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join command

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The external join command performs the relational database operation "join" of two files. Files are joined on a common key field (column) that should exist in both files. Also both files must be sorted on the key field in the same order. join examines one line at a time from each file. If a certain segment of the lines match, they are combined into one line. Only one instance of the same segment is printed. The files are assumed to be sorted in the same order.

During join operation if the contents in the key field of both files match, the two lines are merged. The merged line is written to the standard output.

Related command are awk, cut, grep, paste, sort, and uniq.  

The join is a classic relational database operation. The join command reads the first line of the first file, then reads lines from the second file until the contents in the key field of the second file match or exceed the contents in the key field of the first file. If a match occurs, the line from the first file and the line from the second file are combined and displayed on the output. If no match occurs, join begins the process again except reading is done from the first file. This process continues until one or both files have been read to the end and no more lines in the other file can match the last line of the file.

An example of a join command might help clarify how the join function works. Let's assume we have the following files.

file1:   file2:  
01123 HP 9000/825 01123 Hewlett Packard
02213 Sun 3/110 02213 Sun Microsystems
03321 Sun 4/110 03321 Sun Microsystems
08412 HP 9000/835 08412 Hewlett Packard

The following command should produce the combined output as shown.

$ join -t"\t" file1 file2

01123 HP 9000/825 Hewlett Packard
02213 Sun3/110 Sun Microsystems
03321 Sun4/110 Sun Microsystems
08412 HP 9000/835 Hewlett Packard

Following is the general format of the join command.

     join [ -an ] [ -e str ] [ -jn m ] [ -o list ] [ -tc ] file1 file2


The following list describes the options and their arguments that may be used to control how join functions.

-an If a field in file n does not have a match in the other file, the line is written to file n. For example, if you use
    join -a1 file1 file2
  and file1 has a line in it with a key field that is not matched in file2, then that line is written to the file named 1.
-e str Replaces empty output fields with the str string. The -o option causes empty fields in certain conditions.
-jn m The join is performed using the mth field of file n. If you do not specify n, the mth field is used from both files. The fields are numbered beginning with 1.
-o list Specifies the fields to be displayed on output after the join has been performed. The list is comprised of n.m descriptions that define which fields from each file are to be displayed. The n refers to the file name and the m refers to the field number. Multiple n.m descriptions may be used in the list. They must be space separated. An example is,
    join -j1 3 -j2 4 -o 1.1 1.3 2.1 1.5 file1 file2
  which uses field 3 of file1 and field 4 of file2 to join the files. The output is field 1 of file1, field 3 of file1, field 1 of file2, and field 5 of file1.
-tc Character c is used as the input and output field separator. For example,
    join -t: file1 file2
  informs join to use a : (colon) field separator in place of a tab.
  The default input field separators are blanks (spaces, tabs, and new-lines). If the default separators are being used, multiple occurrences count as only one field separator. Leading separators are ignored. The default output separator is a space.

The following list describes the arguments that may be passed to the join command.

- A hyphen may be used in place of file1 to cause join to read the standard input as file1.
file1 The first file to be joined to the second file. The file must be sorted in ASCII collating sequence on the fields that are used to join the file to file2.
file2 The second file to be joined to the first file. The file must be sorted in ASCII collating sequence on the fields that are used to join the file to file1.

If fields are separated by spaces and tabs you need to use the sort -b command for sorting the file. If you use the -t option you need to use the sort command without the -b option.

Unfortunately, the comm, join, sort, and uniq commands have different format conventions for referencing data.

Numeric filenames may conflict with the -o option and the field descriptions that relate to it. One solution is to mv your files to temporary names beginning with a nonnumeric character. Then after the join, move the files back to their original names.

You use the join command to join related files together based on a key field. The output is a subset of the fields and lines from both files. It is useful in combining two related tables (files) together to create one larger table.

The line segment (or field) is chosen using three switches. The -1 switch selects the field number from the first file. The -2 switch selects the field number from the second. The -t switch specifies the character that separates one field from another. If these switches aren't used, join separates fields by spaces and examines the first field on each line.

Suppose the data in the robots.txt file was separated into two files, one with the pricing information (robots1.txt) and one with the quantity and account information (robots2.txt).

$ cat robots1.txt
Birchwood China Hutch,475.99
Bookcase Oak Veneer,205.99
Small Bookcase Oak Veneer,205.99
Reclining Chair,1599.99
Bunk Bed,705.99
$ cat robots2.txt
Birchwood China Hutch,1,756
Bookcase Oak Veneer,1,756
Small Bookcase Oak Veneer,1,756
Reclining Chair,1,757
Bunk Bed,1,757

To join these two files together, use a comma as a field separator and compare field 1 of the first file with field 1 of the second.

$ join -1 1 -2 1 -t, robots1.txt robots2.txt
Birchwood China Hutch,475.99,1,756
Bookcase Oak Veneer,205.99,1,756
Small Bookcase Oak Veneer,205.99,1,756
Reclining Chair,1599.99,1,757
Bunk Bed,705.99,1,757

If either file contains a line with a unique field, the field is discarded. Lines are joined only if matching fields are found in both files. To print unpaired lines, use -a 1 to print the unique lines in the first file or -a 2 to print the unique lines in the second file. The lines are printed as they appear in the files.

The sense of matching can be reversed with the -v switch. -v 1 prints the unique lines in the first file and -v 2 prints the unique lines in the second file.

The tests are case-insensitive when the --ignore-case (or -i) switch is used.

The fields can be rearranged using the -o (output) switch. Use a comma-separated field list to order the fields. A field is specified using the file number (1 or 2), a period and the field number from that file. A zero is a short form of the join field.

$ join -1 1 -2 1 -t, -o "1.2,2.3,2.2,0" robots1.txt robots2.txt
475.99,756,1,Birchwood China Hutch
205.99,756,1,Bookcase Oak Veneer
205.99,756,1,Small Bookcase Oak Veneer
1599.99,757,1,Reclining Chair
705.99,757,1,Bunk Bed



To join the /etc/passwd and /etc/group file together to show the user name, user ID, group name, group ID, group name, and home directory (both files should be sorted in ASCII collating sequence).

1. First you need to sort the passwd file on the group ID and user ID fields:
sort -t':' +3.0n -4.0n +2.0n -3.0n /etc/passwd > mypasswd
2. Sort the group file on the group ID field. sort -t: +3.0n -4.0n /etc/group > mygroup
3. Now you can join two files:.
   > join -t: -j1 4 -j2 3 -o 1.1 1.3 2.1 2.3 1.6 mypasswd

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The join command can be used to merge two files (one can be standard input) to create a third file (can be standard output). Each line in the file is merged on the basis of a field that has the same value in both input files to create one line in the output file. The fields in each file are separated by either a space or tab character.

Following is a list of flags that can be used with the join command:

Examples Let us assume we have two files, file1 and file2, whose contents are shown as follows:

more file1
computer1 16MB 1.2GB 17inch CDROM
computer2 8MB 840MB 14inch
computer3 12MB 1.6GB 17inch
computer4 4MB 270MB 14inch

more file2
computer1 1stfloor office5
computer3 2ndfloor office9A
computer4 1stfloor office2
computer5 3rdfloor office1

If you want to join the two files and display only the matching lines, execute the following command:

join file1 file2
computer1 16MB 1.2GB 17inch CDROM 1stfloor office5
computer3 12MB 1.6GB 17inch 2ndfloor office9A
computer4 4MB 270MB 14inch CDROM 1stfloor office2

If you want to join the two files and display the matching lines as well as the nonmatching lines from the specified file, use the -a flag in the following command:

join -a1 file1 file2
computer1 16MB 1.2GB 17inch CDROM 1stfloor office5
computer2 8MB 840MB 14inch
computer3 12MB 1.6GB 17inch 2ndfloor office9A
computer4 4MB 270MB 14inch CDROM 1stfloor office2

The above example displays the line with computer2 from file1 because it does not have a matching line in file2. If you want to display only the lines that do not match lines from the specified file, use the -v flag in the following command:

join -v2 file1 file2
computer5 3rdfloor office1

The above example displays the line with computer5 from file2 because it does not have a matching line in file1.

If you want to display only certain fields from the input files to the output file, use the -o flag as in the following command:

join -o 1.1 2.2 2.3 1.5 file1 file2
computer1 1stfloor office5 CDROM
computer3 2ndfloor office9A
computer4 1stfloor office2 CDROM

In the above example, the line with computer3 is displayed with one field short because that field is not present in the input file. You can insert a fixed legend in the empty field in the output by using the -e flag in the following command:

join -o 1.1 2.2 2.3 1.5 -e"NO CDROM" file1 file2
computer1 1stfloor office5 CDROM
computer3 2ndfloor office9A NO CDROM
computer4 1stfloor office2 NO CDROM



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