join is a command in Unix-like

Unix-like

A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification....

 operating systems that merges the lines of two sorted text file

Text file

A text file is a kind of computer file that is structured as a sequence of lines of electronic text. A text file exists within a computer file system...

s based on the presence of a common field

Field (computer science)

In computer science, data that has several parts can be divided into fields. Relational databases arrange data as sets of database records, also called rows. Each record consists of several fields; the fields of all records form the columns....

. It is similar to the join operator used in relational database

Relational database

A relational database is a database that conforms to relational model theory. The software used in a relational database is called a relational database management system . Colloquial use of the term "relational database" may refer to the RDBMS software, or the relational database itself...

s but operating on text files.

The join command takes as input two text files and a number of 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

Character (computing)

In computer and machine-based telecommunications terminology, a character is a unit of information that roughly corresponds to a grapheme, grapheme-like unit, or symbol, such as in an alphabet or syllabary in the written form of a natural language....

 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 printing using redirection.

As an example, the two following files list the known fathers and the mothers of some people. Note that both files have been sorted on the join field — this is a requirement of the program.

george jim
mary john

albert martha
george sophie

The join of these two files (with no argument) would produce:

george jim sophie

Indeed, only "george" is common as a first word of both files.

External links

• man page
• join examples