NGLess one liners

ngless can be used as a traditional command line transformer utility, using the -e argument to pass an inline script on the command line.

The -p (or --print-last) argument tells ngless to output the value of the last expression to stdout.

Converting a SAM file to a FASTQ file

$ ngless -pe 'as_reads(samfile("file.sam"))' > file.fq

This is equivalent to the full script:

ngless "1.4" # <- version declaration, optional on the command line
samcontents = samfile("file.sam") # <- load a SAM/BAM file
reads = as_reads(samcontents) # <- just get the reads (w quality scores)
write(reads, ofname=STDOUT) # <- write them to STDOUT (default format: FASTQ)

This only works if the data in the samfile is single ended as we pipe out a single FQ file. Otherwise, you can always do:

ngless "1.4"
write(as_read(samfile("file.sam")),
        ofile="output.fq")

which will write 3 files: output.1.fq, output.2.fq, and output.singles.fq (the first two for the paired-end reads and the last one for reads without a mate).

Getting aligned reads from a SAM file as FASTQ file

Building on the previous example. We can add a select() call to only output unmapped reads:

$ ngless -pe 'as_reads(select(samfile("file.sam"), keep_if=[{mapped}]))' > file.fq

This is equivalent to the full script:

ngless "1.4" # <- version declaration, optional on the command line
samcontents = samfile("file.sam") # <- load a SAM/BAM file
samcontents = select(samcontents, keep_if=[{mapped}]) # <- select only *mapped* reads
reads = as_reads(samcontents) # <- just get the reads (w quality scores)
write(reads, ofname=STDOUT) # <- write them to STDOUT (default format: FASTQ)

Reading from STDIN

For a true Unix-like utility, the input should be read from standard input. This can be achieved with the special file STDIN. So the previous example now reads:

$ cat file.sam | ngless -pe 'as_reads(select(samfile(STDIN), keep_if=[{mapped}]))' > file.fq

Obviously, this example would more interesting if the input were to come from another programme (not just cat).