ITS IS A VERY GOOD QUESTION TO ASKED.

Most of People in Govt Service dont even know about the permanent and temporary posts. I am actually liable to be paid for rendering this legal advice.

Reference from civil servants act:
"
Section 2, Definitions:
(e) "permanent post" means a post sanctioned without limit of time;
(i) "temporary post" means a post other than a permanent post. ............

3. Terms and conditions.– [(i)] The terms and conditions of a civil
servant shall be as provided in this Act and the rules.
*[(ii) The terms and conditions of service of any person to whom this
Act applies shall not be varied to his disadvantage]
........
"

When a Civil Servant is to be recruited, he is given offer letter that has terms and conditions. Accepting the said offer constitutes a contract between the Civil Servant and the Government Dept.
In reference to the Civil Servants act, if the offer letter does not state the duration of job or does not set any other time limit regards the service then the Civil Servant shall be deemed to be on permanent post subject to clearing of probation.

Kindly note that the terms and conditions can not be changed to a civil servant's disadvantage ( ref to section 3 of Civ Serv Act.)

EVEN WHEN THE ADVERTISEMENT STATES THAT THE JOB IS TEMPORARY OR TEMPORARY LIKELY TO BE PERMANENT IT IS THE OFFER LETTER THAT SPECIFIES THE ACTUAL STATUS OF JOB. (MOST OF THE TIMES OFFER LETTERS ARE ISSUES AS FOR PERMANENT POST)