curl

This curl example fetches a new access token when half of the time until expiration has passed.
#!/bin/sh

login=$( jq -n -c --arg username "$USERNAME" --arg password "$PASSWORD" '{"username": $username, "password": $password}' )
cookies=$( mktemp cookies.XXXXXXXXXX )

refresh() {
  if [ `date --utc --iso-8601=seconds` \< "$refresh" ]; then
    # no refresh needed
    return
  fi
  local response
  local body
  local now
  local expiration
  local seconds
  response=$( curl -w'\n%{response_code}\n' -sS -j -c "$cookies" -H "X-Okapi-Tenant: $TENANT" -H "Content-type: application/json" -d "$login" $OKAPI/authn/login-with-expiry )
  body=$( printf '%s\n' "$response" | head -n -1 )
  if printf '%s' "$response" | tail -1 | grep --invert-match 201; then
    printf '%s' "$body"
    rm "$cookies"
    exit 1
  fi
  now=$( date --utc +%s )
  expiration=$( date --date=`printf '%s' "$body" | jq -r .accessTokenExpiration` --utc +%s )
  # refresh after half of the time
  seconds=$( expr \( $expiration - $now \) / 2 )
  refresh=$( date --date="$seconds seconds" --utc --iso-8601=seconds )
}

for offset in `seq 0 10 40`; do
  refresh;
  curl -w"\n" -sS -D - -b "$cookies" -H "X-Okapi-Tenant: $TENANT" -H "X-Okapi-URL: $OKAPIURL" \
    -H "Content-type: application/json" -H "Accept: application/json" \
    --data-urlencode "query=$1" \
    --data-urlencode "limit=10" \
    --data-urlencode "offset=$offset" \
    --get "$OKAPI/item-storage/items"
done

rm "$cookies"