Source code in the unix programming language

# This while loop repeats for each line of the file.
# This loop is inside a pipeline; many shells will
# run this loop inside a subshell.
cat input.txt |
while IFS= read -r line ; do
  printf '%s\n' "$line"
done

# This loop runs in the current shell, and can read both
# the old standard input (fd 1) and input.txt (fd 3).
exec 3
while IFS= read -r line <&3 ; do
  printf '%s\n' "$line"
done
exec 3>&-

# The old Bourne Shell interprets 'IFS= read' as 'IFS= ; read'.
# It requires extra code to restore the original value of IFS.
exec 3
oldifs=$IFS
while IFS= ; read -r line <&3 ; do
  IFS=$oldifs
  printf '%s\n' "$line"
done
IFS=$oldifs
exec 3>&-