join

Join is a command line tool in UNIX-like operating systems used for joining lines of two files based on a common field. It is primarily used to merge lines based on a common field between two files. The common field can be specified using options like -1, -2, or -j. Join reads files line by line, and based on the common field, it combines the corresponding lines from both files into a single output line.

The join command requires the input files to be sorted in ascending order based on the common field. If the files are not sorted, join may produce incorrect results. Additionally, if the common field is not present in either file, the join command will not merge any lines. By default, join uses whitespace as the field separator, but it can be changed using the -t option.

Join is often used for merging CSV files or any files with structured data. By specifying the desired fields to include in the output, one can extract specific information from the merged lines. Join also supports several options to control the behavior, such as handling unpairable lines, specifying different fields to be compared, and outputting unpairable lines.

Join is a powerful tool for data manipulation and is commonly used in conjunction with other command line tools like sort, cut, and awk. It provides efficiency in joining files and can be automated in shell scripts or used in pipelines for more complex data processing tasks.