需要用到一些测试数据，我准备拿17号染色体的40437407-40486397这约48Kb碱基区域来举例子，就需要把这个区域的bam提取出来。

我分别找了以前处理的wgs,wes,rna-seq,chip-seq公共数据，原始bam非常大，尤其是WGS的，45G的bam文件，所以只能抽取17号染色体的40437407-40486397这约48Kb碱基区域，以前我做mpileup或者其它都是用的-r 参数，所以我想当然的使用下面的代码：

samtools view -h -r chr17:40437407-40486397 your.sorted.merge.bam |samtools view -bS - >wes.bam

发现始终不对，让我着实郁闷，我就Google了一下，https://www.biostars.org/p/48719/

才明白，samtools的view命令的-r参数不再是用来指定坐标了！

samtools view -h  control_1.sort.bam   "chr17:40437407-40486397"  |samtools view -bS - >RNA-seq.bam

所以我修改了命令，完成了提取指定区域比对的reads的bam文件这个需求！

samtools view -h

Usage: samtools view [options] <in.bam>|<in.sam>|<in.cram> [region ...]

Options:
-b output BAM
-C output CRAM (requires -T)
-1 use fast BAM compression (implies -b)
-u uncompressed BAM output (implies -b)
-h include header in SAM output
-H print SAM header only (no alignments)
-c print only the count of matching records
-o FILE output file name [stdout]
-U FILE output reads not selected by filters to FILE [null]
-t FILE FILE listing reference names and lengths (see long help) [null]
-L FILE only include reads overlapping this BED FILE [null]
-r STR only include reads in read group STR [null]
-R FILE only include reads with read group listed in FILE [null]
-q INT only include reads with mapping quality >= INT [0]
-l STR only include reads in library STR [null]
-m INT only include reads with number of CIGAR operations consuming
query sequence >= INT [0]
-f INT only include reads with all bits set in INT set in FLAG [0]
-F INT only include reads with none of the bits set in INT set in FLAG [0]
-x STR read tag to strip (repeatable) [null]
-B collapse the backward CIGAR operation
-s FLOAT integer part sets seed of random number generator [0];
rest sets fraction of templates to subsample [no subsampling]
-@, --threads INT
number of BAM/CRAM compression threads [0]
-? print long help, including note about region specification
-S ignored (input format is auto-detected)
--input-fmt-option OPT[=VAL]
Specify a single input file format option in the form
of OPTION or OPTION=VALUE
-O, --output-fmt FORMAT[,OPT[=VAL]]...
Specify output format (SAM, BAM, CRAM)
--output-fmt-option OPT[=VAL]
Specify a single output file format option in the form
of OPTION or OPTION=VALUE
-T, --reference FILE
Reference sequence FASTA FILE [null]

接下来我们就看看如何读取这些bam文件的
library(pasillaBamSubset)
un1 <- untreated1_chr4() # single-end reads library(GenomicAlignments) reads1 <- readGAlignments(un1) cvg1 <- coverage(reads1) 查看reads1这个结果，可以看到把这个bam文件都读成了一个数据对象GAlignments object，
针对着个数据对象有很多操作，其中一个coverage操作是来自于GenomicFeatures
或者GenomicAlignments函数的，可以算出测序覆盖情况。
可以看到这个bam文件里面的比对情况大多几种在4号染色体里面
> cvg1$chr4
integer-Rle of length 1351857 with 122061 runs
Lengths: 891 27 5 12 13 45 5 12 13 ... 5 1 1 3 10
Values : 0 1 2 3 4 5 4 3 2 ... 12 11 10 6
> mean(cvg1$chr4)
[1] 11.33746
> max(cvg1$chr4)[1] 5627
可以看到平均测序深度是11.3X，最大测序深度是5627X