Show mixing can happen with echo/printf - even if the argument is less than 4 KB

racecondition.sh

#!/bin/bash

# Show that mixing of output can occur with 'echo' with less than 4 KB argument

echo testing echo - any output indicates that mixing has taken place.
# Generate 20000 lines with: echo aa...a &
#                            echo bb...b &
# where aa...a = 4000 a's, and bb...b = 4000 b's.
perl -e '$a="a"x4000; $b="b"x4000; for(1..10000) { print "echo $a &\n echo $b &\n" }' |
  # Execute them. Because of '&' the commands will run in parallel
  bash |
  # Remove duplicate letters: Each line should now be one single letter (a or b)
  # If you get lines with multiple letters (e.g. ab, aba, ba, bab, baba)
  # then you have output mixing from different commands
  tr -s ab |
  # Ignore all lines that contain only 'a' or 'b':
  grep -Ev '^.$'
  # Any output from this will indicate that mixing has taken place.

echo testing printf - any output indicates that mixing has taken place.
# Generate 20000 lines with: printf "aa...a\n" &
#                            printf "bb...b\n" &
# where aa...a = 4000 a's, and bb...b = 4000 b's.
perl -e '$a=("a"x4000)."\\n"; $b=("b"x4000)."\\n"; for(1..10000) { print "printf \"$a\" & \n printf \"$b\" & \n" }' |
    # Execute them. Because of '&' the commands will run in parallel
    bash |
    # Remove duplicate letters: Each line should now be one single letter (a or b)
    # If you get lines with multiple letters (e.g. ab, aba, ba, bab, baba)
    # then you have output mixing from different commands
    tr -s ab |
    # Ignore all lines that contain only 'a' or 'b':
    grep -Ev '^.$'
    # Any output from this will indicate that mixing has taken place.

# So what _is_ the safe limit?
# Testing the above with 1007/1008 instead of 4000 suggests that 1007 is the limit for printf, and 1008 for echo
# The limit for /bin/echo and /usr/bin/printf is 4095/4096