How to parallelize for-loop in bash limiting number of processes

bash 4.4 will have an interesting new type of parameter expansion that simplifies Charles Duffy's answer.

#!/bin/bash

num_procs=$1
num_iters=$2
num_jobs="\j"  # The prompt escape for number of jobs currently running
for ((i=0; i<num_iters; i++)); do
  while (( ${num_jobs@P} >= num_procs )); do
    wait -n
  done
  python foo.py "$i" arg2 &
done

GNU, macOS/OSX, FreeBSD and NetBSD can all do this with xargs -P, no bash versions or package installs required. Here's 4 processes at a time:

printf "%s\0" {1..10} | xargs -0 -I @ -P 4 python foo.py @ arg2

As a very simple implementation, depending on a version of bash new enough to have wait -n (to wait until only the next job exits, as opposed to waiting for all jobs):

#!/bin/bash
#      ^^^^ - NOT /bin/sh!

num_procs=$1
num_iters=$2

declare -A pids=( )

for ((i=0; i<num_iters; i++)); do
  while (( ${#pids[@]} >= num_procs )); do
    wait -n
    for pid in "${!pids[@]}"; do
      kill -0 "$pid" &>/dev/null || unset "pids[$pid]"
    done
  done
  python foo.py "$i" arg2 & pids["$!"]=1
done

If running on a shell without wait -n, one can (very inefficiently) replace it with a command such as sleep 0.2, to poll every 1/5th of a second.

Since you're actually reading input from a file, another approach is to start N subprocesses, each of processes only lines where (linenum % N == threadnum):

num_procs=$1
infile=$2
for ((i=0; i<num_procs; i++)); do
  (
    while read -r line; do
      echo "Thread $i: processing $line"
    done < <(awk -v num_procs="$num_procs" -v i="$i" \
                 'NR % num_procs == i { print }' <"$infile")
  ) &
done
wait # wait for all the $num_procs subprocesses to finish

A relatively simple way to accomplish this with only two additional lines of code. Explanation is inline.

NUM_PROCS=$1
NUM_ITERS=$2

for ((i=0; i<$NUM_ITERS; i++)); do
    python foo.py $i arg2 &
    let 'i>=NUM_PROCS' && wait -n # wait for one process at a time once we've spawned $NUM_PROC workers
done
wait # wait for all remaining workers

Are you aware that if you are allowed to write and run your own scripts, then you can also use GNU Parallel? In essence it is a Perl script in one single file.

From the README:

= Minimal installation =

If you just need parallel and do not have 'make' installed (maybe the system is old or Microsoft Windows):

wget http://git.savannah.gnu.org/cgit/parallel.git/plain/src/parallel
chmod 755 parallel
cp parallel sem
mv parallel sem dir-in-your-$PATH/bin/

seq $2 | parallel -j$1 python foo.py {} arg2

parallel --embed (available since 20180322) even makes it possible to distribute GNU Parallel as part of a shell script (i.e. no extra files needed):

parallel --embed >newscript

Then edit the end of newscript.

This isn't the simplest solution, but if your version of bash doesn't have "wait -n" and you don't want to use other programs like parallel, awk etc, here is a solution using while and for loops.

num_iters=10
total_threads=4
iter=1
while [[ "$iter" -lt "$num_iters" ]]; do
    iters_remainder=$(echo "(${num_iters}-${iter})+1" | bc)
    if [[ "$iters_remainder" -lt "$total_threads" ]]; then
        threads=$iters_remainder
    else
        threads=$total_threads
    fi
    for ((t=1; t<="$threads"; t++)); do
        (
            # do stuff
        ) &
        ((++iter))
    done 
    wait
done