I try to impliment 'gatk MergeBamAlignment', it required three arguments in description: output file, reference sequence and unmapped bam file. I enter the next command: ./miniconda3/pkgs/gatk4-188.8.131.52-hdfd78af_1/bin/gatk MergeBamAlignment -O ./exp/2step -R ./references/Homo_sapiens/hg38.fa -UNMAPPED_BAM ./exp/1step/lv_1u.bam. But it returns: Either ALIGNED_BAM or the combination of READ1_ALIGNED_BAM and READ2_ALIGNED_BAM must be supplied.
I can not understand why aligned_bam is required. Which is algorithem of recieving such bam-file?