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PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface
may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface
may not be implemented on Linux.
NAME
uniq — report or filter out repeated lines in a file
SYNOPSIS
uniq [−c|−d|−u] [−f fields] [−s char] [input_file [output_file]]
DESCRIPTION
The uniq utility shall read an input file comparing adjacent lines, and write one copy of each input line
on the output. The second and succeeding copies of repeated adjacent input lines shall not be written.
The trailing <newline> of each line in the input shall be ignored when doing comparisons.
Repeated lines in the input shall not be detected if they are not adjacent.
OPTIONS
The uniq utility shall conform to the Base Definitions volume of POSIX.1‐2008, Section 12.2, Utility
Syntax Guidelines, except that '+' may be recognized as an option delimiter as well as '−'.
The following options shall be supported:
−c Precede each output line with a count of the number of times the line occurred in the input.
−d Suppress the writing of lines that are not repeated in the input.
−f fields Ignore the first fields fields on each input line when doing comparisons, where fields is a
positive decimal integer. A field is the maximal string matched by the basic regular
expression:
[[:blank:]]*[^[:blank:]]*
If the fields option-argument specifies more fields than appear on an input line, a null string
shall be used for comparison.
−s chars Ignore the first chars characters when doing comparisons, where chars shall be a positive
decimal integer. If specified in conjunction with the −f option, the first chars characters
after the first fields fields shall be ignored. If the chars option-argument specifies more
characters than remain on an input line, a null string shall be used for comparison.
−u Suppress the writing of lines that are repeated in the input.
OPERANDS
The following operands shall be supported:
input_file
A pathname of the input file. If the input_file operand is not specified, or if the input_file
is '−', the standard input shall be used.
output_file
A pathname of the output file. If the output_file operand is not specified, the standard output
shall be used. The results are unspecified if the file named by output_file is the file named
by input_file.
STDIN
The standard input shall be used only if no input_file operand is specified or if input_file is '−'. See
the INPUT FILES section.
INPUT FILES
The input file shall be a text file.
ENVIRONMENT VARIABLES
The following environment variables shall affect the execution of uniq:
LANG Provide a default value for the internationalization variables that are unset or null. (See the
Base Definitions volume of POSIX.1‐2008, Section 8.2, Internationalization Variables for the
precedence of internationalization variables used to determine the values of locale
categories.)
LC_ALL If set to a non-empty string value, override the values of all the other internationalization
variables.
LC_COLLATE
Determine the locale for ordering rules.
LC_CTYPE Determine the locale for the interpretation of sequences of bytes of text data as characters
(for example, single-byte as opposed to multi-byte characters in arguments and input files) and
which characters constitute a <blank> in the current locale.
LC_MESSAGES
Determine the locale that should be used to affect the format and contents of diagnostic
messages written to standard error.
NLSPATH Determine the location of message catalogs for the processing of LC_MESSAGES.
ASYNCHRONOUS EVENTS
Default.
STDOUT
The standard output shall be used if no output_file operand is specified, and shall be used if the
output_file operand is '−' and the implementation treats the '−' as meaning standard output. Otherwise,
the standard output shall not be used. See the OUTPUT FILES section.
STDERR
The standard error shall be used only for diagnostic messages.
OUTPUT FILES
If the −c option is specified, the output file shall be empty or each line shall be of the form:
"%d %s", <number of duplicates>, <line>
otherwise, the output file shall be empty or each line shall be of the form:
"%s", <line>
EXTENDED DESCRIPTION
None.
EXIT STATUS
The following exit values shall be returned:
0 The utility executed successfully.
>0 An error occurred.
CONSEQUENCES OF ERRORS
Default.
The following sections are informative.
APPLICATION USAGE
The sort utility can be used to cause repeated lines to be adjacent in the input file.
EXAMPLES
The following input file data (but flushed left) was used for a test series on uniq:
#01 foo0 bar0 foo1 bar1
#02 bar0 foo1 bar1 foo1
#03 foo0 bar0 foo1 bar1
#04
#05 foo0 bar0 foo1 bar1
#06 foo0 bar0 foo1 bar1
#07 bar0 foo1 bar1 foo0
What follows is a series of test invocations of the uniq utility that use a mixture of uniq options
against the input file data. These tests verify the meaning of adjacent. The uniq utility views the
input data as a sequence of strings delimited by '\n'. Accordingly, for the fieldsth member of the
sequence, uniq interprets unique or repeated adjacent lines strictly relative to the fields+1th member.
1. This first example tests the line counting option, comparing each line of the input file data
starting from the second field:
uniq −c −f 1 uniq_0I.t
1 #01 foo0 bar0 foo1 bar1
1 #02 bar0 foo1 bar1 foo1
1 #03 foo0 bar0 foo1 bar1
1 #04
2 #05 foo0 bar0 foo1 bar1
1 #07 bar0 foo1 bar1 foo0
The number '2', prefixing the fifth line of output, signifies that the uniq utility detected a pair
of repeated lines. Given the input data, this can only be true when uniq is run using the −f 1 option
(which shall cause uniq to ignore the first field on each input line).
2. The second example tests the option to suppress unique lines, comparing each line of the input file
data starting from the second field:
uniq −d −f 1 uniq_0I.t
#05 foo0 bar0 foo1 bar1
3. This test suppresses repeated lines, comparing each line of the input file data starting from the
second field:
uniq −u −f 1 uniq_0I.t
#01 foo0 bar0 foo1 bar1
#02 bar0 foo1 bar1 foo1
#03 foo0 bar0 foo1 bar1
#04
#07 bar0 foo1 bar1 foo0
4. This suppresses unique lines, comparing each line of the input file data starting from the third
character:
uniq −d −s 2 uniq_0I.t
In the last example, the uniq utility found no input matching the above criteria.
RATIONALE
Some historical implementations have limited lines to be 1080 bytes in length, which does not meet the
implied {LINE_MAX} limit.
Earlier versions of this standard allowed the −number and +number options. These options are no longer
specified by POSIX.1‐2008 but may be present in some implementations.
FUTURE DIRECTIONS
None.
SEE ALSO
comm, sort
The Base Definitions volume of POSIX.1‐2008, Chapter 8, Environment Variables, Section 12.2, Utility
Syntax Guidelines
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition,
Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers, Inc
and The Open Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the event
of any discrepancy between this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at
http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are most likely to have been introduced
during the conversion of the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .