join

Additional Notes

join merges lines from two sorted text files based on a common field, similar to a relational database join. Each file should have one line per record with fields separated by whitespace or a custom delimiter.

Both input files must be sorted on the join field. If they are not sorted, use sort before join.

Syntax

join [options] FILE1 FILE2

Parameters

• FILE1: First input file.
• FILE2: Second input file.
• Use - to read from standard input for either file.

Common Options

• -1 FIELD: Join on FIELD of file 1 (default 1).
• -2 FIELD: Join on FIELD of file 2 (default 1).
• -j FIELD: Join on FIELD of both files.
• -t CHAR: Use CHAR as the input and output field separator.
• -o FORMAT: Control which fields to output.
• -a FILENUM: Print unpairable lines from file FILENUM (1 or 2).
• -v FILENUM: Like -a but suppresses matched output lines.
• -e STRING: Replace missing fields with STRING.

Examples

join names.txt scores.txt

Join two files on the first field (default).

join -t',' -1 2 -2 1 file1.csv file2.csv

Join CSV files on field 2 of file1 and field 1 of file2.

join -a 1 left.txt right.txt

Include all lines from the first file, even without a match.

sort -k 1,1 file1.txt | join - file2.txt

Join sorted piped input with a file.

Output Field Format

With -o, control which fields appear:

• 0: The join field (printed once).
• 1.1, 1.2, ...: Fields from file 1.
• 2.1, 2.2, ...: Fields from file 2.

join -o 0,1.2,2.2 file1.txt file2.txt

Show join field, field 2 from file1, field 2 from file2.

Practical Notes

• Both input files must be sorted on the join field using the same collation order.
• Use sort -k 1,1 to sort on the first field before joining.
• Whitespace handling can be tricky; use -t with a specific delimiter for reliable results.
• join is useful for combining lookup tables, configuration mappings, and log analysis. It complements tools like awk, paste, and comm.