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https://github.com/DarwinAwardWinner/mergesam

Automate common sam & bam conversions
https://github.com/DarwinAwardWinner/mergesam

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Automate common sam & bam conversions

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# mergesam: Automate common sam & bam conversions



Mergesam is a a tool to save you some time when interconverting sam

and bam formats. When was the last time you sorted a bam file and

*didn't* immediately index it afterward? For that matter, when was the

last time you didn't sort a bam file? Mergesam aims to have sensible

defaults and flexible options, in order to streamline your path from

sam to ready-to-use bam and back.



## Default: merge, sort, and index.



A common pattern that I have encountered when manipulating sam and bam

files is to merge several such files into one bam file, then sort that

bam file, then index it. With the mergesam script, this becomes a

single simple command:



    mergesam.py -o output.bam input1.sam input2.sam input3.sam



The result is that `output.bam` is a *sorted* bam file, with an

accompanying index file `output.bam.bai`. An equivalent series of

shell commands is:



    samtools view -h input1.sam > temp.sam

    samtools view input2.sam >> temp.sam

    samtools view input3.sam >> temp.sam

    samtools view -ubS temp.sam > temp.bam

    samtools sort temp.bam output

    samtools index output.bam

    rm temp.sam temp.bam



One could also implement it as a single pipeline, with no intermediate

files:



    {

        samtools view -h input1.sam

        samtools view input2.sam

        samtools view input3.sam

    } | samtools view -ubS | samtools sort - output

    samtools index output.bam



If you find yourself frequently writing code like the above while

manipulating sam and bam files, you might want to use mergesam

instead.



## Fully pipe compliant



Mergesam will happily read from standard input and write to standard

output. In fact, this is the default:



    cat input.sam | mergesam.py > output.bam



You can also specify standard input and output explicitly:



    cat input.sam | mergesam.py -o - - > output.bam



Unfortunately, when writing to standard output, it is impossible to

index the resulting file, since mergesam does not know where the

output file is (or even if one exists). So if you want indexing, use

the `-o` option with a filename, not a dash.



By the way, mergesam will never send binary (bam) output directly to

your terminal unless you ask it to, using the `--force_bam_tty`

option. Binary data will only be sent to standard output if it has

been redirected or piped into another command, so don't worry about

clobbering your terminal by accident.



## Variations: unsorted, unindexed, uncompressed



Since you usually want your bam files to be sorted and indexed,

sorting and indexing are both enabled by default. If you don't want

either, you can use the `--nosort` or `--noindex` flags (Note that

using both is redundant, since sorting is a prerequisite for

indexing.) There is also the `--sortbyname` flag, which corrseponds to

`samtools sort -n` -- sorting by read name instead of reference

coordinate. Lastly, you can output uncompressed bam with the

`--uncompressed` option (but note that sorted bam output is always

compressed).



## Input format is detected automatically



If the input files are already bam files, the command is the same.

Input file types are automatically detected:



    mergesam.py -o output.bam input1.bam input2.bam input3.bam



In fact, even if `input.bam` was actually a *sam* file, the above

command would still work. It looks at the contents of the file to

figure out what format it is. The one exception to this rule is that

*standard input is always sam format*, since one cannot detect the

format of an input stream without losing some of the input in the

process. If you need to pipe bam data into mergesam, you'll need to

pass it through `samtools view -h`.



    bam_producer_command | samtools view -h | mergesam.py -o output.bam



## Specifying a reference



If the sam files have no headers, you'll need a reference:



    mergesam.py -o output.bam input.bam -r reference.fa.fai



You can also supply the name of a fasta file, in which case the fasta

index will automatically be created and used.



    mergesam.py -o output.bam input.bam -r reference.fa



If you don't have permission to create the index file in the same

directory as the fasta file, the index will be created in a temporary

directory and deleted when the program is finished.



## SAM output



To output sam instead of bam, just use the `--sam` flag:



    mergesam.py --sam -o output.sam input.bam -r reference.fa



Note that sam output is neither sorted nor indexed. By default, the

sam output includes the header. To suppress it, use `--noheader`.