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

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 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.

The utility shall conform to the Base Definitions volume of
POSIX.1‐2008, except for Guideline 9, and the option should
follow the and options. In addition, may be recognized as an
option delimiter as well as The following options shall be

−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 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 except that a warning message shall not be
          sent to standard error if disorder or, with 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

−u        Unique: suppress all but one in each set of lines
          having equal keys.  If used with the 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 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 shall be significant in comparisons.
          The behavior is undefined for a sort key to which
          or also applies.

−f        Consider all lowercase characters that have
          uppercase equivalents, according to the current
          setting of to be the uppercase equivalent for the
          purposes of comparison.

−i        Ignore all characters that are non‐printable,
          according to the current setting of The behavior
          is undefined for a sort key for which also

−n        Restrict the sort key to an initial numeric
          string, consisting 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

−r        Reverse the sense of comparisons.  The treatment
          of field separators can be altered using the

−b        Ignore leading <blank> characters when determining
          the starting and ending positions of a restricted
          sort key. If the option is specified before the
          first option, it shall be applied to all options.
          Otherwise, the option can be attached
          independently to each or option‐argument (see


−t char   Use as the field separator character; shall not be
          considered to be part of a field (although it can
          be included in a sort key). Each occurrence of
          shall be significant (for example, <char><char>
          delimits an empty field). If 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 argument is a restricted sort key field
          definition. The format of this definition is:

                    where and define a key field restricted
                    to a portion of the line (see the
                    EXTENDED DESCRIPTION section), and is a
                    modifier from the list of characters The
                    modifier shall behave like the option,
                    but shall apply only to the or 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
                    or both. If any modifier is attached to
                    a or to a no option shall apply to
                    either. Implementations shall support at
                    least nine occurrences of the option,
                    which shall be significant in command
                    line order. If no 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 option is
                    specified, lines that otherwise compare
                    equal shall be ordered as if none of the
                    options or were present (but with 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

The following operand shall be supported:

file      A pathname of a file to be sorted, merged, or
          checked. If no operands are specified, or if a
          operand is the standard input shall be used.


The standard input shall be used only if no operands are
specified, or if a operand is See the INPUT FILES section.

The input files shall be text files, except that the 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

LANG      Provide a default value for the
          internationalization variables that are unset or
          null. (See the Base Definitions volume of
          POSIX.1‐2008, 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

          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 and

          Determine the locale that should be used to affect
          the format and contents of diagnostic messages
          written to standard error.

          Determine the locale for the definition of the
          radix character and thousands separator for the

NLSPATH   Determine the location of message catalogs for the
          processing of


Unless the or options are in effect, the standard output
shall contain the sorted input.

The standard error shall be used for diagnostic messages.
When is specified, if disorder is detected (or if 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 option is in effect, the sorted input shall be
written to the file

The notation:

          −k field_start[type][,field_end[type]]
shall define a key field that begins at and ends at
inclusive, unless falls beyond the end of the line or after
in which case the key field is empty. A missing shall mean
the last character of the line.  A field comprises a maximal
sequence of non‐separating characters and, in the absence of
option any preceding field separator.  The portion of the
option‐argument shall have the form:

Fields and characters within fields shall be numbered
starting with 1.  The and pieces, interpreted as positive
decimal integers, shall specify the first character to be
used as part of a sort key. If is omitted, it shall refer to
the first character of the field.  The portion of the
option‐argument shall have the form:

The shall be as described above for The piece, interpreted
as a non‐negative decimal integer, shall specify the last
character to be used as part of the sort key. If evaluates
to zero or is omitted, it shall refer to the last character
of the field specified by If the option or 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 and

The following exit values shall be returned:

 0    All input files were output successfully, or was
      specified and the input file was correctly sorted.

 1    Under the option, the file was not ordered as
      specified, or if the and options were both specified,
      two input lines were found with equal keys.

>1    An error occurred.


The default value for <blank>, has different properties
from, for example, "<space>". If a line contains:



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 is not used. For example, this command returns an exit
status of zero, meaning the input was already sorted:

     sort −c −k 2 <<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 This is historical
practice and allows usage such as:

     sort −t "|" −k 2n <<eof
     Columbia|100385|South Carolina
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 and
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 option.  The option might not work as expected in locales
where there is not a one‐to‐one mapping between an uppercase
and a lowercase letter.

 1. The following command sorts the contents of with the
    second field as the sort key:

             sort −k 2,2 infile

 2. The following command sorts, in reverse order, the
    contents of and placing the output in 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 and using
    the second non‐<blank> of the second field as the sort

             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 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
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 option was omitted;
it is not standard practice on most systems and is
inconsistent with using to sort several files individually
and then merge them together. The text concerning 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 option was
omitted because of non‐portability. The option, present in
System V, was omitted because of non‐portability in
international usage.  An undocumented option exists in some
implementations. It is used to specify a directory for
intermediate files. Implementations are encouraged to
support the use of the environment variable instead of
adding an option to support this functionality.  The option
was added to satisfy two objections. First, the zero‐based
counting used by is not consistent with other utility
conventions. Second, it did not meet syntax guideline
requirements.  Historical documentation indicates that
‘‘setting implies The description of already states that
optional leading <blank>s are tolerated in doing the
comparison. If is enabled, rather than implied, by 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 utility, the implication has been
omitted from this volume of POSIX.1‐2008, and an application
wishing to achieve the previously mentioned side‐effects has
to code the flag explicitly.  Earlier versions of this
standard allowed the 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 and 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
was specified and disorder was detected, and when and 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 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 or
options or type modifiers were in use. POSIX.1‐2008 requires
a message to be produced in this case. POSIX.1‐2008 also
contains the option giving users the ability to choose
either behavior.  When a disorder or duplicate is found when
the 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.


The Base Definitions volume of POSIX.1‐2008, The System
Interfaces volume of POSIX.1‐2008,

Portions of this text are reprinted and reproduced in
electronic form from IEEE Std 1003.1, 2013 Edition, Standard
for Information Technology ‐‐ Portable Operating System
Interface (POSIX), The Open Group Base Specifications Issue
7, Copyright (C) 2013 by the Institute of Electrical and
Electronics Engineers, Inc and The Open Group.  (This is
POSIX.1‐2008 with the 2013 Technical Corrigendum 1 applied.)
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 .

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