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.TH JOIN 1
.CT 1 files
.SH NAME
join \(mi 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
.I file1
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 by blank, tab or newline;
output fields by blank.
In this case, multiple separators count as one, and
leading separators are discarded.
.PP
These options are recognized:
.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 -e \ s
Replace empty output fields by string
.IR s .
.TP
.BI -j n\ m
Join on the
.IR m th
field of file
.IR n .
If
.I n
is missing, use the
.IR m th
field in each file.
.TP
.BI -o \ field \ \fR...
Each output line comprises the named fields, each designated
as 0, meaning the join field, or as
.IR n . m ,
where
.I n
is a file number and
.I m
is a field number.
.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/password | join -t: -a1 -e "" - bdays
Add birthdays to password information, leaving unknown
birthdays empty.
The layout of 
.F /etc/passwd
is given in
.IR passwd (5);
.B bdays
contains sorted lines like
.LR "ken:Feb\ 4" .
.TP
.L
tr : ' ' </etc/passwd | sort +2 >temp
.br
.ns
.TP
.L
join -j1 3 -j2 3 -o 1.1 2.1 temp temp | awk '$1 < $2'
Print all pairs of users with identical userids.
.SH "SEE ALSO"
.IR sort (1), 
.IR comm (1), 
.IR awk (1)
.SH BUGS
With default field separation,
the collating sequence is that of
.LR "sort\ -b" ;
with
.BR -t ,
the sequence is that of a plain sort.
.br
The field conventions of
.I join, sort, comm, uniq, look
and
.IR awk (1)
are wildly incongruent.

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