Bash Shell – What is the IFS Variable?

The trap is that

IFS=; while read..

sets the IFS for the whole shell environment outside the loop, whereas

while IFS= read

redefines it only for the read invocation (except in the Bourne shell). You can check that doing a loop like

while IFS= read xxx; ... done

then after such loop, echo "blabalbla $IFS ooooooo" prints

blabalbla
 ooooooo

whereas after

IFS=; read xxx; ... done

the IFS stays redefined: now echo "blabalbla $IFS ooooooo" prints

blabalbla  ooooooo

So if you use the second form, you have to remember to reset : IFS=$' \t\n'.

_{The second part of this question has been merged here, so I've removed the related answer from here.}

1. Yes, they are the same.
2. Yes.
3. In bash, and similar shells, you could do something like IFS=$' \t\n'. Otherwise, you could insert the literal control codes by using [space] CTRL+V [tab] CTRL+V [enter]. If you are planning to do this, however, it's better to use another variable to temporarily store the old IFS value, and then restore it afterwards (or temporarily override it for one command by using the var=foo command syntax).
4. • The first code snippet will put the entire line read, verbatim, into $line, as there are no field separators to perform word splitting for. Bear in mind however that since many shells use cstrings to store strings, the first instance of a NUL may still cause the appearance of it being prematurely terminated.
   • The second code snippet may not put an exact copy of the input into $line. For example, if there are multiple consecutive field separators, they will be made into a single instance of the first element. This is often recognised as loss of surrounding whitespace.
   • The third code snippet will do the same as the second, except it will only split on a space (not the usual space, tab, or newline).