JOIN(1)

NAME
join - join lines of two files on a common field

SYNOPSIS
join [OPTION]... FILE1 FILE2

DESCRIPTION

For each pair of input lines with identical join fields, write a line to standard output. The default join field is the first, delimited by blanks.

When FILE1 or FILE2 (not both) is -, read standard input.

-a FILENUM
    also print unpairable lines from file FILENUM, where FILENUM is 1 or 2, corresponding to FILE1 or FILE2

-e STRING
    replace missing (empty) input fields with STRING; I.e., missing fields specified with '-12jo' options

-i, --ignore-case
    ignore differences in case when comparing fields

-j FIELD
    equivalent to '-1 FIELD -2 FIELD'

-o FORMAT
    obey FORMAT while constructing output line

-t CHAR
    use CHAR as input and output field separator

-v FILENUM
    like -a FILENUM, but suppress joined output lines

-1 FIELD
    join on this FIELD of file 1

-2 FIELD
    join on this FIELD of file 2

--check-order
    check that the input is correctly sorted, even if all input lines are pairable

--nocheck-order
    do not check that the input is correctly sorted

--header
    treat the first line in each file as field headers, print them without trying to pair them

-z, --zero-terminated
    line delimiter is NUL, not newline

--help
    display this help and exit

--version
    output version information and exit

Unless -t CHAR is given, leading blanks separate fields and are ignored, else fields are separated by CHAR. Any FIELD is a field number counted from 1. FORMAT is one or more comma or blank separated specifications, each being 'FILENUM.FIELD' or '0'. Default FORMAT outputs the join field, the remaining fields from FILE1, the remaining fields from FILE2, all separated by CHAR. If FORMAT is the keyword 'auto', then the first line of each file determines the number of fields output for each line.

Important: FILE1 and FILE2 must be sorted on the join fields. E.g., use "sort -k 1b,1" if 'join' has no options, or use "join -t ''" if 'sort' has no options. Comparisons honor the rules specified by 'LC_COLLATE'. If the input is not sorted and some lines cannot be joined, a warning message will be given.

AUTHOR
Written by Mike Haertel.

REPORTING BUGS
Report bugs to: bug-coreutils@gnu.org
GNU coreutils home page: <https://www.gnu.org/software/coreutils/>
General help using GNU software: <https://www.gnu.org/gethelp/>
Report any translation bugs to <https://translationproject.org/team/>

COPYRIGHT
Copyright © 2026 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later <https://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law.

SEE ALSO
comm(1), uniq(1)

Full documentation <https://www.gnu.org/software/coreutils/join>
or available locally via: info '(coreutils) join invocation'