UNIQ
Section: POSIX Programmer's Manual (1P)
Updated: 2017
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The Linux implementation of this interface may differ (consult
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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.1-2017,
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:]]*[ha[: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.1-2017,
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.
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
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.
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.1-2017,
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 1003.1-2017, 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
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https://www.kernel.org/doc/man-pages/reporting_bugs.html .