How to influence a bash while loop with a timed global variable?

Question

I have a while loop in a bash script which should do something different at the beginning and at every 5 second interval. Any previous loop is allowed to complete. The 5s interval is indicated by the do_different global variable set by the heartbeat function. An additional complication is that a normal while loop completes in an unknown amount of time (simplified with RANDOM in below script). Using cron is not an option, neither is timing the random process.

I already unsuccessfully tried using a pipe as well as process substitution. The whole script may be re-factored.

#!/bin/bash

function heartbeat {
    do_different=true
    while sleep 5s
    do
        do_different=true
    done
}

heartbeat &

while true
do
    if $do_different
    then
        echo 'Something different'
        do_different=false
        i=0
    else
        # process of random duration; not important
        r=$(( 1 + RANDOM % 3 ))
        sleep "${r}s"
        i=$((i + r))
        echo "$i"
    fi
done

Answer 1

First of all, in case it isn't obvious, the script in the question fails because heartbeat runs in a child process, and therefore cannot change shell variables in the parent shell's memory.

Here's an approach that's closer to the spirit of the OP's attempt:

#!/bin/bash

trap 'do_different=true' USR1

heartbeat() {
    while sleep 5
    do                  # If the parent process has gone away, the child should terminate.
        kill -USR1 "$$"  ||  exit
    done
}

heartbeat &
heartbeat_pid=$!
trap 'kill "$heartbeat_pid"' 0

do_different=true

while true
do
    if "$do_different"
    then
        echo 'Something different'
        do_different=false
        i=0
    else
        # process of random duration; not important
        r=$(( 1 + RANDOM % 3 ))
        sleep "$r"
        i=$((i + r))
        echo "$i"
    fi
done

The modified heartbeat sends SIGUSR1 signals to the main (parent) shell process.  This, and SIGUSR2, are reserved for user/application use (and should never be generated by the system).  The trap command allows a shell script to catch signals. The trap 'do_different=true' USR1 command tells the shell to catch the SIGUSR1 signal (which arrives every five seconds) and set the do_different flag when it occurs.

heartbeat_pid, obviously, is the process ID (PID) of the heartbeat child process.  The command trap 'kill "$heartbeat_pid"' 0 defines an action to occur upon "receipt" of the pseudo-signal 0, which refers to script exit.  Think of this as the shell putting a sticky note on the door saying "After you leave, remember to buy groceries on the way home."  This action will be invoked if the script reaches the end or executes an exit statement (neither of which can happen with this script, since it is an infinite loop), or if it is terminated by an interrupt signal (SIGINT, which is generated by Ctrl+C).  This is a safety net; the heartbeat process is already written to terminate when the parent process goes away.

Answer 2

I would use the date utility to get the current time in seconds.

#!/bin/bash
lastTime=-5

while true
do
    currentTime=$(date +%s)
    elapsedTime=$((currentTime - lastTime))
    if [[ $elapsedTime -ge 5 ]]
    then
        echo 'Something different'
        lastTime=$currentTime
        i=0
    else
        # process of random duration; not important
        r=$(( 1 + RANDOM % 3 ))
        sleep ${r}s
        i=$((i + r))
        echo $i
    fi
done

Edit: Changed lastTime's initial value so that it also "does something different" at the beginning.

Answer 3

Here is the code I am running now, based on airfishey's answer with some corrections.

#!/bin/bash

t_lastdue=$(date --date='5seconds ago' +%s)

while true
do
    t_now=$(date +%s)
    t_elapsed=$((t_now - t_lastdue))
    if [ $t_elapsed -ge 5 ]
    then
        echo 'Something different'
        t_lastdue=$((t_lastdue + 5))
        i=0
    else
        # process of random duration; not important
        r=$(( 1 + RANDOM % 3 ))
        sleep "${r}s"
        i=$((i + r))
        echo "$i"
    fi
done

Answer 4

Since you're using bash anyway, you should be using $SECONDS:

#!/bin/bash
while [ "$SECONDS" -lt 5 ] || {
      SECONDS=$((i=0))
      echo Something different
};do  sleep "$((r=(1+RANDOM)%3))"
      echo  "$((i+=r))"
done

From man bash:

$SECONDS

• This variable expands to the number of seconds since the shell was started. Assignment to this variable resets the count to the value assigned, and the expanded value becomes the value assigned plus the number of seconds since the assignment.

And here's how you can do it and handle drift:

S=SECONDS
while   [ "$(($S<5||(i=0*($S-=5))))" -ne 0 ] ||
        echo Something different
do      sleep "$((r=(1+RANDOM)%3))"
        echo  "$((i+=r))"
done