exec 3<> /tmp/foo # open fd 3.
echo a >&3 # write to it
exec 3>&- # close fd 3.

FILE=/tmp/foo
echo a > "$FILE"

while IFS= read -r line; do
  printf '%sn' "$line"
  if IFS= read -r line; then printf '%sn' "$line" >&3; fi
done >odd.txt 3>even.txt

{ while … done | odd-filter >filtered-odd.txt; } 3>&1 | even-filter >filtered-even.txt

exec 8<&0 9>&1
exec >output12
command1
exec <input23
command2
exec >&9
command3
exec <&8

(
rm -f fifo
mkfifo fifo
exec 3<fifo   # open fifo for reading
trap "exit" 1 2 3 15
exec cat fifo | nl
) &
bpid=$!

(
exec 3>fifo  # open fifo for writing
trap "exit" 1 2 3 15
while true;
do
    echo "blah" > fifo
done
)
#kill -TERM $bpid

#!/bin/bash

log() {
    echo $* >&3
}
info() {
    echo $* >&4
}
err() {
    echo $* >&2
}
debug() {
    echo $* >&5
}

VERBOSE=1

while [[ $# -gt 0 ]]; do
    ARG=$1
    shift
    case $ARG in
        "-vv")
            VERBOSE=3
        ;;
        "-v")
            VERBOSE=2
        ;;
        "-q")
            VERBOSE=0
        ;;
        # More flags
        *)
        echo -n
        # Linear args
        ;;
    esac
done

for i in 1 2 3; do
    fd=$(expr 2 + $i)
    if [[ $VERBOSE -ge $i ]]; then
        eval "exec $fd>&1"
    else
        eval "exec $fd> /dev/null"
    fi
done

err "This will _always_ show up."
log "This is normally displayed, but can be prevented with -q"
info "This will only show up if -v is passed"
debug "This will show up for -vv"

env -i bash --norc   # clean up environment
set +o history
read -s -p "Enter your password: " passwd
exec 3<<<"$passwd"
mycommand <&3  # cat /dev/stdin in mycommand

arg1 string to echo
arg2 flag 0,1 print or not print to 3rd fd stdout descriptor
function ecko3 {
if [ "$2" -eq 1 ]; then
    exec 3>$(tty)
    echo -en "$1" | tee >(cat - >&3)
    exec 3>&-
else
    echo -en "$1"
fi
}

#exit if we can't obtain the lock
set -e

#open file descriptor 3 for writing
exec 3> /tmp/file.lock

#create an exclusive lock on the file using file descriptor 3
#exit if lock could not be obtained
flock -n 3

#execute serial code

#close the open file handle which releases the file lock
exec 3>&-