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Top Forums UNIX for Dummies Questions & Answers Explanation of the sort command Post 302758741 by bakunin on Sunday 20th of January 2013 05:39:36 PM
Old 01-20-2013
Quote:
Originally Posted by jimbojames
I was confused with what the
Code:
4,4n

was doing, but your explanation has helped with this, although I still don't understand what the 4 before the comma is doing.
"sort" interprets lines as "fields" being delimited by "delimiters". Per default this delimiter is whitespace.

Per default "sort" sorts on field 1, then on field 2, then field 3 and so on until the end of the line. Lines with equal values of field 1 will be sorted on the contents of field 2, lines with equal fields 1 & 2 sorted on field 3, etc. If you want another field or part of it) as the primary key you will have to use the "-k" option and a field number. But this leaves the question, how lines with equal sort keys should be handled.

Per default sort will use the fields starting with field 1 as secondary sort key in this case:

Code:
sort -k4

will produce the sort order: f4, f1, f2, f3, f4, ....

By defining, where the key should end you can change this default behavior:

Code:
sort -k4,4

Sorting occurs exclusively on field 4, because the ",4" says the ending of the sort key is field 4.

The man page of sort should explain this in greater detail.

I hope this helps.

bakunin
 

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SORT(1) 						      General Commands Manual							   SORT(1)

NAME
sort - sort or merge files SYNOPSIS
sort [ -_________x ] [ +pos1 [ -pos2 ] ] ... [ -o name ] [ -T directory ] [ name ] ... DESCRIPTION
Sort sorts lines of all the named files together and writes the result on the standard output. The name `-' means the standard input. If no input files are named, the standard input is sorted. The default sort key is an entire line. Default ordering is lexicographic by bytes in machine collating sequence. The ordering is affected globally by the following options, one or more of which may appear. b Ignore leading blanks (spaces and tabs) in field comparisons. d `Dictionary' order: only letters, digits and blanks are significant in comparisons. f Fold upper case letters onto lower case. i Ignore characters outside the ASCII range 040-0176 in nonnumeric comparisons. n An initial numeric string, consisting of optional blanks, optional minus sign, and zero or more digits with optional decimal point, is sorted by arithmetic value. Option n implies option b. r Reverse the sense of comparisons. tx `Tab character' separating fields is x. The notation +pos1 -pos2 restricts a sort key to a field beginning at pos1 and ending just before pos2. Pos1 and pos2 each have the form m.n, optionally followed by one or more of the flags bdfinr, where m tells a number of fields to skip from the beginning of the line and n tells a number of characters to skip further. If any flags are present they override all the global ordering options for this key. If the b option is in effect n is counted from the first nonblank in the field; b is attached independently to pos2. A missing .n means .0; a missing -pos2 means the end of the line. Under the -tx option, fields are strings separated by x; otherwise fields are nonempty nonblank strings separated by blanks. When there are multiple sort keys, later keys are compared only after all earlier keys compare equal. Lines that otherwise compare equal are ordered with all bytes significant. These option arguments are also understood: c Check that the input file is sorted according to the ordering rules; give no output unless the file is out of sort. m Merge only, the input files are already sorted. o The next argument is the name of an output file to use instead of the standard output. This file may be the same as one of the inputs. T The next argument is the name of a directory in which temporary files should be made. u Suppress all but one in each set of equal lines. Ignored bytes and bytes outside keys do not participate in this comparison. Examples. Print in alphabetical order all the unique spellings in a list of words. Capitalized words differ from uncapitalized. sort -u +0f +0 list Print the password file (passwd(5)) sorted by user id number (the 3rd colon-separated field). sort -t: +2n /etc/passwd Print the first instance of each month in an already sorted file of (month day) entries. The options -um with just one input file make the choice of a unique representative from a set of equal lines predictable. sort -um +0 -1 dates FILES
/usr/tmp/stm*, /tmp/*: first and second tries for temporary files SEE ALSO
uniq(1), comm(1), rev(1), join(1) DIAGNOSTICS
Comments and exits with nonzero status for various trouble conditions and for disorder discovered under option -c. BUGS
Very long lines are silently truncated. SORT(1)
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