https://unix.stackexchange.com/questions/209123/understanding-ifs-read-r-line

two categories for IFS  and there are differences between space and others
 A: IFS default white spaces char 
 B: others

VAR=value command --> means "modify the environment of command so that VAR will have the value value". 
Basically, the command command will see VAR as having the value value, but any command executed 
after that will still see VAR as having its previous value. In other words, that variable will be modified only for that statement.

# default IFS (white spaces), and reset(clear) IFS
printf '   a b    c  ' | {       read -r line ; printf "[$line]\n"; }# [a b    c] note: get rid of spaces
printf '   a b    c  ' | { IFS=  read -r line ; printf "[$line]\n"; }# [   a b    c  ] keep the spaces

printf ':a:b :c:::'    | { IFS=: read -r n i j; printf "n=%s,i=%s,j=%s" "$n" "$i" "$j"; } # n=,i=a,j=b :c:::

echo ' where are my spaces? ' | { read -r line; printf %s\\n "$line"; }