#!/bin/bash
 
THREADS=5
TASKS=10
 
# execution function
e()
{
  echo "t:${1} (${SECONDS}s) - execute task ${2}"
  sleep 1
}
 
# thread function
t()
{
  thread_id=${1}
  echo "t:${thread_id} (${SECONDS}s) started"
  a=0
  while [[ ${a} -lt ${TASKS} ]]; do
    b=$(( ${a} % ${THREADS} ))
    if [[ ${b} = ${thread_id} ]]; then
      e ${thread_id} ${a}
    fi
  a=$(( ${a} + 1 ))
  done
  echo "t:${thread_id} (${SECONDS}s) done"
}
 
# run threads
i=0
while [[ ${i} -lt ${THREADS} ]]; do
  t ${i} &
  i=$(( ${i} + 1 ))
done
 
# wait until all threads are done
wait
 
echo "done (${SECONDS}s)"

THREADS=5; TASKS=10
t:0 (0s) started
t:0 (0s) - execute task 0
t:1 (0s) started
t:1 (0s) - execute task 1
t:2 (0s) started
t:2 (0s) - execute task 2
t:4 (0s) started
t:4 (0s) - execute task 4
t:3 (0s) started
t:3 (0s) - execute task 3
t:0 (1s) - execute task 5
t:1 (1s) - execute task 6
t:2 (1s) - execute task 7
t:4 (1s) - execute task 9
t:3 (1s) - execute task 8
t:0 (2s) done
t:1 (2s) done
t:2 (2s) done
t:4 (2s) done
t:3 (2s) done
done (2s)

THREADS=1; TASKS=5
t:0 (0s) started
t:0 (0s) - execute task 0
t:0 (1s) - execute task 1
t:0 (2s) - execute task 2
t:0 (3s) - execute task 3
t:0 (4s) - execute task 4
t:0 (5s) done
done (5s)

THREADS=4; TASKS=2
t:1 (0s) started
t:1 (0s) - execute task 1
t:2 (0s) started
t:2 (0s) done
t:3 (0s) started
t:3 (0s) done
t:0 (0s) started
t:0 (0s) - execute task 0
t:1 (1s) done
t:0 (1s) done
done (1s)