join Command in Linux

The join command in Linux merges lines from two files based on a common key field.

• It matches records from both files where the key field values are identical.
• By default, the first column in each file is used as the join key unless specified otherwise.
• You can customize field separators, output format, and include unmatched lines using various options.

Join Command Example

Let’s assume we have two text files, file1.txt and file2.txt, and we want to merge their contents based on a common field using the join command. Displaying the contents of first file:

Commands:

cat file1.txt

Displaying contents of second file:

cat file2.txt

Output:

Now, we use the join command to combine the contents of these two files and display the output as a single merged file.Using join command:

join file1.txt file2.txt

Output:

• By default join command takes the first column as the key to join as in the above case.
• So, the output contains the key followed by all the matching columns from the first file 'file1.txt', followed by all the columns of second file 'file2.txt'.
• Now, if we wanted to create a new file with the joined contents, we could use the following command:

$join file1.txt file2.txt > newjoinfile.txt

Output:

This will direct the output of joined files into a new file 'newjoinfile.txt' containing the same output as the example above.

Options of join Command in Linux & Examples

The join command combines lines from two files based on a common field (key). By default, it uses the first field in both files to match.

join [OPTION]... FILE1 FILE2

1. -1 and -2: Specify Join Fields

• By default, join uses the first field (column).
• You can specify which column to join on using -1 for file1 and -2 for file2.

Command:

join -1 1 -2 1 file1.txt file2.txt

Output:

2. -t: Specify a Delimiter

• If your fields are separated by something other than spaces (like commas or colons), use -t.

Example:

cat file1.txt
cat file2.txt

Command:

join -t ',' file1.txt file2.txt

Output:

3. -a: Include Unpairable Lines (from both files)

• By default, join shows only lines that match in both files.
• Use -a 1 or -a 2 to include lines that don’t have a match in file1 or file2.

Example:

cat file1.txt && cat file2.txt

Output:

Command:

join -a 1 file1.txt file2.txt

4. -v: Show Only Unmatched Lines

Use -v 1 or -v 2 to display only lines not matching from a particular file.

join -v 2 file1.txt file2.txt

5. -o: Select Specific Output Fields

This option lets you control which fields are displayed in the output.

Example:

join -o 1.2,2.2 file1.txt file2.txt

Output:

6. -i: Ignore Case While Matching

Makes matching case-insensitive.

Example:

If two files, file1.txt and file2.txt, contain entries with case-sensitive text, the -i option in the join command can be used to merge them while ignoring case differences.

Command:

cat file1.txt && cat file2.txt
join -i file1.txt file2.txt