faidx.5

'\" t
.TH faidx 5 "June 2018" "htslib" "Bioinformatics formats"
.SH NAME
faidx \- an index enabling random access to FASTA and FASTQ files
.\"
.\" Copyright (C) 2013, 2015, 2018 Genome Research Ltd.
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.\" Author: John Marshall <jm18@sanger.ac.uk>
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.SH SYNOPSIS
.IR file.fa .fai,
.IR file.fasta .fai,
.IR file.fq .fai,
.IR file.fastq .fai
.SH DESCRIPTION
Using an \fBfai index\fP file in conjunction with a FASTA/FASTQ file containing
reference sequences enables efficient access to arbitrary regions within
those reference sequences.
The index file typically has the same filename as the corresponding FASTA/FASTQ
file, with \fB.fai\fP appended.
.P
An \fBfai index\fP file is a text file consisting of lines each with
five TAB-delimited columns for a FASTA file and six for FASTQ:
.TS
lbl.
NAME	Name of this reference sequence
LENGTH	Total length of this reference sequence, in bases
OFFSET	Offset in the FASTA/FASTQ file of this sequence's first base
LINEBASES	The number of bases on each line
LINEWIDTH	The number of bytes in each line, including the newline
QUALOFFSET	Offset of sequence's first quality within the FASTQ file
.TE
.P
The \fBNAME\fP and \fBLENGTH\fP columns contain the same
data as would appear in the \fBSN\fP and \fBLN\fP fields of a
SAM \fB@SQ\fP header for the same reference sequence.
.P
The \fBOFFSET\fP column contains the offset within the FASTA/FASTQ file, in
bytes starting from zero, of the first base of this reference sequence, i.e., of
the character following the newline at the end of the header line (the 
"\fB>\fP" line in FASTA, "\fB@\fP" in FASTQ). Typically the lines of a
\fBfai index\fP file appear in the order in which the reference sequences
appear in the FASTA/FASTQ file, so \fB.fai\fP files are typically sorted
according to this column.
.P
The \fBLINEBASES\fP column contains the number of bases in each of the sequence
lines that form the body of this reference sequence, apart from the final line
which may be shorter.
The \fBLINEWIDTH\fP column contains the number of \fIbytes\fP in each of
the sequence lines (except perhaps the final line), thus differing from
\fBLINEBASES\fP in that it also counts the bytes forming the line terminator.
.P
The \fBQUALOFFSET\fP works the same way as \fBOFFSET\fP but for the first
quality score of this reference sequence.  This would be the first character
following the newline at the end of the "\fB+\fP" line.  For FASTQ files only.
.SS FASTA Files
In order to be indexed with \fBsamtools faidx\fP, a FASTA file must be a text
file of the form
.LP
.RS
.RI > name
.RI [ description ...]
.br
ATGCATGCATGCATGCATGCATGCATGCAT
.br
GCATGCATGCATGCATGCATGCATGCATGC
.br
ATGCAT
.br
.RI > name
.RI [ description ...]
.br
ATGCATGCATGCAT
.br
GCATGCATGCATGC
.br
[...]
.RE
.LP
In particular, each reference sequence must be "well-formatted", i.e., all
of its sequence lines must be the same length, apart from the final sequence
line which may be shorter.
(While this sequence line length must be the same within each sequence,
it may vary between different reference sequences in the same FASTA file.)
.P
This also means that although the FASTA file may have Unix- or Windows-style
or other line termination, the newline characters present must be consistent,
at least within each reference sequence.
.P
The \fBsamtools\fP implementation uses the first word of the "\fB>\fP" header
line text (i.e., up to the first whitespace character, having skipped any
initial whitespace after the ">") as the \fBNAME\fP column.
.SS FASTQ Files
FASTQ files for indexing work in the same way as the FASTA files.
.LP
.RS
.RI @ name
.RI [ description...]
.br
ATGCATGCATGCATGCATGCATGCATGCAT
.br
GCATGCATGCATGCATGCATGCATGCATGC
.br
ATGCAT
.br
.RI +
.br
FFFA@@FFFFFFFFFFHHB:::@BFFFFGG
.br
HIHIIIIIIIIIIIIIIIIIIIIIIIFFFF
.br
8011<<
.br
.RI @ name
.RI [ description...]
.br
ATGCATGCATGCAT
.br
GCATGCATGCATGC
.br
.RI +
.br
IIA94445EEII==
.br
=>IIIIIIIIICCC
.br
[...]
.RE
.LP
Quality lines must be wrapped at the same length as the corresponding
sequence lines.
.SH EXAMPLE
For example, given this FASTA file
.LP
.RS
>one
.br
ATGCATGCATGCATGCATGCATGCATGCAT
.br
GCATGCATGCATGCATGCATGCATGCATGC
.br
ATGCAT
.br
>two another chromosome
.br
ATGCATGCATGCAT
.br
GCATGCATGCATGC
.br
.RE
.LP
formatted with Unix-style (LF) line termination, the corresponding fai index
would be
.RS
.TS
lnnnn.
one	66	5	30	31
two	28	98	14	15
.TE
.RE
.LP
If the FASTA file were formatted with Windows-style (CR-LF) line termination,
the fai index would be
.RS
.TS
lnnnn.
one	66	6	30	32
two	28	103	14	16
.TE
.RE
.LP
An example FASTQ file
.LP
.RS
@fastq1
.br
ATGCATGCATGCATGCATGCATGCATGCAT
.br
GCATGCATGCATGCATGCATGCATGCATGC
.br
ATGCAT
.br
+
.br
FFFA@@FFFFFFFFFFHHB:::@BFFFFGG
.br
HIHIIIIIIIIIIIIIIIIIIIIIIIFFFF
.br
8011<<
.br
@fastq2
.br
ATGCATGCATGCAT
.br
GCATGCATGCATGC
.br
+
.br
IIA94445EEII==
.br
=>IIIIIIIIICCC
.br
.RE
.LP
Formatted with Unix-style line termination would give this fai index
.RS
.TS
lnnnnn.
fastq1	66	8	30	31	79
fastq2	28	156	14	15	188
.TE
.RE
.SH SEE ALSO
.IR samtools (1)
.TP
https://en.wikipedia.org/wiki/FASTA_format
.TP
https://en.wikipedia.org/wiki/FASTQ_format

Further description of the FASTA and FASTQ formats