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+'\" et
+.TH SORT "1P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
+.\"
+.SH 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.
+.\"
+.SH NAME
+sort
+\(em sort, merge, or sequence check text files
+.SH SYNOPSIS
+.LP
+.nf
+sort \fB[\fR-m\fB] [\fR-o \fIoutput\fB] [\fR-bdfinru\fB] [\fR-t \fIchar\fB] [\fR-k \fIkeydef\fB]\fR... \fB[\fIfile\fR...\fB]\fR
+.P
+sort \fB[\fR-c|-C\fB] [\fR-bdfinru\fB] [\fR-t \fIchar\fB] [\fR-k \fIkeydef\fB] [\fIfile\fB]\fR
+.fi
+.SH DESCRIPTION
+The
+.IR sort
+utility shall perform one of the following functions:
+.IP " 1." 4
+Sort lines of all the named files together and write the result to
+the specified output.
+.IP " 2." 4
+Merge lines of all the named (presorted) files together and write the
+result to the specified output.
+.IP " 3." 4
+Check that a single input file is correctly presorted.
+.P
+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\(hy2017,
+.IR "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.
+.SH OPTIONS
+The
+.IR sort
+utility shall conform to the Base Definitions volume of POSIX.1\(hy2017,
+.IR "Section 12.2" ", " "Utility Syntax Guidelines",
+except for Guideline 9, and the
+.BR \-k
+.IR keydef
+option should follow the
+.BR \-b ,
+.BR \-d ,
+.BR \-f ,
+.BR \-i ,
+.BR \-n ,
+and
+.BR \-r
+options. In addition,
+.BR '\(pl'
+may be recognized as an option delimiter as well as
+.BR '\-' .
+.P
+The following options shall be supported:
+.IP "\fB\-c\fP" 10
+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
+.BR \-u ,
+a duplicate key) is detected, a warning message shall be sent to
+standard error indicating where the disorder or duplicate key
+was found.
+.IP "\fB\-C\fP" 10
+Same as
+.BR \-c ,
+except that a warning message shall not be sent to standard error
+if disorder or, with
+.BR \-u ,
+a duplicate key is detected.
+.IP "\fB\-m\fP" 10
+Merge only; the input file shall be assumed to be already sorted.
+.IP "\fB\-o\ \fIoutput\fR" 10
+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
+.IR file s.
+.IP "\fB\-u\fP" 10
+Unique: suppress all but one in each set of lines having equal keys.
+If used with the
+.BR \-c
+option, check that there are no lines with duplicate keys, in addition
+to checking that the input file is sorted.
+.P
+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
+.BR \-k ),
+the specified ordering options shall override all global ordering
+options for that key.
+.IP "\fB\-d\fP" 10
+Specify that only
+<blank>
+characters and alphanumeric characters, according to the current
+setting of
+.IR LC_CTYPE ,
+shall be significant in comparisons. The behavior is undefined for a
+sort key to which
+.BR \-i
+or
+.BR \-n
+also applies.
+.IP "\fB\-f\fP" 10
+Consider all lowercase characters that have uppercase equivalents,
+according to the current setting of
+.IR LC_CTYPE ,
+to be the uppercase equivalent for the purposes of comparison.
+.IP "\fB\-i\fP" 10
+Ignore all characters that are non-printable, according to the current
+setting of
+.IR LC_CTYPE .
+The behavior is undefined for a sort key for which
+.BR \-n
+also applies.
+.IP "\fB\-n\fP" 10
+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.
+.IP "\fB\-r\fP" 10
+Reverse the sense of comparisons.
+.P
+The treatment of field separators can be altered using the options:
+.IP "\fB\-b\fP" 10
+Ignore leading
+<blank>
+characters when determining the starting and ending positions of a
+restricted sort key. If the
+.BR \-b
+option is specified before the first
+.BR \-k
+option, it shall be applied to all
+.BR \-k
+options. Otherwise, the
+.BR \-b
+option can be attached independently to each
+.BR \-k
+.IR field_start
+or
+.IR field_end
+option-argument (see below).
+.IP "\fB\-t\ \fIchar\fR" 10
+Use
+.IR char
+as the field separator character;
+.IR char
+shall not be considered to be part of a field (although it can be
+included in a sort key). Each occurrence of
+.IR char
+shall be significant (for example, <\fIchar\fR><\fIchar\fR> delimits an
+empty field). If
+.BR \-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-\c
+<blank>
+shall be a field separator.
+.P
+Sort keys can be specified using the options:
+.IP "\fB\-k\ \fIkeydef\fR" 10
+The
+.IR keydef
+argument is a restricted sort key field definition. The format of this
+definition is:
+.RS 10
+.sp
+.RS 4
+.nf
+
+\fIfield_start\fB[\fItype\fB][\fR,\fIfield_end\fB[\fItype\fB]]\fR
+.fi
+.P
+.RE
+.P
+where
+.IR field_start
+and
+.IR field_end
+define a key field restricted to a portion of the line (see the
+EXTENDED DESCRIPTION section), and
+.IR type
+is one or more modifiers from the list of characters
+.BR 'b' ,
+.BR 'd' ,
+.BR 'f' ,
+.BR 'i' ,
+.BR 'n' ,
+.BR 'r' .
+The
+.BR 'b'
+modifier shall behave like the
+.BR \-b
+option, but shall apply only to the
+.IR field_start
+or
+.IR 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
+.IR field_start ,
+.IR field_end ,
+or both. If any modifier is attached to a
+.IR field_start
+or to a
+.IR field_end ,
+no option shall apply to either. Implementations shall support at
+least nine occurrences of the
+.BR \-k
+option, which shall be significant in command line order. If no
+.BR \-k
+option is specified, a default sort key of the entire line shall be
+used.
+.P
+When there are multiple key fields, later keys shall be compared only
+after all earlier keys compare equal. Except when the
+.BR \-u
+option is specified, lines that otherwise compare equal shall be
+ordered as if none of the options
+.BR \-d ,
+.BR \-f ,
+.BR \-i ,
+.BR \-n ,
+or
+.BR \-k
+were present (but with
+.BR \-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.
+.RE
+.SH OPERANDS
+The following operand shall be supported:
+.IP "\fIfile\fR" 10
+A pathname of a file to be sorted, merged, or checked. If no
+.IR file
+operands are specified, or if a
+.IR file
+operand is
+.BR '\-' ,
+the standard input shall be used. If
+.IR sort
+encounters an error when opening or reading a
+.IR file
+operand, it may exit without writing any output to standard output or
+processing later operands.
+.SH STDIN
+The standard input shall be used only if no
+.IR file
+operands are specified, or if a
+.IR file
+operand is
+.BR '\-' .
+See the INPUT FILES section.
+.SH "INPUT FILES"
+The input files shall be text files, except that the
+.IR sort
+utility shall add a
+<newline>
+to the end of a file ending with an incomplete last line.
+.SH "ENVIRONMENT VARIABLES"
+The following environment variables shall affect the execution of
+.IR sort :
+.IP "\fILANG\fP" 10
+Provide a default value for the internationalization variables that are
+unset or null. (See the Base Definitions volume of POSIX.1\(hy2017,
+.IR "Section 8.2" ", " "Internationalization Variables"
+for the precedence of internationalization variables used to determine
+the values of locale categories.)
+.IP "\fILC_ALL\fP" 10
+If set to a non-empty string value, override the values of all the
+other internationalization variables.
+.IP "\fILC_COLLATE\fP" 10
+.br
+Determine the locale for ordering rules.
+.IP "\fILC_CTYPE\fP" 10
+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
+.BR \-b ,
+.BR \-d ,
+.BR \-f ,
+.BR \-i ,
+and
+.BR \-n
+options.
+.IP "\fILC_MESSAGES\fP" 10
+.br
+Determine the locale that should be used to affect the format and
+contents of diagnostic messages written to standard error.
+.IP "\fILC_NUMERIC\fP" 10
+.br
+Determine the locale for the definition of the radix character and
+thousands separator for the
+.BR \-n
+option.
+.IP "\fINLSPATH\fP" 10
+Determine the location of message catalogs for the processing of
+.IR LC_MESSAGES .
+.SH "ASYNCHRONOUS EVENTS"
+Default.
+.SH STDOUT
+Unless the
+.BR \-o
+or
+.BR \-c
+options are in effect, the standard output shall contain the sorted
+input.
+.SH STDERR
+The standard error shall be used for diagnostic messages. When
+.BR \-c
+is specified, if disorder is detected (or if
+.BR \-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.
+.SH "OUTPUT FILES"
+If the
+.BR \-o
+option is in effect, the sorted input shall be written to the file
+.IR output .
+.SH "EXTENDED DESCRIPTION"
+The notation:
+.sp
+.RS 4
+.nf
+
+-k \fIfield_start\fB[\fItype\fB][\fR,\fIfield_end\fB[\fItype\fB]]\fR
+.fi
+.P
+.RE
+.P
+shall define a key field that begins at
+.IR field_start
+and ends at
+.IR field_end
+inclusive, unless
+.IR field_start
+falls beyond the end of the line or after
+.IR field_end ,
+in which case the key field is empty. A missing
+.IR field_end
+shall mean the last character of the line.
+.P
+A field comprises a maximal sequence of non-separating characters and,
+in the absence of option
+.BR \-t ,
+any preceding field separator.
+.P
+The
+.IR field_start
+portion of the
+.IR keydef
+option-argument shall have the form:
+.sp
+.RS 4
+.nf
+
+\fIfield_number\fB[\fR.\fIfirst_character\fB]\fR
+.fi
+.P
+.RE
+.P
+Fields and characters within fields shall be numbered starting with 1.
+The
+.IR field_number
+and
+.IR first_character
+pieces, interpreted as positive decimal integers, shall specify the
+first character to be used as part of a sort key. If
+.IR .first_character
+is omitted, it shall refer to the first character of the field.
+.P
+The
+.IR field_end
+portion of the
+.IR keydef
+option-argument shall have the form:
+.sp
+.RS 4
+.nf
+
+\fIfield_number\fB[\fR.\fIlast_character\fB]\fR
+.fi
+.P
+.RE
+.P
+The
+.IR field_number
+shall be as described above for
+.IR field_start.
+The
+.IR 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
+.IR last_character
+evaluates to zero or
+.IR .last_character
+is omitted, it shall refer to the last character of the field specified
+by
+.IR field_number .
+.P
+If the
+.BR \-b
+option or
+.BR b
+type modifier is in effect, characters within a field shall be counted
+from the first non-\c
+<blank>
+in the field. (This shall apply separately to
+.IR first_character
+and
+.IR last_character .)
+.SH "EXIT STATUS"
+The following exit values shall be returned:
+.IP "\00" 6
+All input files were output successfully, or
+.BR \-c
+was specified and the input file was correctly sorted.
+.IP "\01" 6
+Under the
+.BR \-c
+option, the file was not ordered as specified, or if the
+.BR \-c
+and
+.BR \-u
+options were both specified, two input lines were found with equal
+keys.
+.IP >1 6
+An error occurred.
+.SH "CONSEQUENCES OF ERRORS"
+The default requirements shall apply, except that if
+.IR sort
+encounters an error when opening or reading a
+.IR file
+operand, it may exit without writing any output to standard output or
+processing later operands.
+.LP
+.IR "The following sections are informative."
+.SH "APPLICATION USAGE"
+The default value for
+.BR \-t ,
+<blank>,
+has different properties from, for example,
+.BR \-t \c
+"<space>". If a line contains:
+.sp
+.RS 4
+.nf
+
+<space><space>foo
+.fi
+.P
+.RE
+.P
+the following treatment would occur with default separation as opposed
+to specifically selecting a
+<space>:
+.TS
+center box tab(@);
+cB | cB | cB
+n | l | l.
+Field@Default@\-t "<space>"
+_
+1@<space><space>foo@\fIempty\fP
+2@\fIempty\fP@\fIempty\fP
+3@\fIempty\fP@foo
+.TE
+.P
+The leading field separator itself is included in a field when
+.BR \-t
+is not used. For example, this command returns an exit status of zero,
+meaning the input was already sorted:
+.sp
+.RS 4
+.nf
+
+sort -c -k 2 <<eof
+y<tab>b
+x<space>a
+eof
+.fi
+.P
+.RE
+.P
+(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
+.BR \-t .
+This is historical practice and allows usage such as:
+.sp
+.RS 4
+.nf
+
+sort -t "|" -k 2n <<eof
+Atlanta|425022|Georgia
+Birmingham|284413|Alabama
+Columbia|100385|South Carolina
+eof
+.fi
+.P
+.RE
+.P
+where the second field can be correctly sorted numerically without
+regard to the non-numeric field separator.
+.P
+The wording in the OPTIONS section clarifies that the
+.BR \-b ,
+.BR \-d ,
+.BR \-f ,
+.BR \-i ,
+.BR \-n ,
+and
+.BR \-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\(hy2017 matches historical practice, not historical documentation.
+The results are unspecified if these options are specified after a
+.BR \-k
+option.
+.P
+The
+.BR \-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.
+.P
+When using
+.IR 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.
+.P
+If the collating sequence of the current locale does not have a total
+ordering of all characters, this can affect the behavior of
+.IR sort
+in the following ways:
+.IP " *" 4
+As
+.IR sort
+.BR \-u
+suppresses lines with duplicate keys, it suppresses lines that collate
+equally but are not identical.
+.IP " *" 4
+The output of
+.IR sort
+(without
+.BR \-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
+.IR sort
+with
+.IR comm
+and
+.IR uniq ;
+see the APPLICATION USAGE for those utilities.
+.SH EXAMPLES
+.IP " 1." 4
+The following command sorts the contents of
+.BR infile
+with the second field as the sort key:
+.RS 4
+.sp
+.RS 4
+.nf
+
+sort -k 2,2 infile
+.fi
+.P
+.RE
+.RE
+.IP " 2." 4
+The following command sorts, in reverse order, the contents of
+.BR infile1
+and
+.BR infile2 ,
+placing the output in
+.BR 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):
+.RS 4
+.sp
+.RS 4
+.nf
+
+sort -r -o outfile -k 2.2,2.2 infile1 infile2
+.fi
+.P
+.RE
+.RE
+.IP " 3." 4
+The following command sorts the contents of
+.BR infile1
+and
+.BR infile2
+using the second non-\c
+<blank>
+of the second field as the sort key:
+.RS 4
+.sp
+.RS 4
+.nf
+
+sort -k 2.2b,2.2b infile1 infile2
+.fi
+.P
+.RE
+.RE
+.IP " 4." 4
+The following command prints the System\ V password file (user
+database) sorted by the numeric user ID (the third
+<colon>-separated
+field):
+.RS 4
+.sp
+.RS 4
+.nf
+
+sort -t : -k 3,3n /etc/passwd
+.fi
+.P
+.RE
+.RE
+.IP " 5." 4
+The following command prints the lines of the already sorted file
+.BR infile ,
+suppressing all but one occurrence of lines having the same third
+field:
+.RS 4
+.sp
+.RS 4
+.nf
+
+sort -um -k 3.1,3.0 infile
+.fi
+.P
+.RE
+.RE
+.SH RATIONALE
+Examples in some historical documentation state that options
+.BR \-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.
+.P
+The
+.BR \-z
+option was omitted; it is not standard practice on most systems and is
+inconsistent with using
+.IR sort
+to sort several files individually and then merge them together. The
+text concerning
+.BR \-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.
+.P
+The
+.BR \-y
+option was omitted because of non-portability. The
+.BR \-M
+option, present in System V, was omitted because of non-portability in
+international usage.
+.P
+An undocumented
+.BR \-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
+.IR TMPDIR
+environment variable instead of adding an option to support this
+functionality.
+.P
+The
+.BR \-k
+option was added to satisfy two objections. First, the zero-based
+counting used by
+.IR sort
+is not consistent with other utility conventions. Second, it did not
+meet syntax guideline requirements.
+.P
+Historical documentation indicates that ``setting
+.BR \-n
+implies
+.BR \-b ''.
+The description of
+.BR \-n
+already states that optional leading <blank>s are tolerated in doing
+the comparison. If
+.BR \-b
+is enabled, rather than implied, by
+.BR \-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
+.IR sort
+utility, the
+.BR \-b
+implication has been omitted from this volume of POSIX.1\(hy2017, and an application wishing to
+achieve the previously mentioned side-effects has to code the
+.BR \-b
+flag explicitly.
+.P
+Earlier versions of this standard allowed the
+.BR \-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.
+.P
+Earlier versions of this standard also allowed the
+.BR \- \c
+.IR number
+and
+.BR \(pl \c
+.IR number
+options. These options are no longer specified by POSIX.1\(hy2008 but may
+be present in some implementations.
+.P
+Historical implementations produced a message on standard error when
+.BR \-c
+was specified and disorder was detected, and when
+.BR \-c
+and
+.BR \-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:
+.sp
+.RS 4
+.nf
+
+sort -c file || echo disorder
+.fi
+.P
+.RE
+.P
+whether or not the
+.IR 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
+.BR \-b ,
+.BR \-d ,
+.BR \-f ,
+.BR \-i ,
+.BR \-n ,
+or
+.BR \- r
+options or
+.IR keydef
+type modifiers were in use. POSIX.1\(hy2008 requires a message to be
+produced in this case. POSIX.1\(hy2008 also contains the
+.BR \-C
+option giving users the ability to choose either behavior.
+.P
+When a disorder or duplicate is found when the
+.BR \-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.
+.P
+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
+.BR '@'
+modifier in the name (since locales without an
+.BR '@'
+modifier should have a total ordering of all characters \(em see the Base Definitions volume of POSIX.1\(hy2017,
+.IR "Section 7.3.2" ", " "LC_COLLATE").
+Note that if the implementation provides a
+.IR "stable sort"
+option as an extension (usually
+.BR \-s ),
+the additional comparison should not be performed when this option has
+been specified.
+.SH "FUTURE DIRECTIONS"
+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.
+.SH "SEE ALSO"
+.IR "\fIcomm\fR\^",
+.IR "\fIjoin\fR\^",
+.IR "\fIuniq\fR\^"
+.P
+The Base Definitions volume of POSIX.1\(hy2017,
+.IR "Section 7.3.2" ", " "LC_COLLATE",
+.IR "Chapter 8" ", " "Environment Variables",
+.IR "Section 12.2" ", " "Utility Syntax Guidelines"
+.P
+The System Interfaces volume of POSIX.1\(hy2017,
+.IR "\fItoupper\fR\^(\|)"
+.\"
+.SH 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 .
+.PP
+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 .