User:Timothee Flutre/Notebook/Postdoc/2013/12/01

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One-liners for high-throughput sequencing data

  • softwares: see BWA, Bowtie, MOSAIK, etc; they take fastq files as input and return bam files as output
   IN1="reads_R1.fastq.gz"
   IN2="reads_R2.fastq.gz"
   OUT="alignments.bam"
  • total number of reads in the fastq file:
   nbLines=$(zcat $IN1 | wc -l); echo "scale=0; "${nbLines}"/4" | bc -l
  • total number of reads in the bam file: should be equal to the nb of reads in the fastq file if no filtering was made
   samtools view $OUT | cut -f1 | sort | uniq | wc -l
  • flag statistics in SAM/BAM:
   samtools flagstat $OUT

which returns something like: 

   4635834 + 0 in total (QC-passed reads + QC-failed reads)
   20290 + 0 secondary
   0 + 0 supplimentary
   0 + 0 duplicates
   4443270 + 0 mapped (95.85%:-nan%)
   4615544 + 0 paired in sequencing
   2307772 + 0 read1
   2307772 + 0 read2
   4299122 + 0 properly paired (93.14%:-nan%)
   4412810 + 0 with itself and mate mapped
   10170 + 0 singletons (0.22%:-nan%)
   57898 + 0 with mate mapped to a different chr
   44330 + 0 with mate mapped to a different chr (mapQ>=5)
  • list of different flags in the bam file: along with their number of occurrences
   samtools view $OUT | cut -f2 | sort | uniq -c | sort -k1,1nr -k2,2n
   # | awk '{sum+=$1}END{print sum}' # <- same as line 1

list of bits used to form the flags

this website is useful to interpret flags

for instance, if there are N entries with flag 99, there should also be N entries with flag 147; idem for 83 and 163; idem for 77 and 141; etc

  • total number of entries in the bam file: same as line 1
   samtools view $OUT | wc -l
  • total number of mapped entries: same as line 5
   samtools view -F 0x4 $OUT | wc -l
  • total number of unmapped entries: same as line 1 - line 5
   samtools view -f 0x4 $OUT | wc -l
  • not-primary/supplementary entries: see details
   samtools view -f 0x0100 $OUT | wc -l
   samtools view -f 0x0800 $OUT | wc -l
  • R1 and R2 entries:
   nbE1=$(samtools view -f 0x0040 -c $OUT)
   nbE2=$(samtools view -f 0x0080 -c $OUT)
   echo "$nbE1 + $nbE2" | bc -l # same as line 1
  • R1 and R2 reads:
   samtools view -f 0x0040 $OUT | cut -f1 | sort | uniq -c > tmp1
   samtools view -f 0x0080 $OUT | cut -f1 | sort | uniq -c > tmp2
   wc -l < tmp1 # same as line 7
   wc -l < tmp2 # same as line 8
   cat tmp1 | awk '{print $1}' | sort | uniq -c
   cat tmp2 | awk '{print $1}' | sort | uniq -c
  • entries which read is properly paired in sequencing: same as line 1 if both input fastq files have the same number of reads (i.e. all reads are properly paired before mapping)
   samtools view -f 0x0001 -c $OUT

Note that this is different from line 6 which is equal to line 1 - line 2.


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