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SORT(1P) POSIX Programmer's Manual SORT(1P)

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
sort — sort, merge, or sequence check text files

SYNOPSIS
sort [−m] [−o output] [−bdfinru] [−t char] [−k keydef]... [file...]

sort [−c|−C] [−bdfinru] [−t char] [−k keydef] [file]

DESCRIPTION
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 per‐
formed using the collating sequence of the current locale.

OPTIONS
The sort utility shall conform to the Base Definitions volume of
POSIX.1‐2008, 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 charac‐
ters, 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 equiva‐
lents, 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, consist‐
ing of optional <blank> characters, optional minus-sign, 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 start‐
ing 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 sepa‐
rator.

Sort keys can be specified using the options:

−k keydef The keydef argument is a restricted sort key field defini‐
tion. 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 sec‐
tion), and type is a modifier 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 com‐
pared 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.

OPERANDS
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.

STDIN
The standard input shall be used only if no file operands are speci‐
fied, or if a file operand is '−'. See the INPUT FILES section.

INPUT FILES
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.

ENVIRONMENT VARIABLES
The following environment variables shall affect the execution of sort:

LANG Provide a default value for the internationalization vari‐
ables that are unset or null. (See the Base Definitions vol‐
ume of POSIX.1‐2008, Section 8.2, Internationalization Vari‐
ables 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 charac‐
ter and thousands separator for the −n option.

NLSPATH Determine the location of message catalogs for the processing
of LC_MESSAGES.

ASYNCHRONOUS EVENTS
Default.

STDOUT
Unless the −o or −c options are in effect, the standard output shall
contain the sorted input.

STDERR
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.

OUTPUT FILES
If the −o option is in effect, the sorted input shall be written to the
file output.

EXTENDED DESCRIPTION
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 miss‐
ing 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_num‐
ber.

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.)

EXIT STATUS
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.

CONSEQUENCES OF ERRORS
Default.

The following sections are informative.

APPLICATION USAGE
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:

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.1‐2008 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.

EXAMPLES
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 data‐
base) 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

RATIONALE
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 doc‐
umentation 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 interna‐
tional usage.

An undocumented −T option exists in some implementations. It is used to
specify a directory for intermediate files. Implementations are encour‐
aged 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 con‐
ventions. 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.1‐2008, and an application wishing to achieve the previously men‐
tioned 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 speci‐
fied and a duplicate key was detected. An earlier version of this stan‐
dard 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 modi‐
fiers 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 speci‐
fying the line number of the offending line. This standard allows
either type of message.

FUTURE DIRECTIONS
None.