Initialises Terraform HTTP backend to Gitlab. Here's a nice script which will ask for details in an interactive way.

main.tf

# More information can be found at https://docs.gitlab.com/ee/user/infrastructure/#gitlab-managed-terraform-state

terraform {
  backend "http" {
  }
}

terraform-gitlab-init.sh

#!/bin/bash

check_password() {
  security find-generic-password \
    -s "$1" > /dev/null 2>&1 && return 0 || return 1
}

get_username() {
  [ -z "$1" ] && { echo >&2 "get_password(): Service name required1"; return 1; }
  check_password "$1" || { echo >&2 "get_password(): No password found!"; return 1; }
  security find-generic-password \
    -D "application password" -s "$1" -g 2>&1 | grep "\"acct\"<blob>=" | cut -d '=' -f2 | cut -d '"' -f2
}

get_password() {
  [ -z "$1" ] && { echo >&2 "get_password(): Service name required1"; return 1; }
  check_password "$1" || { echo >&2 "get_password(): No password found!"; return 1; }
  security find-generic-password \
    -D "application password" -s "$1" -g 2>&1 | grep "password: " | cut -d '"' -f2
}

set_password() {
  [ -z "$1" ] && { echo >&2 "set_password(): Service name required1"; return 1; }
  [ -z "$2" ] && { echo >&2 "set_password(): Username required1"; return 1; }
  [ -z "$3" ] && { echo >&2 "set_password(): Password required1"; return 1; }
  
  security add-generic-password \
   -s "$1" \
   -a "$2" \
   -w "$3" \
   -j "Gitlab Terraform Setup Script Access token" \
   -D "application password" \
   -U
}

ask_user_details() {
  while read -p "Gitlab Username [$INPUT_TF_USERNAME]: " TF_USERNAME; do
    if [ -z "$INPUT_TF_USERNAME" ] && [ -z "$TF_USERNAME" ]; then
      echo "ERROR: gitlab Username cannot be empty!"
      continue
    elif [ -z $INPUT_TF_USERNAME ] && [ ! -z $TF_USERNAME ]; then
      INPUT_TF_USERNAME="$TF_USERNAME"
      break
    elif [ ! -z $INPUT_TF_USERNAME ] && [ -z $TF_USERNAME ]; then
      TF_USERNAME="$INPUT_TF_USERNAME"
      break
    elif [ ! -z $INPUT_TF_USERNAME ] && [ ! -z $TF_USERNAME ]; then
      INPUT_TF_USERNAME="$TF_USERNAME"
      break
    fi
  done

  while read -p "Access Token [$INPUT_TF_ACCESS_TOKEN]: " TF_ACCESS_TOKEN; do
    if [ -z "$INPUT_TF_ACCESS_TOKEN" ] && [ -z "$TF_ACCESS_TOKEN" ]; then
      echo "ERROR: Access Token cannot be empty!"
      continue
    elif [ -z $INPUT_TF_ACCESS_TOKEN ] && [ ! -z $TF_ACCESS_TOKEN ]; then
      INPUT_TF_ACCESS_TOKEN="$TF_ACCESS_TOKEN"
      break
    elif [ ! -z $INPUT_TF_ACCESS_TOKEN ] && [ -z $TF_ACCESS_TOKEN ]; then
      TF_ACCESS_TOKEN="$INPUT_TF_ACCESS_TOKEN"
      break
    elif [ ! -z $INPUT_TF_ACCESS_TOKEN ] && [ ! -z $TF_ACCESS_TOKEN ]; then
      INPUT_TF_ACCESS_TOKEN="$TF_ACCESS_TOKEN"
      break
    fi
  done

  while read -p "Project ID [$INPUT_PROJECT_ID]: " TF_PROJECT_ID; do  
    if [ -z "$INPUT_PROJECT_ID" ] && [ -z "$TF_PROJECT_ID" ]; then
      echo "ERROR: Project ID cannot be empty!"
      continue
    else
      if [[ ! $TF_PROJECT_ID =~ ^[0-9]+$ ]] && [[ ! $INPUT_PROJECT_ID =~ ^[0-9]+$ ]]; then
        echo "ERROR: Project ID can only contain digits!"
        continue
      elif [ -z $INPUT_PROJECT_ID ] && [ ! -z $TF_PROJECT_ID ]; then
        INPUT_PROJECT_ID="$TF_PROJECT_ID"
        break
      elif [ ! -z $INPUT_PROJECT_ID ] && [ -z $TF_PROJECT_ID ]; then
        TF_PROJECT_ID="$INPUT_PROJECT_ID"
        break
      elif [ ! -z $INPUT_PROJECT_ID ] && [ ! -z $TF_PROJECT_ID ]; then
        INPUT_PROJECT_ID="$TF_PROJECT_ID"
        break
      fi
    fi
  done
}

ask_state_name() {
  while read -p "Terraform State Name [$INPUT_TF_STATE_NAME]: " TF_STATE_NAME; do
    if [ -z "$INPUT_TF_STATE_NAME" ] && [ -z "$TF_STATE_NAME" ]; then
      echo "ERROR: State name cannot be empty!"
      continue
    elif [ -z $INPUT_TF_STATE_NAME ] && [ ! -z $TF_STATE_NAME ]; then
      INPUT_TF_STATE_NAME="$TF_STATE_NAME"
      break
    elif [ ! -z $INPUT_TF_STATE_NAME ] && [ -z $TF_STATE_NAME ]; then
      TF_STATE_NAME="$INPUT_TF_STATE_NAME"
      break
    elif [ ! -z $INPUT_TF_ACCESS_TOKEN ] && [ ! -z $TF_STATE_NAME ]; then
      INPUT_TF_STATE_NAME="$TF_STATE_NAME"
      break
    fi
  done
}

check_access_token() {
  request_cmd="$(curl -i -o - --silent -X GET --header "Accept: application/json" --header "PRIVATE-TOKEN: $1" "https://gitlab.com/api/v4/projects")"
  http_status=$(echo "$request_cmd" | grep HTTP |  awk '{print $2}')

  if [ $http_status -ge 200 ] && [ $http_status -le 299 ]; then
    return 0
  else
    return $http_status
  fi
}

get_project_details() {
  HTTP_RESPONSE="$(curl --write-out "\nHTTPSTATUS:%{http_code}" --silent -X GET --header "Accept: application/json" --header "PRIVATE-TOKEN: $1" "https://gitlab.com/api/v4/projects/$2")"
  HTTP_STATUS=$(echo $HTTP_RESPONSE | tr -d '\n' | sed -e 's/.*HTTPSTATUS://')

  if [ $HTTP_STATUS -ge 200 ] && [ $HTTP_STATUS -le 299 ]; then
    echo "$HTTP_RESPONSE" | sed -e 's/^HTTPSTATUS\:.*//g'
    return 0
  else
    return $HTTP_STATUS
  fi
}

INPUT_TF_USERNAME="${GITLAB_USERNAME:-"$1"}"
INPUT_TF_ACCESS_TOKEN="${GITLAB_ACCESS_TOKEN:-"$2"}"
INPUT_PROJECT_ID="$3"
INPUT_TF_STATE_NAME="$4"

if [ -z "$INPUT_TF_ACCESS_TOKEN" ]; then
  if check_password "gitlab"; then
    INPUT_TF_ACCESS_TOKEN=`get_password "gitlab"`
    SAVED_TF_ACCESS_TOKEN="$INPUT_TF_ACCESS_TOKEN"
  fi
fi

if [ -z "$INPUT_TF_USERNAME" ]; then
  if check_password "gitlab"; then
    INPUT_TF_USERNAME=`get_username "gitlab"`
    SAVED_TF_USERNAME="$INPUT_TF_USERNAME"
  fi
fi

DETAILS_CORRECT=""
PROJECT_DETAILS=""
while true; do
  ask_user_details
  check_access_token "$TF_ACCESS_TOKEN" || { echo >&2 "ERROR: Invalid Access Token"; continue; }
  PROJECT_DETAILS=`get_project_details "$TF_ACCESS_TOKEN" "$TF_PROJECT_ID"` || { echo >&2 "ERROR: Invalid Project ID"; continue; }
  [ -z $INPUT_TF_STATE_NAME ] && INPUT_TF_STATE_NAME="`echo "$PROJECT_DETAILS" | jq -r '.name'`-dev"
  ask_state_name
  read -p "Are these details correct? [Yn]: " -n 1 -r DETAILS_CORRECT
  [ -z $DETAILS_CORRECT ] && DETAILS_CORRECT="y"
  if [[ $DETAILS_CORRECT =~ ^[Yy]$ ]]; then
    printf "\n"
    break
  else
    printf "\n"
  fi
done

if [[ ( "$SAVED_TF_USERNAME" != "$TF_USERNAME" && -z "$GITLAB_USERNAME" ) ||
     ( "$SAVED_TF_ACCESS_TOKEN" != "$TF_ACCESS_TOKEN" && -z "$GITLAB_ACCESS_TOKEN" ) ]]; then
  read -p "Saved Gitlab Username or Access Token has changed. Update keychain? [Y]: " -n 1 -r ANSWER
  if [[ ! $ANSWER =~ ^[Nn]$ ]]; then
    set_password "gitlab" "$TF_USERNAME" "$TF_ACCESS_TOKEN"
  fi
  echo ""
fi

TF_ADDRESS="https://gitlab.com/api/v4/projects/${TF_PROJECT_ID}/terraform/state/${TF_STATE_NAME}"

echo "Now initailising repo..."
[ -d "$PWD/.terraform" ] && rm -Rf "$PWD/.terraform"
echo terraform init \
    -backend-config="address=${TF_ADDRESS}" \
    -backend-config="lock_address=${TF_ADDRESS}/lock" \
    -backend-config="unlock_address=${TF_ADDRESS}/lock" \
    -backend-config="username=${TF_USERNAME}" \
    -backend-config="password=${TF_ACCESS_TOKEN}" \
    -backend-config="lock_method=POST" \
    -backend-config="unlock_method=DELETE" \
    -backend-config="retry_wait_min=5"