8.3.1 General options

The program accepts the following options. Also see Common options.

-a file-number

Print a line for each unpairable line in file file-number (either ‘1’ or ‘2’), in addition to the normal output.


Fail with an error message if either input file is wrongly ordered.


Do not check that both input files are in sorted order. This is the default.

-e string

Replace those output fields that are missing in the input with string. I.e., missing fields specified with the -12jo options.


Treat the first line of each input file as a header line. The header lines will be joined and printed as the first output line. If -o is used to specify output format, the header line will be printed according to the specified format. The header lines will not be checked for ordering even if --check-order is specified. Also if the header lines from each file do not match, the heading fields from the first file will be used.


Ignore differences in case when comparing keys. With this option, the lines of the input files must be ordered in the same way. Use ‘sort -f’ to produce this ordering.

-1 field

Join on field field (a positive integer) of file 1.

-2 field

Join on field field (a positive integer) of file 2.

-j field

Equivalent to -1 field -2 field.

-o field-list
-o auto

If the keyword ‘auto’ is specified, infer the output format from the first line in each file. This is the same as the default output format but also ensures the same number of fields are output for each line. Missing fields are replaced with the -e option and extra fields are discarded.

Otherwise, construct each output line according to the format in field-list. Each element in field-list is either the single character ‘0’ or has the form m.n where the file number, m, is ‘1’ or ‘2’ and n is a positive field number.

A field specification of ‘0’ denotes the join field. In most cases, the functionality of the ‘0’ field spec may be reproduced using the explicit m.n that corresponds to the join field. However, when printing unpairable lines (using either of the -a or -v options), there is no way to specify the join field using m.n in field-list if there are unpairable lines in both files. To give join that functionality, POSIX invented the ‘0’ field specification notation.

The elements in field-list are separated by commas or blanks. Blank separators typically need to be quoted for the shell. For example, the commands ‘join -o 1.2,2.2’ and ‘join -o '1.2 2.2'’ are equivalent.

All output lines – including those printed because of any -a or -v option – are subject to the specified field-list.

-t char

Use character char as the input and output field separator. Treat as significant each occurrence of char in the input file. Use ‘sort -t char’, without the -b option of ‘sort’, to produce this ordering. If ‘join -t ''’ is specified, the whole line is considered, matching the default operation of sort. If ‘-t '\0'’ is specified then the ASCII NUL character is used to delimit the fields.

-v file-number

Print a line for each unpairable line in file file-number (either ‘1’ or ‘2’), instead of the normal output.


Delimit items with a zero byte rather than a newline (ASCII LF). I.e., treat input as items separated by ASCII NUL and terminate output items with ASCII NUL. This option can be useful in conjunction with ‘perl -0’ or ‘find -print0’ and ‘xargs -0’ which do the same in order to reliably handle arbitrary file names (even those containing blanks or other special characters). With -z the newline character is treated as a field separator.

An exit status of zero indicates success, and a nonzero value indicates failure.

If the --check-order option is given, unsorted inputs will cause a fatal error message. If the option --nocheck-order is given, unsorted inputs will never cause an error message. If neither of these options is given, wrongly sorted inputs are diagnosed only if an input file is found to contain unpairable lines, and when both input files are non empty. If an input file is diagnosed as being unsorted, the join command will exit with a nonzero status (and the output should not be used).

Forcing join to process wrongly sorted input files containing unpairable lines by specifying --nocheck-order is not guaranteed to produce any particular output. The output will probably not correspond with whatever you hoped it would be.