Join (Unix)
Command in Unix-like operating systems
join is a command in Unix and Unix-like operating systems that merges the lines of two sorted text files based on the presence of a common field. It is similar to the join operator used in relational databases (specifically, it implements the sort-merge join algorithm) but operating on text files.
01Overview
The join command takes as input two text files and several options. If no command-line argument is given, this command looks for a pair of lines from the two files having the same first field (a sequence of characters that are different from space), and outputs a line composed of the first field followed by the rest of the two lines.
The program arguments specify which character to be used in place of space to separate the fields of the line, which field to use when looking for matching lines, and whether to output lines that do not match. The output can be stored to another file rather than printed using redirection.
As an example, the two following files list the known fathers and the mothers of some people. Both files have been sorted on the join field, this is a requirement of the program.
george jim kumar gunaware albert martha george sophieThe join of these two files (with no argument) would produce:
george jim sophieIndeed, only "george" is common as a first word of both files.
02Examples
To find Pythagorean triples of the form (k, k+1, n).
$ join <(for i in {1..9999} ; do printf -- '%010d\t%s\t%s\t%s\n' "$((2*i*(i+1)+1))" "$i" "$((i+1))" ; done) <(for i in {1..9999} ; do printf -- '%010d\t%s\n' "$((i**2))" "$i" ; done) 0000000025 3 4 5 0000000841 20 21 29 0000028561 119 120 169 0000970225 696 697 985 0032959081 4059 4060 574103History
join is intended to be a relation database operator. It is part of the X/Open Portability Guide since issue 2 of 1987. It was inherited into the first version of POSIX.1 and the Single Unix Specification.
The version of join bundled in GNU coreutils was written by Mike Haertel. The command is available as a separate package for Microsoft Windows as part of the UnxUtils collection of native Win32 ports of common GNU Unix-like utilities.
Sources and credits
This article is adapted from the Wikipedia article “Join (Unix)”, written by its contributors and licensed under CC BY-SA 4.0. Fathomly has changed the layout, removed citation markers, navigation and maintenance notices, and adjusted punctuation. This adapted version is shared under the same license. For references, see the original article.
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