Bash retry function

retry.sh

# echo, but to stderr
function echoerr {
  echo "$@" 1>&2;
}
# Retry a command up to a specific numer of times until it exits successfully,
# with exponential back off.
#
#  $ retry 5 echo Hello
#  Hello
#
#  $ retry 5 false
#  Retry 1/5 exited 1, retrying in 15 seconds...
#  Retry 2/5 exited 1, retrying in 30 seconds...
#  Retry 3/5 exited 1, retrying in 60 seconds...
#  Retry 4/5 exited 1, retrying in 120 seconds...
#  Retry 5/5 exited 1, no more retries left.
#  
function retry {
  local WAIT_MODIFIER=15
  local retries=$1
  shift

  local count=0
  until "$@"; do
    exit=$?
    wait=$((2 ** $count * ${WAIT_MODIFIER:-1}))
    count=$(($count + 1))
    if [ $count -lt $retries ]; then
      echoerr "Retry $count/$retries exited $exit, retrying in $wait seconds..."
      sleep $wait
    else
      echoerr "Retry $count/$retries exited $exit, no more retries left."
      return $exit
    fi
  done
  return 0
}