|
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. |
|
|
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 |
|
|
|
SORT(1P) POSIX Programmer's Manual SORT(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.
sort — sort, merge, or sequence check text files
sort [-m] [-o output] [-bdfinru] [-t char] [-k keydef]... [file...]
sort [-c|-C] [-bdfinru] [-t char] [-k keydef] [file]
The sort utility shall perform one of the following functions:
1. Sort lines of all the named files together and write the
result to the specified output.
2. Merge lines of all the named (presorted) files together and
write the result to the specified output.
3. Check that a single input file is correctly presorted.
Comparisons shall be based on one or more sort keys extracted from
each line of input (or, if no sort keys are specified, the entire
line up to, but not including, the terminating <newline>), and
shall be performed using the collating sequence of the current
locale. If this collating sequence does not have a total ordering
of all characters (see the Base Definitions volume of
POSIX.1‐2017, Section 7.3.2, LC_COLLATE), any lines of input that
collate equally should be further compared byte-by-byte using the
collating sequence for the POSIX locale.
The sort utility shall conform to the Base Definitions volume of
POSIX.1‐2017, Section 12.2, Utility Syntax Guidelines, except for
Guideline 9, and the -k keydef option should follow the -b, -d,
-f, -i, -n, and -r options. In addition, '+' may be recognized as
an option delimiter as well as '-'.
The following options shall be supported:
-c Check that the single input file is ordered as specified
by the arguments and the collating sequence of the
current locale. Output shall not be sent to standard
output. The exit code shall indicate whether or not
disorder was detected or an error occurred. If disorder
(or, with -u, a duplicate key) is detected, a warning
message shall be sent to standard error indicating where
the disorder or duplicate key was found.
-C Same as -c, except that a warning message shall not be
sent to standard error if disorder or, with -u, a
duplicate key is detected.
-m Merge only; the input file shall be assumed to be
already sorted.
-o output Specify the name of an output file to be used instead of
the standard output. This file can be the same as one of
the input files.
-u Unique: suppress all but one in each set of lines having
equal keys. If used with the -c option, check that
there are no lines with duplicate keys, in addition to
checking that the input file is sorted.
The following options shall override the default ordering rules.
When ordering options appear independent of any key field
specifications, the requested field ordering rules shall be
applied globally to all sort keys. When attached to a specific key
(see -k), the specified ordering options shall override all global
ordering options for that key.
-d Specify that only <blank> characters and alphanumeric
characters, according to the current setting of
LC_CTYPE, shall be significant in comparisons. The
behavior is undefined for a sort key to which -i or -n
also applies.
-f Consider all lowercase characters that have uppercase
equivalents, according to the current setting of
LC_CTYPE, to be the uppercase equivalent for the
purposes of comparison.
-i Ignore all characters that are non-printable, according
to the current setting of LC_CTYPE. The behavior is
undefined for a sort key for which -n also applies.
-n Restrict the sort key to an initial numeric string,
consisting of optional <blank> characters, optional
<hyphen-minus> character, and zero or more digits with
an optional radix character and thousands separators (as
defined in the current locale), which shall be sorted by
arithmetic value. An empty digit string shall be treated
as zero. Leading zeros and signs on zeros shall not
affect ordering.
-r Reverse the sense of comparisons.
The treatment of field separators can be altered using the
options:
-b Ignore leading <blank> characters when determining the
starting and ending positions of a restricted sort key.
If the -b option is specified before the first -k
option, it shall be applied to all -k options.
Otherwise, the -b option can be attached independently
to each -k field_start or field_end option-argument (see
below).
-t char Use char as the field separator character; char shall
not be considered to be part of a field (although it can
be included in a sort key). Each occurrence of char
shall be significant (for example, <char><char> delimits
an empty field). If -t is not specified, <blank>
characters shall be used as default field separators;
each maximal non-empty sequence of <blank> characters
that follows a non-<blank> shall be a field separator.
Sort keys can be specified using the options:
-k keydef The keydef argument is a restricted sort key field
definition. The format of this definition is:
field_start[type][,field_end[type]]
where field_start and field_end define a key field
restricted to a portion of the line (see the EXTENDED
DESCRIPTION section), and type is one or more modifiers
from the list of characters 'b', 'd', 'f', 'i', 'n',
'r'. The 'b' modifier shall behave like the -b option,
but shall apply only to the field_start or field_end to
which it is attached. The other modifiers shall behave
like the corresponding options, but shall apply only to
the key field to which they are attached; they shall
have this effect if specified with field_start,
field_end, or both. If any modifier is attached to a
field_start or to a field_end, no option shall apply to
either. Implementations shall support at least nine
occurrences of the -k option, which shall be significant
in command line order. If no -k option is specified, a
default sort key of the entire line shall be used.
When there are multiple key fields, later keys shall be
compared only after all earlier keys compare equal.
Except when the -u option is specified, lines that
otherwise compare equal shall be ordered as if none of
the options -d, -f, -i, -n, or -k were present (but with
-r still in effect, if it was specified) and with all
bytes in the lines significant to the comparison. The
order in which lines that still compare equal are
written is unspecified.
The following operand shall be supported:
file A pathname of a file to be sorted, merged, or checked.
If no file operands are specified, or if a file operand
is '-', the standard input shall be used. If sort
encounters an error when opening or reading a file
operand, it may exit without writing any output to
standard output or processing later operands.
The standard input shall be used only if no file operands are
specified, or if a file operand is '-'. See the INPUT FILES
section.
The input files shall be text files, except that the sort utility
shall add a <newline> to the end of a file ending with an
incomplete last line.
The following environment variables shall affect the execution of
sort:
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_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 the behavior of character
classification for the -b, -d, -f, -i, and -n options.
LC_MESSAGES
Determine the locale that should be used to affect the
format and contents of diagnostic messages written to
standard error.
LC_NUMERIC
Determine the locale for the definition of the radix
character and thousands separator for the -n option.
NLSPATH Determine the location of message catalogs for the
processing of LC_MESSAGES.
Default.
Unless the -o or -c options are in effect, the standard output
shall contain the sorted input.
The standard error shall be used for diagnostic messages. When -c
is specified, if disorder is detected (or if -u is also specified
and a duplicate key is detected), a message shall be written to
the standard error which identifies the input line at which
disorder (or a duplicate key) was detected. A warning message
about correcting an incomplete last line of an input file may be
generated, but need not affect the final exit status.
If the -o option is in effect, the sorted input shall be written
to the file output.
The notation:
-k field_start[type][,field_end[type]]
shall define a key field that begins at field_start and ends at
field_end inclusive, unless field_start falls beyond the end of
the line or after field_end, in which case the key field is empty.
A missing field_end shall mean the last character of the line.
A field comprises a maximal sequence of non-separating characters
and, in the absence of option -t, any preceding field separator.
The field_start portion of the keydef option-argument shall have
the form:
field_number[.first_character]
Fields and characters within fields shall be numbered starting
with 1. The field_number and first_character pieces, interpreted
as positive decimal integers, shall specify the first character to
be used as part of a sort key. If .first_character is omitted, it
shall refer to the first character of the field.
The field_end portion of the keydef option-argument shall have the
form:
field_number[.last_character]
The field_number shall be as described above for field_start. The
last_character piece, interpreted as a non-negative decimal
integer, shall specify the last character to be used as part of
the sort key. If last_character evaluates to zero or
.last_character is omitted, it shall refer to the last character
of the field specified by field_number.
If the -b option or b type modifier is in effect, characters
within a field shall be counted from the first non-<blank> in the
field. (This shall apply separately to first_character and
last_character.)
The following exit values shall be returned:
0 All input files were output successfully, or -c was
specified and the input file was correctly sorted.
1 Under the -c option, the file was not ordered as specified,
or if the -c and -u options were both specified, two input
lines were found with equal keys.
>1 An error occurred.
The default requirements shall apply, except that if sort
encounters an error when opening or reading a file operand, it may
exit without writing any output to standard output or processing
later operands.
The following sections are informative.
The default value for -t, <blank>, has different properties from,
for example, -t"<space>". If a line contains:
<space><space>foo
the following treatment would occur with default separation as
opposed to specifically selecting a <space>:
┌───────┬───────────────────┬──────────────┐
│ Field │ Default │ -t "<space>" │
├───────┼───────────────────┼──────────────┤
│ 1 │ <space><space>foo │ empty │
│ 2 │ empty │ empty │
│ 3 │ empty │ foo │
└───────┴───────────────────┴──────────────┘
The leading field separator itself is included in a field when -t
is not used. For example, this command returns an exit status of
zero, meaning the input was already sorted:
sort -c -k 2 <<eof
y<tab>b
x<space>a
eof
(assuming that a <tab> precedes the <space> in the current
collating sequence). The field separator is not included in a
field when it is explicitly set via -t. This is historical
practice and allows usage such as:
sort -t "|" -k 2n <<eof
Atlanta|425022|Georgia
Birmingham|284413|Alabama
Columbia|100385|South Carolina
eof
where the second field can be correctly sorted numerically without
regard to the non-numeric field separator.
The wording in the OPTIONS section clarifies that the -b, -d, -f,
-i, -n, and -r options have to come before the first sort key
specified if they are intended to apply to all specified keys. The
way it is described in this volume of POSIX.1‐2017 matches
historical practice, not historical documentation. The results
are unspecified if these options are specified after a -k option.
The -f option might not work as expected in locales where there is
not a one-to-one mapping between an uppercase and a lowercase
letter.
When using sort 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.
If the collating sequence of the current locale does not have a
total ordering of all characters, this can affect the behavior of
sort in the following ways:
* As sort -u suppresses lines with duplicate keys, it suppresses
lines that collate equally but are not identical.
* The output of sort (without -u) can contain identical lines
that are not adjacent, if it does not implement the
recommended further byte-by-byte comparison of lines that
collate equally. This affects the use of sort with comm and
uniq; see the APPLICATION USAGE for those utilities.
1. The following command sorts the contents of infile with the
second field as the sort key:
sort -k 2,2 infile
2. The following command sorts, in reverse order, the contents of
infile1 and infile2, placing the output in outfile and using
the second character of the second field as the sort key
(assuming that the first character of the second field is the
field separator):
sort -r -o outfile -k 2.2,2.2 infile1 infile2
3. The following command sorts the contents of infile1 and
infile2 using the second non-<blank> of the second field as
the sort key:
sort -k 2.2b,2.2b infile1 infile2
4. The following command prints the System V password file (user
database) sorted by the numeric user ID (the third
<colon>-separated field):
sort -t : -k 3,3n /etc/passwd
5. The following command prints the lines of the already sorted
file infile, suppressing all but one occurrence of lines
having the same third field:
sort -um -k 3.1,3.0 infile
Examples in some historical documentation state that options -um
with one input file keep the first in each set of lines with equal
keys. This behavior was deemed to be an implementation artifact
and was not standardized.
The -z option was omitted; it is not standard practice on most
systems and is inconsistent with using sort to sort several files
individually and then merge them together. The text concerning -z
in historical documentation appeared to require implementations to
determine the proper buffer length during the sort phase of
operation, but not during the merge.
The -y option was omitted because of non-portability. The -M
option, present in System V, was omitted because of non-
portability in international usage.
An undocumented -T option exists in some implementations. It is
used to specify a directory for intermediate files.
Implementations are encouraged to support the use of the TMPDIR
environment variable instead of adding an option to support this
functionality.
The -k option was added to satisfy two objections. First, the
zero-based counting used by sort is not consistent with other
utility conventions. Second, it did not meet syntax guideline
requirements.
Historical documentation indicates that ``setting -n implies -b''.
The description of -n already states that optional leading
<blank>s are tolerated in doing the comparison. If -b is enabled,
rather than implied, by -n, this has unusual side-effects. When a
character offset is used in a column of numbers (for example, to
sort modulo 100), that offset is measured relative to the most
significant digit, not to the column. Based upon a recommendation
from the author of the original sort utility, the -b implication
has been omitted from this volume of POSIX.1‐2017, and an
application wishing to achieve the previously mentioned side-
effects has to code the -b flag explicitly.
Earlier versions of this standard allowed the -o option to appear
after operands. Historical practice allowed all options to be
interspersed with operands. This version of the standard allows
implementations to accept options after operands but conforming
applications should not use this form.
Earlier versions of this standard also allowed the -number and
+number options. These options are no longer specified by
POSIX.1‐2008 but may be present in some implementations.
Historical implementations produced a message on standard error
when -c was specified and disorder was detected, and when -c and
-u were specified and a duplicate key was detected. An earlier
version of this standard contained wording that did not make it
clear that this message was allowed and some implementations
removed this message to be sure that they conformed to the
standard's requirements. Confronted with this difference in
behavior, interactive users that wanted to be sure that they got
visual feedback instead of just exit code 1 could have used a
command like:
sort -c file || echo disorder
whether or not the sort utility provided a message in this case.
But, it was not easy for a user to find where the disorder or
duplicate key occurred on implementations that do not produce a
message, especially when some parts of the input line were not
part of the key and when one or more of the -b, -d, -f, -i, -n, or
-r options or keydef type modifiers were in use. POSIX.1‐2008
requires a message to be produced in this case. POSIX.1‐2008 also
contains the -C option giving users the ability to choose either
behavior.
When a disorder or duplicate is found when the -c option is
specified, some implementations print a message containing the
first line that is out of order or contains a duplicate key;
others print a message specifying the line number of the offending
line. This standard allows either type of message.
Implementations are encouraged to perform the recommended further
byte-by-byte comparison of lines that collate equally, even though
this may affect efficiency. The impact on efficiency can be
mitigated by only performing the additional comparison if the
current locale's collating sequence does not have a total ordering
of all characters (if the implementation provides a way to query
this) or by only performing the additional comparison if the
locale name associated with the LC_COLLATE category has an '@'
modifier in the name (since locales without an '@' modifier should
have a total ordering of all characters — see the Base Definitions
volume of POSIX.1‐2017, Section 7.3.2, LC_COLLATE). Note that if
the implementation provides a stable sort option as an extension
(usually -s), the additional comparison should not be performed
when this option has been specified.
A future version of this standard may require that if the
collating sequence of the current locale does not have a total
ordering of all characters, any lines of input that collate
equally when comparing them as whole lines are further compared
byte-by-byte using the collating sequence for the POSIX locale.
comm(1p), join(1p), uniq(1p)
The Base Definitions volume of POSIX.1‐2017, Section 7.3.2,
LC_COLLATE, Chapter 8, Environment Variables, Section 12.2,
Utility Syntax Guidelines
The System Interfaces volume of POSIX.1‐2017, toupper(3p)
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 SORT(1P)
Pages that refer to this page: comm(1p), join(1p), uniq(1p)
|
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. |
|