pitfall of while read #bash

pitfall-of-while-read-in-bash.bash

# A background process started within a loop may compete with
# the loop for stdin, which causes the `while read` loop stop
# after the 1st round.
 
cat << EOF >| tasklist.txt
1
2
3
EOF
 
# Solution 1: remove stdin for background process.
while read -r line; do
  echo "$line"
  # use htop as an example for background process
  </dev/null htop &
  # or
  # htop </dev/null &
  sleep 1
done < tasklist.txt
 
# Solution 2: read lines into an array and do a `for` loop.
# `mapfile/readarray` is available in Bash 4 to read lines
# from file into an array.
# -t indicates stripping the **trailing** newline.
mapfile -t lines < tasklist.txt
for line in "${lines[@]}"; do
  echo "$line"
  htop &
  sleep 1
done
unset line