Provided by: samtools_1.22.1-1_amd64 
NAME
samtools - Utilities for the Sequence Alignment/Map (SAM) format
SYNOPSIS
samtools addreplacerg -r 'ID:fish' -r 'LB:1334' -r 'SM:alpha' -o output.bam input.bam
samtools ampliconclip -b bed.file input.bam
samtools ampliconstats primers.bed in.bam
samtools bedcov aln.sorted.bam
samtools calmd in.sorted.bam ref.fasta
samtools cat out.bam in1.bam in2.bam in3.bam
samtools checksum in.bam
samtools collate -o aln.name_collated.bam aln.sorted.bam
samtools consensus -o out.fasta in.bam
samtools coverage aln.sorted.bam
samtools cram-size -v -o out.size in.cram
samtools depad input.bam
samtools depth aln.sorted.bam
samtools dict -a GRCh38 -s "Homo sapiens" ref.fasta
samtools faidx ref.fasta
samtools fasta input.bam > output.fasta
samtools fastq input.bam > output.fastq
samtools fixmate in.namesorted.sam out.bam
samtools flags PAIRED,UNMAP,MUNMAP
samtools flagstat aln.sorted.bam
samtools fqidx ref.fastq
samtools head in.bam
samtools idxstats aln.sorted.bam
samtools import input.fastq > output.bam
samtools index aln.sorted.bam
samtools markdup in.algnsorted.bam out.bam
samtools merge out.bam in1.bam in2.bam in3.bam
samtools mpileup -f ref.fasta -r chr3:1,000-2,000 in1.bam in2.bam
samtools phase input.bam
samtools quickcheck in1.bam in2.cram
samtools reference -o ref.fa in.cram
samtools reheader in.header.sam in.bam > out.bam
samtools reset -o /tmp/reset.bam processed.bam
samtools samples input.bam
samtools sort -T /tmp/aln.sorted -o aln.sorted.bam aln.bam
samtools split merged.bam
samtools stats aln.sorted.bam
samtools targetcut input.bam
samtools tview aln.sorted.bam ref.fasta
samtools view -bt ref_list.txt -o aln.bam aln.sam.gz
DESCRIPTION
Samtools is a set of utilities that manipulate alignments in the SAM (Sequence Alignment/Map), BAM, and
CRAM formats. It converts between the formats, does sorting, merging and indexing, and can retrieve
reads in any regions swiftly.
Samtools is designed to work on a stream. It regards an input file `-' as the standard input (stdin) and
an output file `-' as the standard output (stdout). Several commands can thus be combined with Unix
pipes. Samtools always output warning and error messages to the standard error output (stderr).
Samtools is also able to open files on remote FTP or HTTP(S) servers if the file name starts with
`ftp://', `http://', etc. Samtools checks the current working directory for the index file and will
download the index upon absence. Samtools does not retrieve the entire alignment file unless it is asked
to do so.
If an index is needed, samtools looks for the index suffix appended to the filename, and if that isn't
found it tries again without the filename suffix (for example in.bam.bai followed by in.bai). However if
an index is in a completely different location or has a different name, both the main data filename and
index filename can be pasted together with ##idx##. For example /data/in.bam##idx##/indices/in.bam.bai
may be used to explicitly indicate where the data and index files reside.
COMMANDS
Each command has its own man page which can be viewed using e.g. man samtools-view or with a recent GNU
man using man samtools view. Below we have a brief summary of syntax and sub-command description.
Options common to all sub-commands are documented below in the GLOBAL COMMAND OPTIONS section.
view samtools view [options] in.sam|in.bam|in.cram [region...]
With no options or regions specified, prints all alignments in the specified input alignment
file (in SAM, BAM, or CRAM format) to standard output in SAM format (with no header by
default).
You may specify one or more space-separated region specifications after the input filename to
restrict output to only those alignments which overlap the specified region(s). Use of region
specifications requires a coordinate-sorted and indexed input file.
Options exist to change the output format from SAM to BAM or CRAM, so this command also acts as
a file format conversion utility.
tview samtools tview [-p chr:pos] [-s STR] [-d display] <in.sorted.bam> [ref.fasta]
Text alignment viewer (based on the ncurses library). In the viewer, press `?' for help and
press `g' to check the alignment start from a region in the format like `chr10:10,000,000' or
`=10,000,000' when viewing the same reference sequence.
quickcheck
samtools quickcheck [options] in.sam|in.bam|in.cram [ ... ]
Quickly check that input files appear to be intact. Checks that beginning of the file contains
a valid header (all formats) containing at least one target sequence and then seeks to the end
of the file and checks that an end-of-file (EOF) is present and intact (BAM only).
Data in the middle of the file is not read since that would be much more time consuming, so
please note that this command will not detect internal corruption, but is useful for testing
that files are not truncated before performing more intensive tasks on them.
This command will exit with a non-zero exit code if any input files don't have a valid header
or are missing an EOF block. Otherwise it will exit successfully (with a zero exit code).
checksum samtools checksum [options] in.sam|in.bam|in.cram
samtools checksum produces a CRC32 based checksum of data contained within a BAM file. This
can either be order and orientation agnostic for purposes of validating all the sequencing data
has passed through the entire pipeline from FASTQ through alignment and sorting, or full
alignment information and order aware for the purposes of validating format conversions and
while file data processing.
head samtools head [options] in.sam|in.bam|in.cram
Prints the input file's headers and optionally also its first few alignment records. This
command always displays the headers as they are in the file, never adding an extra @PG header
itself.
index samtools index [-bc] [-m INT] aln.sam.gz|aln.bam|aln.cram [out.index]
Index a coordinate-sorted SAM, BAM or CRAM file for fast random access. Note for SAM this only
works if the file has been BGZF compressed first. (Starting from Samtools 1.16, this command
can also be given several alignment filenames, which are indexed individually.)
This index is needed when region arguments are used to limit samtools view and similar commands
to particular regions of interest.
If an output filename is given, the index file will be written to out.index. Otherwise, for a
CRAM file aln.cram, index file aln.cram.crai will be created; for a BAM or SAM file aln.bam,
either aln.bam.bai or aln.bam.csi will be created, depending on the index format selected.
sort samtools sort [-l level] [-m maxMem] [-o out.bam] [-O format] [-n] [-t tag] [-T tmpprefix] [-@
threads] [in.sam|in.bam|in.cram]
Sort alignments by leftmost coordinates, or by read name when -n is used. An appropriate @HD-
SO sort order header tag will be added or an existing one updated if necessary.
The sorted output is written to standard output by default, or to the specified file (out.bam)
when -o is used. This command will also create temporary files tmpprefix.%d.bam as needed when
the entire alignment data cannot fit into memory (as controlled via the -m option).
Consider using samtools collate instead if you need name collated data without a full
lexicographical sort.
Note that if the sorted output file is to be indexed with samtools index, the default
coordinate sort must be used. Thus the -n and -t options are incompatible with samtools index.
collate samtools collate [options] in.sam|in.bam|in.cram [<prefix>]
Shuffles and groups reads together by their names. A faster alternative to a full query name
sort, collate ensures that reads of the same name are grouped together in contiguous groups,
but doesn't make any guarantees about the order of read names between groups.
The output from this command should be suitable for any operation that requires all reads from
the same template to be grouped together.
cram-size samtools cram-size [options] in.cram
Produces a summary of CRAM block Content ID numbers and their associated Data Series stored
within them. Optionally a more detailed breakdown of how each data series is encoded per
container may also be listed using the -e or --encodings option.
idxstats samtools idxstats in.sam|in.bam|in.cram
Retrieve and print stats in the index file corresponding to the input file. Before calling
idxstats, the input BAM file should be indexed by samtools index.
If run on a SAM or CRAM file or an unindexed BAM file, this command will still produce the same
summary statistics, but does so by reading through the entire file. This is far slower than
using the BAM indices.
The output is TAB-delimited with each line consisting of reference sequence name, sequence
length, # mapped reads and # unmapped reads. It is written to stdout.
flagstat samtools flagstat in.sam|in.bam|in.cram
Does a full pass through the input file to calculate and print statistics to stdout.
Provides counts for each of 13 categories based primarily on bit flags in the FLAG field. Each
category in the output is broken down into QC pass and QC fail, which is presented as "#PASS +
#FAIL" followed by a description of the category.
flags samtools flags INT|STR[,...]
Convert between textual and numeric flag representation.
FLAGS:
0x1 PAIRED paired-end (or multiple-segment) sequencing technology
0x2 PROPER_PAIR each segment properly aligned according to the aligner
0x4 UNMAP segment unmapped
0x8 MUNMAP next segment in the template unmapped
0x10 REVERSE SEQ is reverse complemented
0x20 MREVERSE SEQ of the next segment in the template is reverse complemented
0x40 READ1 the first segment in the template
0x80 READ2 the last segment in the template
0x100 SECONDARY secondary alignment
0x200 QCFAIL not passing quality controls
0x400 DUP PCR or optical duplicate
0x800 SUPPLEMENTARY supplementary alignment
stats samtools stats [options] in.sam|in.bam|in.cram [region...]
samtools stats collects statistics from BAM files and outputs in a text format. The output can
be visualized graphically using plot-bamstats.
bedcov samtools bedcov [options] region.bed in1.sam|in1.bam|in1.cram[...]
Reports the total read base count (i.e. the sum of per base read depths) for each genomic
region specified in the supplied BED file. The regions are output as they appear in the BED
file and are 0-based. Counts for each alignment file supplied are reported in separate
columns.
depth samtools depth [options] [in1.sam|in1.bam|in1.cram [in2.sam|in2.bam|in2.cram] [...]]
Computes the read depth at each position or region.
ampliconstats
samtools ampliconstats [options] primers.bed in.sam|in.bam|in.cram[...]
samtools ampliconstats collects statistics from one or more input alignment files and produces
tables in text format. The output can be visualized graphically using plot-ampliconstats.
The alignment files should have previously been clipped of primer sequence, for example by
samtools ampliconclip and the sites of these primers should be specified as a bed file in the
arguments.
mpileup samtools mpileup [-EB] [-C capQcoef] [-r reg] [-f in.fa] [-l list] [-Q minBaseQ] [-q minMapQ]
in.bam [in2.bam [...]]
Generate textual pileup for one or multiple BAM files. For VCF and BCF output, please use the
bcftools mpileup command instead. Alignment records are grouped by sample (SM) identifiers in
@RG header lines. If sample identifiers are absent, each input file is regarded as one sample.
See the samtools-mpileup man page for a description of the pileup format and options.
consensus samtools consensus [options] in.bam
Generate consensus from a SAM, BAM or CRAM file based on the contents of the alignment records.
The consensus is written either as FASTA, FASTQ, or a pileup oriented format.
The default output for FASTA and FASTQ formats include one base per non-gap consensus. Hence
insertions with respect to the aligned reference will be included and deletions removed. This
behaviour can be adjusted.
Two consensus calling algorithms are offered. The default computes a heterozygous consensus in
a Bayesian manner, derived from the "Gap5" consensus algorithm. A simpler base frequency
counting method is also available.
reference samtools reference [options] in.bam
Generate a reference from a SAM, BAM or CRAM file based on the contents of the SEQuence field
and the MD:Z: auxiliary tags, or from the embedded reference blocks within a CRAM file
(provided it was constructed using the embed_ref=1 option).
coverage samtools coverage [options] [in1.sam|in1.bam|in1.cram [in2.sam|in2.bam|in2.cram] [...]]
Produces a histogram or table of coverage per chromosome.
merge samtools merge [-nur1f] [-h inh.sam] [-t tag] [-R reg] [-b list] out.bam in1.bam [in2.bam
in3.bam ... inN.bam]
Merge multiple sorted alignment files, producing a single sorted output file that contains all
the input records and maintains the existing sort order.
If -h is specified the @SQ headers of input files will be merged into the specified header,
otherwise they will be merged into a composite header created from the input headers. If the
@SQ headers differ in order this may require the output file to be re-sorted after merge.
The ordering of the records in the input files must match the usage of the -n and -t command-
line options. If they do not, the output order will be undefined. See sort for information
about record ordering.
split samtools split [options] merged.sam|merged.bam|merged.cram
Splits a file by read group, producing one or more output files matching a common prefix (by
default based on the input filename) each containing one read-group.
cat samtools cat [-b list] [-h header.sam] [-o out.bam] in1.bam in2.bam [ ... ]
Concatenate BAMs or CRAMs. Although this works on either BAM or CRAM, all input files must be
the same format as each other. The sequence dictionary of each input file must be identical,
although this command does not check this. This command uses a similar trick to reheader which
enables fast BAM concatenation.
import samtools import [options] in.fastq [ ... ]
Converts one or more FASTQ files to unaligned SAM, BAM or CRAM. These formats offer a richer
capability of tracking sample meta-data via the SAM header and per-read meta-data via the
auxiliary tags. The fastq command may be used to reverse this conversion.
fastq/a samtools fastq [options] in.bam
samtools fasta [options] in.bam
Converts a BAM or CRAM into either FASTQ or FASTA format depending on the command invoked. The
files will be automatically compressed if the file names have a .gz, .bgz, or .bgzf extension.
The input to this program must be collated by name. Use samtools collate or samtools sort -n
to ensure this.
faidx samtools faidx <ref.fasta> [region1 [...]]
Index reference sequence in the FASTA format or extract subsequence from indexed reference
sequence. If no region is specified, faidx will index the file and create <ref.fasta>.fai on
the disk. If regions are specified, the subsequences will be retrieved and printed to stdout in
the FASTA format.
The input file can be compressed in the BGZF format.
FASTQ files can be read and indexed by this command. Without using --fastq any extracted
subsequence will be in FASTA format.
fqidx samtools fqidx <ref.fastq> [region1 [...]]
Index reference sequence in the FASTQ format or extract subsequence from indexed reference
sequence. If no region is specified, fqidx will index the file and create <ref.fastq>.fai on
the disk. If regions are specified, the subsequences will be retrieved and printed to stdout in
the FASTQ format.
The input file can be compressed in the BGZF format.
samtools fqidx should only be used on fastq files with a small number of entries. Trying to
use it on a file containing millions of short sequencing reads will produce an index that is
almost as big as the original file, and searches using the index will be very slow and use a
lot of memory.
dict samtools dict ref.fasta|ref.fasta.gz
Create a sequence dictionary file from a fasta file.
calmd samtools calmd [-Eeubr] [-C capQcoef] aln.bam ref.fasta
Generate the MD tag. If the MD tag is already present, this command will give a warning if the
MD tag generated is different from the existing tag. Output SAM by default.
Calmd can also read and write CRAM files although in most cases it is pointless as CRAM
recalculates MD and NM tags on the fly. The one exception to this case is where both input and
output CRAM files have been / are being created with the no_ref option.
fixmate samtools fixmate [-rpcm] [-O format] in.nameSrt.bam out.bam
Fill in mate coordinates, ISIZE and mate related flags from a name-sorted alignment.
markdup samtools markdup [-l length] [-r] [-s] [-T] [-S] in.algsort.bam out.bam
Mark duplicate alignments from a coordinate sorted file that has been run through samtools
fixmate with the -m option. This program relies on the MC and ms tags that fixmate provides.
rmdup samtools rmdup [-sS] <input.srt.bam> <out.bam>
This command is obsolete. Use markdup instead.
addreplacerg
samtools addreplacerg [-r rg-line | -R rg-ID] [-m mode] [-l level] [-o out.bam] in.bam
Adds or replaces read group tags in a file.
reheader samtools reheader [-iP] in.header.sam in.bam
Replace the header in in.bam with the header in in.header.sam. This command is much faster
than replacing the header with a BAM→SAM→BAM conversion.
By default this command outputs the BAM or CRAM file to standard output (stdout), but for CRAM
format files it has the option to perform an in-place edit, both reading and writing to the
same file. No validity checking is performed on the header, nor that it is suitable to use
with the sequence data itself.
targetcut samtools targetcut [-Q minBaseQ] [-i inPenalty] [-0 em0] [-1 em1] [-2 em2] [-f ref] in.bam
This command identifies target regions by examining the continuity of read depth, computes
haploid consensus sequences of targets and outputs a SAM with each sequence corresponding to a
target. When option -f is in use, BAQ will be applied. This command is only designed for
cutting fosmid clones from fosmid pool sequencing [Ref. Kitzman et al. (2010)].
phase samtools phase [-AF] [-k len] [-b prefix] [-q minLOD] [-Q minBaseQ] in.bam
Call and phase heterozygous SNPs.
depad samtools depad [-SsCu1] [-T ref.fa] [-o output] in.bam
Converts a BAM aligned against a padded reference to a BAM aligned against the depadded
reference. The padded reference may contain verbatim "*" bases in it, but "*" bases are also
counted in the reference numbering. This means that a sequence base-call aligned against a
reference "*" is considered to be a cigar match ("M" or "X") operator (if the base-call is "A",
"C", "G" or "T"). After depadding the reference "*" bases are deleted and such aligned
sequence base-calls become insertions. Similarly transformations apply for deletions and
padding cigar operations.
ampliconclip
samtools ampliconclip [-o out.file] [-f stat.file] [--soft-clip] [--hard-clip] [--both-ends]
[--strand] [--clipped] [--fail] [--no-PG] -b bed.file in.file
Clip reads in a SAM compatible file based on data from a BED file.
samples samtools samples [-o out.file] [-i] [-T TAG] [-f refs.fasta] [-F refs_list] [-X]
Prints the samples from alignment files
reset samtools reset [-o FILE] [-x/--remove-tag tag_list] [--keep-tag tag_list] [--reject-PG pgid]
[--no-RG] [--no-PG] [...]
Removes alignment information from records, producing an unaligned SAM, BAM or CRAM file.
Flags are reset, header tags are updated or removed as appropriate, and auxiliary tags are
removed or retained as specified. Note that the sort order is unchanged.
SAMTOOLS OPTIONS
These are options that are passed after the samtools command, before any sub-command is specified.
help, --help
Display a brief usage message listing the samtools commands available. If the name of a command
is also given, e.g., samtools help view, the detailed usage message for that particular command is
displayed.
--version
Display the version numbers and copyright information for samtools and the important libraries
used by samtools.
--version-only
Display the full samtools version number in a machine-readable format.
GLOBAL COMMAND OPTIONS
Several long-options are shared between multiple samtools sub-commands: --input-fmt, --input-fmt-option,
--output-fmt, --output-fmt-option, --reference, --write-index, and --verbosity. The input format is
auto-detected and specifying the format is unnecessary, so this option is rarely offered. Note that not
all subcommands have all options. Consult the subcommand help for more details.
Format strings recognised are "sam", "sam.gz", "bam" and "cram". They may be followed by a comma
separated list of options as key or key=value. See below for examples.
The fmt-option arguments accept either a single option or option=value. Note that some options only work
on some file formats and only on read or write streams. If value is unspecified for a boolean option,
the value is assumed to be 1. The valid options are as follows.
level=INT
Output only. Specifies the compression level from 1 to 9, or 0 for uncompressed. If the output
format is SAM, this also enables BGZF compression, otherwise SAM defaults to uncompressed.
nthreads=INT
Specifies the number of threads to use during encoding and/or decoding. For BAM this will be
encoding only. In CRAM the threads are dynamically shared between encoder and decoder.
filter=STRING
Apply filter STRING to all incoming records, rejecting any that do not satisfy the expression. See
the FILTER EXPRESSIONS section below for specifics.
reference=fasta_file
Specifies a FASTA reference file for use in CRAM encoding or decoding. It usually is not required
for decoding except in the situation of the MD5 not being obtainable via the REF_PATH or REF_CACHE
environment variables.
decode_md=0|1
CRAM input only; defaults to 1 (on). CRAM does not typically store MD and NM tags, preferring to
generate them on the fly. When this option is 0 missing MD, NM tags will not be generated. It can
be particularly useful when combined with a file encoded using store_md=1 and store_nm=1.
store_md=0|1
CRAM output only; defaults to 0 (off). CRAM normally only stores MD tags when the reference is
unknown and lets the decoder generate these values on-the-fly (see decode_md).
store_nm=0|1
CRAM output only; defaults to 0 (off). CRAM normally only stores NM tags when the reference is
unknown and lets the decoder generate these values on-the-fly (see decode_md).
ignore_md5=0|1
CRAM input only; defaults to 0 (off). When enabled, md5 checksum errors on the reference sequence
and block checksum errors within CRAM are ignored. Use of this option is strongly discouraged.
required_fields=bit-field
CRAM input only; specifies which SAM columns need to be populated. By default all fields are used.
Limiting the decode to specific columns can have significant performance gains. The bit-field is a
numerical value constructed from the following table.
0x1 SAM_QNAME
0x2 SAM_FLAG
0x4 SAM_RNAME
0x8 SAM_POS
0x10 SAM_MAPQ
0x20 SAM_CIGAR
0x40 SAM_RNEXT
0x80 SAM_PNEXT
0x100 SAM_TLEN
0x200 SAM_SEQ
0x400 SAM_QUAL
0x800 SAM_AUX
0x1000 SAM_RGAUX
name_prefix=string
CRAM input only; defaults to output filename. Any sequences with auto-generated read names will use
string as the name prefix.
multi_seq_per_slice=0|1
CRAM output only; defaults to 0 (off). By default CRAM generates one container per reference
sequence, except in the case of many small references (such as a fragmented assembly).
version=major.minor
CRAM output only. Specifies the CRAM version number. Acceptable values are "2.1", "3.0", and "3.1".
seqs_per_slice=INT
CRAM output only; defaults to 10000.
slices_per_container=INT
CRAM output only; defaults to 1. The effect of having multiple slices per container is to share the
compression header block between multiple slices. This is unlikely to have any significant impact
unless the number of sequences per slice is reduced. (Together these two options control the
granularity of random access.)
embed_ref=0|1
CRAM output only; defaults to 0 (off). If 1, this will store portions of the reference sequence in
each slice, permitting decode without having requiring an external copy of the reference sequence.
no_ref=0|1
CRAM output only; defaults to 0 (off). If 1, sequences will be stored verbatim with no reference
encoding. This can be useful if no reference is available for the file.
use_bzip2=0|1
CRAM output only; defaults to 0 (off). Permits use of bzip2 in CRAM block compression.
use_lzma=0|1
CRAM output only; defaults to 0 (off). Permits use of lzma in CRAM block compression.
use_arith=0|1
CRAM ≥ 3.1 output only; enables use of arithmetic entropy coding in CRAM block compression. This is
off by default, but enabled for archive mode. This is significantly slower but sometimes smaller
than the standard rANS entropy encoder.
use_fqz=0|1
CRAM ≥ 3.1 output only; enables and disables the fqzcomp quality compression method. This is on by
default for version 3.1 and above only when the small and archive profiles are in use.
use_tok=0|1
CRAM ≥ 3.1 output only; enables and disables the name tokeniser compression method. This is on by
default for version 3.1 and above.
lossy_names=0|1
CRAM output only; defaults to 0 (off). If 1, templates with all members within the same CRAM slice
will have their read names removed. New names will be automatically generated during decoding. Also
see the name_prefix option.
fast, normal, small, archive
CRAM output only. Set the CRAM compression profile. This is a simplified way of setting many output
options at once. It changes the following options according to the profile in use. The "normal"
profile is the default.
Option fast normal small archive
level 1 5 6 7
use_bzip2 off off on on
use_lzma off off off on if level>7
use_tok(*) off on on on
use_fqz(*) off off on on
use_arith(*) off off off on
seqs_per_slice 10000 10000 25000 100000
(*) use_tok, use_fqz and use_arith are only enabled for CRAM version 3.1 and above.
The level listed is only the default value, and will not be set if it has been explicitly changed
already. Additionally bases_per_slice is set to 500*seqs_per_slice unless previously explicitly set.
fastq_name2
FASTQ input only. Indicates that the names are not the first word in the header, but the second.
This is a FASTQ variant commonly used in the SRA and ENA archives.
fastq_casava
FASTQ input and output only. The Illumina CASAVA identifiers are stored in the second word of the
FASTQ header lines and store read meta-data. The CASAVA tag defines the data held in the READ1,
READ2 and QCFAIL flags and the barcode auxiliary tag ("BC" by default). This option may be used to
both read and write CASAVA identifiers.
fastq_barcode=TAG
FASTQ input and output only. When the fastq_casava option is used, this controls the name of the
barcode aux tag to be used. TAG defaults to "BC" if not specified.
fastq_aux=LIST
FASTQ input and output only. Processes SAM format auxiliary tags following the other fields on the
record identifier lines. If no =LIST is specified or LIST is "1" then all aux tags listed are copied
to/from the SAM record. Otherwise it is a comma separated list of 2-letter tag types and is used to
control which tags are processed with any others being omitted.
Note as commas are used to separate options in the --output-fmt string detailing file format and
options combined together, you will need to use the --output-fmt-option option if you want to specify
a comma separated list of tag types.
fastq_rnum
FASTQ output only. If set, paired reads will have "/1" and "/2" appended to their read names. This
has no effect on unpaired reads. When reading FASTQ these suffixes are automatically detected and
processed irrespective of the fastq_rnum option.
For example:
samtools view --input-fmt-option decode_md=0
--output-fmt cram,version=3.0 --output-fmt-option embed_ref
--output-fmt-option seqs_per_slice=2000 -o foo.cram foo.bam
samtools view -O cram,small -o bar.cram bar.bam
The --write-index option enables automatic index creation while writing out BAM, CRAM or bgzf SAM files.
Note to get compressed SAM as the output format you need to manually request a compression level,
otherwise all SAM files are uncompressed. By default SAM and BAM will use CSI indices while CRAM will
use CRAI indices. If you need to create BAI indices note that it is possible to specify the name of the
index being written to, and hence the format, by using the filename##idx##indexname notation.
For example: to convert a BAM to a compressed SAM with CSI indexing:
samtools view -h -O sam,level=6 --write-index in.bam -o out.sam.gz
To convert a SAM to a compressed BAM using BAI indexing:
samtools view --write-index in.sam -o out.bam##idx##out.bam.bai
The --verbosity INT option sets the verbosity level for samtools and HTSlib. The default is 3
(HTS_LOG_WARNING); 2 reduces warning messages and 0 or 1 also reduces some error messages, while values
greater than 3 produce increasing numbers of additional warnings and logging messages.
FILTER EXPRESSIONS
Filter expressions are used as an on-the-fly checking of incoming SAM, BAM or CRAM records, discarding
records that do not match the specified expression.
The language used is primarily C style, but with a few differences in the precedence rules for bit
operators and the inclusion of regular expression matching.
The operator precedence, from strongest binding to weakest, is:
Grouping (, ) E.g. "(1+2)*3"
Values: literals, vars Numbers, strings and variables
Unary ops: +, -, !, ~ E.g. -10 +10, !10 (not), ~5 (bit not)
Math ops: *, /, % Multiply, division and (integer) modulo
Math ops: +, - Addition / subtraction
Bit-wise: & Integer AND
Bit-wise ^ Integer XOR
Bit-wise | Integer OR
Conditionals: >, >=, <, <=
Equality: ==, !=, =~, !~ =~ and !~ match regular expressions
Boolean: &&, || Logical AND / OR
Expressions are computed using floating point mathematics, so "10 / 4" evaluates to 2.5 rather than 2.
They may be written as integers in decimal or "0x" plus hexadecimal, and floating point with or without
exponents.However operations that require integers first do an implicit type conversion, so "7.9 % 5" is
2 and "7.9 & 4.1" is equivalent to "7 & 4", which is 4. Strings are always specified using double
quotes. To get a double quote in a string, use backslash. Similarly a double backslash is used to get a
literal backslash. For example ab\"c\\d is the string ab"c\d.
Comparison operators are evaluated as a match being 1 and a mismatch being 0, thus "(2 > 1) + (3 < 5)"
evaluates as 2. All comparisons involving undefined (null) values are deemed to be false.
The variables are where the file format specifics are accessed from the expression. The variables
correspond to SAM fields, for example to find paired alignments with high mapping quality and a very
large insert size, we may use the expression "mapq >= 30 && (tlen >= 100000 || tlen <= -100000)". Valid
variable names and their data types are:
endpos int Alignment end position (1-based)
flag int Combined FLAG field
flag.paired int Single bit, 0 or 1
flag.proper_pair int Single bit, 0 or 2
flag.unmap int Single bit, 0 or 4
flag.munmap int Single bit, 0 or 8
flag.reverse int Single bit, 0 or 16
flag.mreverse int Single bit, 0 or 32
flag.read1 int Single bit, 0 or 64
flag.read2 int Single bit, 0 or 128
flag.secondary int Single bit, 0 or 256
flag.qcfail int Single bit, 0 or 512
flag.dup int Single bit, 0 or 1024
flag.supplementary int Single bit, 0 or 2048
hclen int Number of hard-clipped bases
library string Library (LB header via RG)
mapq int Mapping quality
mpos int Synonym for pnext
mrefid int Mate reference number (0 based)
mrname string Synonym for rnext
ncigar int Number of cigar operations
pnext int Mate's alignment position (1-based)
pos int Alignment position (1-based)
qlen int Alignment length: no. query bases
qname string Query name
qual string Quality values (raw, 0 based)
refid int Integer reference number (0 based)
rlen int Alignment length: no. reference bases
rname string Reference name
rnext string Mate's reference name
sclen int Number of soft-clipped bases
seq string Sequence
tlen int Template length (insert size)
[XX] int / string XX tag value
Flags are returned either as the whole flag value or by checking for a single bit. Hence the filter
expression flag.dup is equivalent to flag & 1024.
"qlen" and "rlen" are measured using the CIGAR string to count the number of query (sequence) and
reference bases consumed. Note "qlen" may not exactly match the length of the "seq" field if the
sequence is "*".
"sclen" and "hclen" are the number of soft and hard-clipped bases respectively. The formula "qlen-sclen"
gives the number of sequence bases used in the alignment, distinguishing between global alignment and
local alignment length.
"endpos" is the (1-based inclusive) position of the rightmost mapped base of the read, as measured using
the CIGAR string, and for mapped reads is equivalent to "pos+rlen-1". For unmapped reads, it is the same
as "pos".
Reference names may be matched either by their string forms ("rname" and "mrname") or as the Nth @SQ line
(counting from zero) as stored in BAM using "tid" and "mtid" respectively.
Auxiliary tags are described in square brackets and these expand to either integer or string as defined
by the tag itself (XX:Z:string or XX:i:int). For example [NM]>=10 can be used to look for alignments
with many mismatches and [RG]=~"grp[ABC]-" will match the read-group string.
If no comparison is used with an auxiliary tag it is taken simply to be a test for the existence of that
tag. So [NM] will return any record containing an NM tag, even if that tag is zero (NM:i:0). In htslib
<= 1.15 negating this with ![NM] gave misleading results as it was true if the tag did not exist or did
exist but was zero. Now this is strictly does-not-exist. An explicit exists([NM]) and !exists([NM])
function has also been added to make this intention clear.
Similarly in htslib <= 1.15 using [NM]!=0 was true both when the tag existed and was not zero as well as
when the tag did not exist. From 1.16 onwards all comparison operators are only true for tags that
exist, so [NM]!=0 works as expected.
Some simple functions are available to operate on strings. These treat the strings as arrays of bytes,
permitting their length, minimum, maximum and average values to be computed. These are useful for
processing Quality Scores.
length(x) Length of the string (excluding nul char)
min(x) Minimum byte value in the string
max(x) Maximum byte value in the string
avg(x) Average byte value in the string
Note that "avg" is a floating point value and it may be NAN for empty strings. This means that
"avg(qual)" does not produce an error for records that have both seq and qual of "*". NAN values will
fail any conditional checks, so e.g. "avg(qual) > 20" works and will not report these records. NAN also
fails all equality, < and > comparisons, and returns zero when given as an argument to the exists
function. It can be negated with !x in which case it becomes true.
Functions that operate on both strings and numerics:
exists(x) True if the value exists (or is explicitly true).
default(x,d) Value x if it exists or d if not.
Functions that apply only to numeric values:
sqrt(x) Square root of x
log(x) Natural logarithm of x
pow(x, y) Power function, x to the power of y
exp(x) Base-e exponential, equivalent to pow(e,x)
ENVIRONMENT VARIABLES
HTS_PATH
A colon-separated list of directories in which to search for HTSlib plugins. If $HTS_PATH starts
or ends with a colon or contains a double colon (::), the built-in list of directories is searched
at that point in the search.
If no HTS_PATH variable is defined, the built-in list of directories specified when HTSlib was
built is used, which typically includes /usr/local/libexec/htslib and similar directories.
REF_PATH
A colon separated (semi-colon on Windows) list of locations in which to look for sequences
identified by their MD5sums. This can be either a list of directories or URLs. Note that if a URL
is included then the colon in http:// and ftp:// and the optional port number will be treated as
part of the URL and not a PATH field separator. Alternatively a double colon may be used to
indicate a single colon character. If REF_PATH includes %nums then it is replaced with the next
num elements of the md5sum. An implicit /%s is also added to each path element if any md5sum
digits are unused. For example "REF_PATH=/some/dir/%4s/%s" or "REF_PATH=/some/dir/%4s" will
search a directory structure with the first 4 characters of the md5sum as a subdirectory and the
remaining 28 as the filename within that directory.
Version 1.21 and earlier defaulted to using the EBI's CRAM reference server if no REF_PATH was
specified. This default has been removed to reduce load on the EBI's service. It is recommended
that a site-wide proxy is set up to allow better sharing of downloaded references, for example the
ref-cache server provided with HTSlib. The original behaviour can be restored by including
http://www.ebi.ac.uk/ena/cram/md5/%s in your REF_PATH. If that is done, it is strongly encouraged
you also specify a local REF_CACHE directory.
See <https://www.htslib.org/doc/reference_seqs.html> and REFERENCE SEQUENCES below for more
information.
REF_CACHE
This can be defined to a single location housing a local cache of references. When REF_CACHE is
set any non-local reference will create a file in the local REF_CACHE named after the sequence
md5sum. This cache will be searched prior to REF_PATH. If you wish to search REF_CACHE but not
to further populate it, add the directory to the start of REF_PATH instead.
As per REF_PATH, the percent notation (e.g. "dir/%2s/%2s/%s") may be used to avoid too many files
within a single directory.
To pre-populate the REF_CACHE a script misc/seq_cache_populate.pl is provided in the Samtools
distribution. This takes a fasta file or a directory of fasta files and generates the MD5sum named
files.
For example if you use seq_cache_populate -subdirs 2 -root /local/ref_cache to create 2 nested
subdirectories (the default), each consuming 2 characters of the MD5sum, then REF_CACHE must be
set to /local/ref_cache/%2s/%2s/%s.
REFERENCE SEQUENCES
The CRAM format requires use of a reference sequence for both reading and writing.
When reading a CRAM the @SQ headers are interrogated to identify the reference sequence MD5sum (M5: tag)
and the local reference sequence filename (UR: tag). Note that non-local URIs in the UR tag are not
used, but file:// is supported. This is a change in behaviour, but not documentation, to htslib 1.21.
To create a CRAM the @SQ headers will also be read to identify the reference sequences, but M5: and UR:
tags may not be present. In this case the -T and -t options of samtools view may be used to specify the
fasta or fasta.fai filenames respectively (provided the .fasta.fai file is also backed up by a .fasta
file).
The search order to obtain a reference is:
1. Use any local file specified by the command line options (eg -T).
2. Look for MD5 via REF_CACHE environment variable.
3. Look for MD5 in each element of the REF_PATH environment variable.
4. Look for a local file listed in the UR: header tag.
EXAMPLES
o Import SAM to BAM when @SQ lines are present in the header:
samtools view -b aln.sam > aln.bam
If @SQ lines are absent:
samtools faidx ref.fa
samtools view -bt ref.fa.fai aln.sam > aln.bam
where ref.fa.fai is generated automatically by the faidx command.
o Convert a BAM file to a CRAM file using a local reference sequence.
samtools view -C -T ref.fa aln.bam > aln.cram
AUTHOR
Heng Li from the Sanger Institute wrote the original C version of samtools. Bob Handsaker from the Broad
Institute implemented the BGZF library. Petr Danecek and Heng Li wrote the VCF/BCF implementation.
James Bonfield from the Sanger Institute developed the CRAM implementation. Other large code
contributions have been made by John Marshall, Rob Davies, Martin Pollard, Andrew Whitwham, Valeriu Ohan,
Vasudeva Sarma (all while primarily at the Sanger Institute), with numerous other smaller but valuable
contributions. See the per-command manual pages for further authorship.
SEE ALSO
samtools-addreplacerg(1), samtools-ampliconclip(1), samtools-ampliconstats(1), samtools-bedcov(1),
samtools-calmd(1), samtools-cat(1), samtools-checksum(1), samtools-collate(1), samtools-consensus(1),
samtools-coverage(1), samtools-cram-size(1), samtools-depad(1), samtools-depth(1), samtools-dict(1),
samtools-faidx(1), samtools-fasta(1), samtools-fastq(1), samtools-fixmate(1), samtools-flags(1),
samtools-flagstat(1), samtools-fqidx(1), samtools-head(1), samtools-idxstats(1), samtools-import(1),
samtools-index(1), samtools-markdup(1), samtools-merge(1), samtools-mpileup(1), samtools-phase(1),
samtools-quickcheck(1), samtools-reference(1), samtools-reheader(1), samtools-reset(1), samtools-
rmdup(1), samtools-sort(1), samtools-split(1), samtools-stats(1), samtools-targetcut(1), samtools-
tview(1), samtools-view(1), bcftools(1), sam(5), tabix(1) ref-cache(1)
Samtools website: <http://www.htslib.org/>
File format specification of SAM/BAM,CRAM,VCF/BCF: <http://samtools.github.io/hts-specs>
Samtools latest source: <https://github.com/samtools/samtools>
HTSlib latest source: <https://github.com/samtools/htslib>
Bcftools website: <http://samtools.github.io/bcftools>
samtools-1.22.1 14 July 2025 samtools(1)