join - relational database operator


join [-a file_number | -v file_number] [-e string] [-o list] [-t char] [-1 field] [-2 field] file1 file2


The join utility performs an ``equality join'' on the specified files and writes the result to the standard output. The ``join field'' is the field in each file by which the files are compared. The first field in each line is used by default. There is one line in the output for each pair of lines in file1 and file2 which have identical join fields. Each output line consists of the join field, the remaining fields from file1 and then the remaining fields from file2.

The defaults are: the join field is the first field in each line; fields in the input are separated by one or more blanks, with leading blanks on the line ignored; fields in the output are separated by a space; each output line consists of the join field, the remaining fields from file1, then the remaining fields from file2.

Many of the options use file and field numbers. Both file numbers and field numbers are 1 based, i.e. the first file on the command line is file number 1 and the first field is field number 1. The following options are available:

-a file_number

In addition to the default output, produce a line for each unpairable line in file file_number. (The argument to -a must not be preceded by a space; see the COMPATIBILITY section.)

-e string

Replace empty output fields with string.

-o field-list

Construct each output line according to the format in field-list. Each element in field-list is either the single character `0' or has the form M.N where the file number, M, is `1' or `2' and N is a positive field number.

A field specification of `0' denotes the join field. In most cases, the functionality of the `0' field spec may be reproduced using the explicit M.N that corresponds to the join field. However, when printing unpairable lines (using either of the -a or -v options), there is no way to specify the join field using M.N in field-list if there are unpairable lines in both files. To give join that functionality, POSIX invented the `0' field specification notation.

The elements in field-list are separated by commas or blanks. Multiple field-list arguments can be given after a single -o option; the values of all lists given with -o are concatenated together. All output lines - including those printed because of any -a or -v option - are subject to the specified field-list.

-t char

Use character char as a field separator for both input and output. Every occurrence of char in a line is significant.

-v file_number

Do not display the default output, but display a line for each unpairable line in file file_number. The options -v 1 and -v 2 may be specified at the same time.

-1 field

Join on the field'th field of file 1.

-2 field

Join on the field'th field of file 2.

When the default field delimiter characters are used, the files to be joined should be ordered in the collating sequence of sort(1), using the -b option, on the fields on which they are to be joined, otherwise join may not report all field matches. When the field delimiter characters are specified by the -t option, the collating sequence should be the same as sort without the -b option.

If one of the arguments file1 or file2 is ``-'', the standard input is used.

The join utility exits 0 on success or >0 if an error occurred.


For compatibility with historic versions of join, the following options are available:

-j1 field

Join on the field'th field of file 1.

-j2 field

Join on the field'th field of file 2.

-j field

Join on the field'th field of both file 1 and file 2.

-o list ...

Historical implementations of join permitted multiple arguments to the -o option. These arguments were of the form ``file_number.field_number'' as described for the current -o option. This has obvious difficulties in the presence of files named ``1.2''.


awk(1), comm(1), paste(1), sort(1), uniq(1)


join has no known bugs. It does not support the following historical switches: -a with no args. Patches are welcome.


The Perl implementation of join was written by Jonathan Feinberg,


This program is copyright (c) Jonathan Feinberg 1999.

This program is free and open software. You may use, modify, distribute, and sell this program (and any modified variants) in any way you wish, provided you do not restrict others from doing the same.