what you are trying to do ? ... edit the question itself with the actual scenario if thats ok with you ?
yeah that is the case ..... i tested it seperately , it does give 0 sometimes ... but thats for sleep the var is set by the for counter which is echoed to the read in while via pipe..... whys that unassigned :(
I dunno. Read the other stuff here to see my motivations for this question. I believe @perreal solved it pretty nicely, I may post back with a non-fifo pipe only solution later if I can figure one out.

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