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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.

However, 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 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.

The program accepts the following options. Also see 2. 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'
`-j1 field'
Join on field field (a positive integer) of file 1.

`-2 field'
`-j2 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. 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 the input and output field separator.

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

In addition, when GNU join is invoked with exactly one argument, the `--help' and `--version' options are recognized. See section 2. Common options.

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