Dear candidate, filling the form and not appearing in exam is not considered as an attempt. However, attempting number of prelims is considered as attempts. If you attempted just one paper of prelims, then also it will be considered as an attempt. Visiting exam hall/ Signing in attendance sheet is considered as attempt. Giving one prelims, mains and interview is equivalent to an attempt or giving just prelims or giving prelims and mains. These all are an attempt. Total six attempts are allowed in UPSC for General category students.
Hope, it helps.
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