Changes

   mkdir result

   mkdir result

  samtools mpileup -r 20:1500000-2500000 -Iuf ref/human_g1k_v37_chr20.fa bams/*bam | bcftools view -bvcg - > result/chr20.bcf

    samtools mpileup -Iuf ref/human_g1k_v37_chr20.fa bams/SAMPLE*bam | bcftools view -bvcg - > result/chr20.mpileup.bcf

   bcftools view result/chr20.bcf  | sed s/AF1/AF/g | grep -v AF=1 > result/chr20.mpileup.vcf

   bcftools view result/chr20.bcf  | sed s/AF1/AF/g | grep -v AF=1 > result/chr20.mpileup.vcf