Question: I would like to show a default value if the user doesn't fill in a field in the user profile.

For example: 

I have a Hair type field. If the user fills it in, i would like the following output:

**Hair type: long.**

It is already working. If the user doesn't fill in the field, i would like the following output:

**Hair type: Not specified.**

I use Display Suite to display the user profile. I don't want to set a default value to the field, because i just want to show the Not specified value on the user profile page.

Which contrib modules or hooks can i use to solve this problem?

Answer: 
To add default value to User Profile field. Go to admin/config/people/account settings/manage fields and edit that particular field. When you on the edit form of that field, there is a field to set `Default Value`. You can add any default value that you want in that field.
You can easily do this using [Rules](https://www.drupal.org/project/rules) module. In user accounts I create a field and I called it `field_hair_type` of type `text`.The Event, condition and action are as follow:

Event
=====

The Event must be `After updating an existing user account` 

[![enter image description here](https://i.stack.imgur.com/9O9tR.png)](https://i.stack.imgur.com/9O9tR.png)

Condition
=========

The condition must be `Data value is empty`. Now using *Data Selector* select the `hair type` field, as follow

[![enter image description here](https://i.stack.imgur.com/o1LoT.png)](https://i.stack.imgur.com/o1LoT.png)

Action
======

The action must be `Set a data value`, again same as **condition** section select the `hair type` field in *Data Selector* and after saving and set the value to `Not Specified.` as follow

[![enter image description here](https://i.stack.imgur.com/tikNM.png)](https://i.stack.imgur.com/tikNM.png)

**Now it is done**