.TH JOIN 1plan9
.CT 1 files
.SH NAME
join \- relational database operator
.SH SYNOPSIS
.B join
[
.I options
]
.I file1 file2
.SH DESCRIPTION
.I Join
forms, on the standard output,
a join
of the two relations specified by the lines of
.I file1
and
.IR file2 .
If one of the file names is
.LR - ,
the standard input is used.
.PP
.I File1
and
.I file2
must be sorted in increasing
.SM ASCII 
collating
sequence on the fields
on which they are to be joined,
normally the first in each line.
.PP
There is one line in the output
for each pair of lines in
.I file1
and
.I file2
that have identical join fields.
The output line normally consists of the common field,
then the rest of the line from
.IR file1 ,
then the rest of the line from
.IR file2 .
.PP
Input fields are normally separated spaces or tabs;
output fields by space.
In this case, multiple separators count as one, and
leading separators are discarded.
.PP
The following options are recognized, with POSIX syntax.
.TP
.BI -a " n
In addition to the normal output,
produce a line for each unpairable line in file
.IR n ,
where
.I n
is 1 or 2.
.TP
.BI -v " n
Like
.BR -a ,
omitting output for paired lines.
.TP
.BI -e " s
Replace empty output fields by string
.IR s .
.TP
.BI -1 " m
.br
.ns
.TP
.BI -2 " m
Join on the
.IR m th
field of
.I file1
or
.IR file2 .
.TP
.BI -j "n m"
Archaic equivalent for
.BI - n " m"\f1.
.TP
.BI -o fields
Each output line comprises the designated fields.
The comma-separated field designators are either
.BR 0 ,
meaning the join field, or have the form
.IR n . m ,
where
.I n
is a file number and
.I m
is a field number.
Archaic usage allows separate arguments for field designators.
.PP
.TP
.BI -t c
Use character
.I c
as the only separator (tab character) on input and output.
Every appearance of
.I c
in a line is significant.
.SH EXAMPLES
.TP
.L
sort /etc/passwd | join -t: -1 1 -a 1 -e "" - bdays
Add birthdays to the
.B /etc/passwd
file, leaving unknown
birthdays empty.
The layout of 
.B /adm/users
is given in
.IR passwd (5);
.B bdays
contains sorted lines like
.LR "ken:Feb\ 4,\ 1953" .
.TP
.L
tr : ' ' </etc/passwd | sort -k 3 3 >temp
.br
.ns
.TP
.L
join -1 3 -2 3 -o 1.1,2.1 temp temp | awk '$1 < $2'
Print all pairs of users with identical userids.
.SH SOURCE
.B \*9/src/cmd/join.c
.SH "SEE ALSO"
.IR sort (1), 
.IR comm (1), 
.IR awk (1)
.SH BUGS
With default field separation,
the collating sequence is that of
.BI "sort -b"
.BI -k y , y\f1;
with
.BR -t ,
the sequence is that of
.BI "sort -t" x
.BI -k y , y\f1.
.PP
One of the files must be randomly accessible.