library(GenomicAlignments)
library(rtracklayer)

bam_files<- list.files()

for(i in 1:length(bam_files)) {
    gr <- readGAlignments(bam_files[i])

    gr.cov <- coverage(gr)

    export.bw(gr.cov,paste0(bam_files[i], ".bigWig") )
   }