Why does unix while read not read last line? [duplicate]

Question

My script

export IFS=":"

cat hello | while read a b c d; do
    echo $a,$b,$c,$d
done

My file hello

1:1:1:1
2:2:2:2
3:3:3:3

My output

1,1,1,1
2,2,2,2

If I put a blank line after 3:3:3:3 in hello then the output becomes

1,1,1,1
2,2,2,2
3,3,3,3

Anyone know how I can fix this problem so I don't need to put a blank line at the end of hello?

Answer 1

What's happening is, the read command fails when the input is not terminated with a newline. Since the newline character is missing at the end of your file, the read fails, so the last iteration of the while loop is skipped.

If you don't want to / cannot make sure that your input file has a newline at the end, you can group your cat with an echo to give the appearance of an input terminated by newline, for example like this:

{ cat hello; echo; } | while read a b c d; do
    echo $a,$b,$c,$d
done

or like this:

(cat hello; echo) | while read a b c d; do
    echo $a,$b,$c,$d
done

Answer 2

This is an extensive reading for the question why?

If you don't want to put a new line at the end, you can do this:

while IFS=: read -r a b c d || [ -n "$a" ]; 
do     
echo $a,$b,$c,$d; 
done < file

Or using grep:

while IFS=: read -r a b c d; 
do     
echo $a,$b,$c,$d; 
done < <(grep "" filee)

Note:

1. There's no need to use cat with while loop.
2. You should almost always use the -r option with read.