One thing to be careful of when doing "while read..." is that a new shell is started on each iteration, so you cannot for example set a variable within the loop that you can use later in the script, as its value will be lost when the shell process exits.
printf "\n\n\n" | while read i; do a="x$a"; echo "$a"; done
x
xx
xxx
The accumulator value even carries over after the while loop:
printf "\n\n\n" | ( while read i; do a="x$a"; echo "$a"; done ; echo "$a" )
x
xx
xxx
xxx
(Technically, whether or not the loop body is executed in a subshell may be implementation dependent. Haven't looked at the POSIX shell spec in a while, but I seem to remember an old ksh that actually used subshells. At any rate, none of the modern sh's and bash force a subshell.)
What is true, however, is that a pipeline will execute in a subshell. Maybe that's what you're getting at here, and it is an important caveat.
a=y; printf "\n\n\n" | while read i; do a="x$a"; echo "$a"; done; echo "$a"
xy
xxy
xxxy
y
Ah, ok. True, when I have had this issue it was after doing something like 'grep "pattern" file | while read ...'. I did not realize it was the pipe that caused this.
