Bash: IFS delimiter

echo > test << EOF
a b c d
aa bb cc dd
a_b_c_d
EOF

for i in `cat test`; do echo $i; done    
a
b
c
d
aa
bb
cc
dd
a_b_c_d

IFS=$'_'
for i in `cat test`; do echo $i; done
a b c d
aa bb cc dd
a
b
c
d

IFS=$'\n'                                                                                                                                                              
for i in `cat test`; do echo $i;done   
a b c d
aa bb cc dd
a_b_c_d

IFS=$' '
for i in `cat test`; do echo $i; done
a
b
c
d
aa
bb
cc
dd
a_b_c_d