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PROLOG | NAME | SYNOPSIS | DESCRIPTION | OPTIONS | OPERANDS | STDIN | INPUT FILES | ENVIRONMENT VARIABLES | ASYNCHRONOUS EVENTS | STDOUT | STDERR | OUTPUT FILES | EXTENDED DESCRIPTION | EXIT STATUS | CONSEQUENCES OF ERRORS | APPLICATION USAGE | EXAMPLES | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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UNIQ(1P) POSIX Programmer's Manual UNIQ(1P)
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.
uniq — report or filter out repeated lines in a file
uniq [-c|-d|-u] [-f fields] [-s char] [input_file [output_file]]
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.
The uniq utility shall conform to the Base Definitions volume of
POSIX.1‐2017, 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.
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.
The standard input shall be used only if no input_file operand is
specified or if input_file is '-'. See the INPUT FILES section.
The input file shall be a text file.
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.1‐2017, 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_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.
Default.
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.
The standard error shall be used only for diagnostic messages.
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>
None.
The following exit values shall be returned:
0 The utility executed successfully.
>0 An error occurred.
Default.
The following sections are informative.
If the collating sequence of the current locale has a total
ordering of all characters, the sort utility can be used to cause
repeated lines to be adjacent in the input file. If the collating
sequence does not have a total ordering of all characters, the
sort utility should still do this but it might not. To ensure that
all duplicate lines are eliminated, and have the output sorted
according the collating sequence of the current locale,
applications should use:
LC_ALL=C sort -u | sort
instead of:
sort | uniq
To remove duplicate lines based on whether they collate equally
instead of whether they are identical, applications should use:
sort -u
instead of:
sort | uniq
When using uniq to process pathnames, it is recommended that
LC_ALL, or at least LC_CTYPE and LC_COLLATE, are set to POSIX or C
in the environment, since pathnames can contain byte sequences
that do not form valid characters in some locales, in which case
the utility's behavior would be undefined. In the POSIX locale
each byte is a valid single-byte character, and therefore this
problem is avoided.
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.
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.
None.
comm(1p), sort(1p)
The Base Definitions volume of POSIX.1‐2017, Chapter 8,
Environment Variables, Section 12.2, Utility Syntax Guidelines
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 by the Institute of Electrical and Electronics Engineers,
Inc and The Open Group. 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.opengroup.org/unix/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 .
IEEE/The Open Group 2017 UNIQ(1P)
Pages that refer to this page: comm(1p), join(1p), sort(1p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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