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Operating on sorted files

These commands work with (or produce) sorted files.

sort: Sort text files

sort sorts, merges, or compares all the lines from the given files, or standard input if none are given or for a file of -. By default, sort writes the results to standard output. Synopsis:

sort [option]... [file]...

sort has three modes of operation: sort (the default), merge, and check for sortedness. The following options change the operation mode:

-c -c Check whether the given files are already sorted: if they are not all sorted, print an error message and exit with a status of 1.

-m -m Merge the given files by sorting them as a group. Each input file must always be individually sorted. It always works to sort instead of merge; merging is provided because it is faster, in the case where it works.

A pair of lines is compared as follows: if any key fields have been specified, sort compares each pair of fields, in the order specified on the command line, according to the associated ordering options, until a difference is found or no fields are left.

If any of the global options Mbdfinr are given but no key fields are specified, sort compares the entire lines according to the global options.

Finally, as a last resort when all keys compare equal (or if no ordering options were specified at all), sort compares the lines byte by byte in machine collating sequence. The last resort comparison honors the -r global option. The -s (stable) option disables this last-resort comparison so that lines in which all fields compare equal are left in their original relative order. If no fields or global options are specified, -s has no effect.

GNU sort (as specified for all GNU utilities) has no limits on input line length or restrictions on bytes allowed within lines. In addition, if the final byte of an input file is not a newline, GNU sort silently supplies one.

If the environment variable TMPDIR is set, sort uses its value as the directory for temporary files instead of /tmp. The -T tempdir option in turn overrides the environment variable.

The following options affect the ordering of output lines. They may be specified globally or as part of a specific key field. If no key fields are specified, global options apply to comparison of entire lines; otherwise the global options are inherited by key fields that do not specify any special options of their own.

-b -b Ignore leading blanks when finding sort keys in each line.

-d -d Sort in phone directory order: ignore all characters except letters, digits and blanks when sorting.

-f -f Fold lowercase characters into the equivalent uppercase characters when sorting so that, for example, b and B sort as equal.

-i -i Ignore characters outside the printable ASCII range 040-0176 octal (inclusive) when sorting.

-M -M An initial string, consisting of any amount of whitespace, followed by three letters abbreviating a month name, is folded to UPPER case and compared in the order JAN FEB DEC. Invalid names compare low to valid names.

-n -n Sort numerically: the number begins each line; specifically, it consists of optional whitespace, an optional - sign, and zero or more digits, optionally followed by a decimal point and zero or more digits.

-r -r Reverse the result of comparison, so that lines with greater key values appear earlier in the output instead of later.

Other options are:

-o output-file -o Write output to output-file instead of standard output. If output-file is one of the input files, sort copies it to a temporary file before sorting and writing the output to output-file.

-t separator -t Use character separator as the field separator when finding the sort keys in each line. By default, fields are separated by the empty string between a non-whitespace character and a whitespace character. That is, given the input line foo bar, sort breaks it into fields foo and bar. The field separator is not considered to be part of either the field preceding or the field following.

-u -u For the default case or the -m option, only output the first of a sequence of lines that compare equal. For the -c option, check that no pair of consecutive lines compares equal.

-k pos1[,pos2] -k The recommended, POSIX, option for specifying a sort field. The field consists of the line between pos1 and pos2 (or the end of the line, if pos2 is omitted), inclusive. Fields and character positions are numbered starting with 1. See below.

+pos1[-pos2] The obsolete, traditional option for specifying a sort field. The field consists of the line between pos1 and up to but not including pos2 (or the end of the line if pos2 is omitted). Fields and character positions are numbered starting with 0. See below.

In addition, when GNU sort is invoked with exactly one argument, options --help and --version are recognized. See section Common options.

Historical (BSD and System V) implementations of sort have differed in their interpretation of some options, particularly -b, -f, and -n. GNU sort follows the POSIX behavior, which is usually (but not always!) like the System V behavior. According to POSIX, -n no longer implies -b. For consistency, -M has been changed in the same way. This may affect the meaning of character positions in field specifications in obscure cases. The only fix is to add an explicit -b.

A position in a sort field specified with the -k or + option has the form f.c, where f is the number of the field to use and c is the number of the first character from the beginning of the field (for +pos) or from the end of the previous field (for -pos). If the .c is omitted, it is taken to be the first character in the field. If the -b option was specified, the .c part of a field specification is counted from the first nonblank character of the field (for +pos) or from the first nonblank character following the previous field (for -pos).

A sort key option may also have any of the option letters Mbdfinr appended to it, in which case the global ordering options are not used for that particular field. The -b option may be independently attached to either or both of the +pos and -pos parts of a field specification, and if it is inherited from the global options it will be attached to both. If a -n or -M option is used, thus implying a -b option, the -b option is taken to apply to both the +pos and the -pos parts of a key specification. Keys may span multiple fields.

Here are some examples to illustrate various combinations of options. In them, the POSIX -k option is used to specify sort keys rather than the obsolete +pos1-pos2 syntax.

  • Sort in descending (reverse) numeric order.
    sort -nr

    Sort alphabetically, omitting the first and second fields. This uses a single key composed of the characters beginning at the start of field three and extending to the end of each line.

    sort -k3
  • Sort numerically on the second field and resolve ties by sorting alphabetically on the third and fourth characters of field five. Use : as the field delimiter.
    sort -t : -k 2,2n -k 5.3,5.4

    Note that if you had written -k 2 instead of -k 2,2 sort would have used all characters beginning in the second field and extending to the end of the line as the primary numeric key. For the large majority of applications, treating keys spanning more than one field as numeric will not do what you expect.

    Also note that the n modifier was applied to the field-end specifier for the first key. It would have been equivalent to specify -k 2n,2 or -k 2n,2n. All modifiers except b apply to the associated field, regardless of whether the modifier character is attached to the field-start and/or the field-end part of the key specifier.

  • Sort the password file on the fifth field and ignore any leading white space. Sort lines with equal values in field five on the numeric user ID in field three.
    sort -t : -k 5b,5 -k 3,3n /etc/passwd

    An alternative is to use the global numeric modifier -n.

    sort -t : -n -k 5b,5 -k 3,3 /etc/passwd

    Finally, to ignore both leading and trailing white space, you could have applied the b modifier to the field-end specifier for the first key,

    sort -t : -n -k 5b,5b -k 3,3 /etc/passwd

    or by using the global -b modifier instead of -n and an explicit n with the second key specifier.

    sort -t : -b -k 5,5 -k 3,3n /etc/passwd

uniq: Uniqify files

uniq writes the unique lines in the given input, or standard input if nothing is given or for an input name of -. Synopsis:

uniq [option]... [input [output]]

By default, uniq prints the unique lines in a sorted file, i.e., discards all but one of identical successive lines. Optionally, it can instead show only lines that appear exactly once, or lines that appear more than once.

The input must be sorted. If your input is not sorted, perhaps you want to use sort -u.

If no output file is specified, uniq writes to standard output.

The program accepts the following options. Also see section Common options.

-n -f n --skip-fields=n -n -f --skip-fields Skip n fields on each line before checking for uniqueness. Fields are sequences of non-space non-tab characters that are separated from each other by at least one spaces or tabs.

+n -s n --skip-chars=n +n -s --skip-chars Skip n characters before checking for uniqueness. If you use both the field and character skipping options, fields are skipped over first.

-c --count -c --count Print the number of times each line occurred along with the line.

-d --repeated -d --repeated Print only duplicate lines.

-u --unique -u --unique Print only unique lines.

-w n --check-chars=n -w --check-chars Compare n characters on each line (after skipping any specified fields and characters). By default the entire rest of the lines are compared.

comm: Compare two sorted files line by line

comm writes to standard output lines that are common, and lines that are unique, to two input files; a file name of - means standard input. Synopsis:

comm [option]... file1 file2

The input files must be sorted before comm can be used.

With no options, comm produces three column output. Column one contains lines unique to file1, column two contains lines unique to file2, and column three contains lines common to both files. Columns are separated by TAB.

-1 -2 -3 The options -1, -2, and -3 suppress printing of the corresponding columns. Also see section Common options.



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