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8.3 join: Join lines on a common field

join writes to standard output a line for each pair of input lines that have identical join fields. Synopsis:

join [option]… file1 file2

Either file1 or file2 (but not both) can be ‘-’, meaning standard input. file1 and file2 should be sorted on the join fields.

Normally, the sort order is that of the collating sequence specified by the LC_COLLATE locale. Unless the ‘-t’ option is given, the sort comparison ignores blanks at the start of the join field, as in sort -b. If the ‘--ignore-case’ option is given, the sort comparison ignores the case of characters in the join field, as in sort -f.

The sort and join commands should use consistent locales and options if the output of sort is fed to join. You can use a command like ‘sort -k 1b,1’ to sort a file on its default join field, but if you select a non-default locale, join field, separator, or comparison options, then you should do so consistently between join and sort.

As a GNU extension, if the input has no unpairable lines the sort order can be any order that considers two fields to be equal if and only if the sort comparison described above considers them to be equal. For example:

$ cat file1
a a1
c c1
b b1
$ cat file2
a a2
c c2
b b2
$ join file1 file2
a a1 a2
c c1 c2
b b1 b2

The defaults are:

The program accepts the following options. Also see Common options.

-a file-number

Print a line for each unpairable line in file file-number (either ‘1’ or ‘2’), in addition to the normal output.

-e string

Replace those output fields that are missing in the input with string.


Ignore differences in case when comparing keys. With this option, the lines of the input files must be ordered in the same way. Use ‘sort -f’ to produce this ordering.

-1 field

Join on field field (a positive integer) of file 1.

-2 field

Join on field field (a positive integer) of file 2.

-j field

Equivalent to ‘-1 field -2 field’.

-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. Blank separators typically need to be quoted for the shell. For example, the commands ‘join -o 1.2,2.2’ and ‘join -o '1.2 2.2'’ are equivalent.

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 the input and output field separator. Treat as significant each occurrence of char in the input file. Use ‘sort -t char’, without the ‘-b’ option of ‘sort’, to produce this ordering.

-v file-number

Print a line for each unpairable line in file file-number (either ‘1’ or ‘2’), instead of the normal output.

An exit status of zero indicates success, and a nonzero value indicates failure.

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