55 ILCS 5/3-8011
Whenever a position in a higher rank is to be filled, the Merit Commission shall certify to the sheriff the names of eligible persons who stand highest upon the promotional register for the rank to which the position belongs. The Commission shall make certifications for promotion on the basis of ascertained merit, seniority of service, and physical and other qualifying examinations.
The sheriff shall appoint from those whose names were certified. If the sheriff rejects all persons so certified, he shall state his reasons for such refusal in writing to the Commission.
All vacancies in all ranks of deputy sheriff above the lowest shall be filled by promotion, except that the Merit Commission may by regulation provide that a former sheriff may be appointed by a successor sheriff of the same county to any rank after he has been certified by the Commission or that applicants who have served with another sheriff's office, a police department, or any other law enforcement agency, may be given credit for time so served and may receive an initial appointment to a rank above the lowest.
Persons appointed to a higher rank shall be on probation in such higher rank for a period of 12 months. Such appointees may be demoted by the sheriff to their former rank at any time during the period of probation, if, in the opinion of the sheriff, they have failed to demonstrate the ability and the qualifications necessary to furnish satisfactory service.
55 ILCS 5/3-8011