IPC properties

echo_with_lock.sh

#!/bin/bash

set -o noclobber
lockfile=${1:-lockfile}

while read line
do
  while ! echo "$$" > "$lockfile"
  do true  #printf "waiting for: $lockfile [$(cat $lockfile)]\n" 1>&2
  done 2>/dev/null

  trap 'rm "$lockfile"; exit $?' INT TERM EXIT
  printf "$line\n"
  rm "$lockfile"
done

fifo_queue.sh

#!/bin/sh
#############################################################################
# Demonstrates how to use a fifo as a persitent queue.
#############################################################################

target=${1:-target}
if [ -e "$target" ]
then rm "$target"
fi
mkfifo "$target"

# Launch a hang process to hold open the fifo and thereby prevent broken pipes
# as multiple processes to access the fifo and then exit.  Set a trap to
# cleanup the hang loop on exit.
trap 'kill -9 "$hangpid"; exit' INT TERM EXIT
while true; do sleep 1; done < "$target" > "$target" & 
hangpid=$!

echo abc > "$target"
echo pqr > "$target"
echo xyz > "$target"

head -n 3 < "target"

flock_no_race.sh

#!/bin/sh
#############################################################################
# Illustrates file locks producing coherent output.  Check with:
#
#   wc -l target    # 30000
#   uniq -c target  # random grouping of a, b, or c lines
#
#############################################################################

target=${1:-target}
lockfile=${1:"$target.lock"}
rm "$target" "$lockfile"

ruby -e "
10000.times do
  puts 'a' * 10
end
" | ./echo_with_lock.sh "$lockfile" >> "$target" &

ruby -e "
10000.times do
  puts 'b' * 10
end
" | ./echo_with_lock.sh "$lockfile" >> "$target" &

ruby -e "
10000.times do
  puts 'c' * 10
end
" | ./echo_with_lock.sh "$lockfile" >> "$target"

! grep -qE 'ab|ba' "$target"

redirect_race.sh

#!/bin/sh
#############################################################################
# Illustrates a race condition when multiple processes append to the same
# target.  Exit status 0 if the target has coherent output, and 1 if the
# output is garbled.
#############################################################################

target=${1:-target}
if [ -e "$target" ]
then rm "$target"
fi

ruby -e "
1000.times do
  puts 'a' * 10
end
" >> "$target" &

ruby -e "
1000.times do
  puts 'b' * 10
end
" >> "$target"

! grep -qE 'ab|ba' "$target"