Simple examples of reading a file in the shell one line at a time.

Reading a file in the shell a single line at a time

cat file.txt | while read line || [[ -n $line ]];
do
   printf '%s\n' "$line"   # do something with $line here
done

Reading a file in the shell a single line at a time an internal field separator

while IFS= read -r line; do
   echo "$line"
done <file

Reading a file in the shell a single line at a time with a file descriptor

# Option 3: Single line at a time with a file descriptor

filename='file.txt'
exec 4<"$filename"
echo Start
while read -u4 p ; do
    echo "$p"
done

Reading a file in the shell a single line at a time with input redirection

# Option 2: Single line at a time with Input redirection
filename='file.txt'
echo Start
while read p; do 
    echo "$p"
done < "$filename"