Why when splitting file in two we add 1 line before dividing the number of lines by 2?

Question

[salamandra]

In this tutorial there's a part in which they try to shuffle the lines of a file and then split the file into two. I do not understand why when splitting the file into 2, instead of dividing the number of lines by two they add 1 and then divide by two. Why is that?

That part of the code is:

nlines=$(samtools view merged.bam | wc -l ) # Number of reads in the BAM file
nlines=$(( (nlines + 1) / 2 )) # half that number
samtools view ${tmpDir}/${NAME1}_${NAME2}_merged.bam | shuf - | split -d -l ${nlines} - "${tmpDir}/${EXPT}" # This will shuffle the lines in the file and split it
 into two SAM files
cat ${tmpDir}/${EXPT}_header.sam ${tmpDir}/${EXPT}00 | samtools view -bS - > ${outputDir}/${EXPT}00.bam
cat ${tmpDir}/${EXPT}_header.sam ${tmpDir}/${EXPT}01 | samtools view -bS - > ${outputDir}/${EXPT}01.bam

[h.mon]

You can try for yourself, run the relevant part with part of the tutorial with nlines=$(( (nlines + 1) / 2 )) and nlines=$(( (nlines ) / 2 )), and see if it makes a difference.

You may also open an issue at the repo with your question. But please first try for yourself and check for differences, if any.

[Joe]

Are there an odd number of lines in the bam?

Answer 1

[salamandra]

Think I know now:

In bash it only gives the integer part of the number. If the original file has for e.g. 3 lines and we want to split in half, we want one file to have 2 lines and the other 1 line, but 3/2=1.5 and bash considers to be =1, so then the file will be split in 3 files each one with one line. If we add 1 before dividing by 2: (3+1)/2=2, so bash will put two lines into a file and the remaining 1 line in the other, as we wanted.

If the file has an even number of lines: for e.g original file has 4 lines, (4+1)/2=2.5 and bash thinks it's 2, so file will be split into files which will contain 2 lines each, so still does what we want for even numbers.