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The GNU Awk User's Guide

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14.2.6 Printing Nonduplicated Lines of Text

The uniq utility reads sorted lines of data on its standard input, and by default removes duplicate lines. In other words, it only prints unique lines--hence the name. uniq has a number of options. The usage is as follows:

uniq [-udc [-n]] [+n] [ input file [ output file ]]

The options for uniq are:

Pnly print only repeated lines.

Print only nonrepeated lines.

Count lines. This option overrides `-d' and `-u'. Both repeated and nonrepeated lines are counted.

Skip n fields before comparing lines. The definition of fields is similar to awk's default: nonwhitespace characters separated by runs of spaces and/or tabs.

Skip n characters before comparing lines. Any fields specified with `-n' are skipped first.

input file
Data is read from the input file named on the command line, instead of from the standard input.

output file
The generated output is sent to the named output file, instead of to the standard output.

Normally uniq behaves as if both the `-d' and `-u' options are provided.

uniq uses the getopt library function (see section Processing Command-Line Options) and the join library function (see section Merging an Array into a String).

The program begins with a usage function and then a brief outline of the options and their meanings in a comment. The BEGIN rule deals with the command-line arguments and options. It uses a trick to get getopt to handle options of the form `-25', treating such an option as the option letter `2' with an argument of `5'. If indeed two or more digits are supplied (Optarg looks like a number), Optarg is concatenated with the option digit and then the result is added to zero to make it into a number. If there is only one digit in the option, then Optarg is not needed. In this case, Optind must be decremented so that getopt processes it next time. This code is admittedly a bit tricky.

If no options are supplied, then the default is taken, to print both repeated and nonrepeated lines. The output file, if provided, is assigned to outputfile. Early on, outputfile is initialized to the standard output, `/dev/stdout':

# uniq.awk --- do uniq in awk
# Requires getopt and join library functions
function usage(    e)
    e = "Usage: uniq [-udc [-n]] [+n] [ in [ out ]]"
    print e > "/dev/stderr"
    exit 1

# -c    count lines. overrides -d and -u
# -d    only repeated lines
# -u    only non-repeated lines
# -n    skip n fields
# +n    skip n characters, skip fields first

    count = 1
    outputfile = "/dev/stdout"
    opts = "udc0:1:2:3:4:5:6:7:8:9:"
    while ((c = getopt(ARGC, ARGV, opts)) != -1) {
        if (c == "u")
        else if (c == "d")
        else if (c == "c")
        else if (index("0123456789", c) != 0) {
            # getopt requires args to options
            # this messes us up for things like -5
            if (Optarg ~ /^[0-9]+$/)
                fcount = (c Optarg) + 0
            else {
                fcount = c + 0
        } else

    if (ARGV[Optind] ~ /^\+[0-9]+$/) {
        charcount = substr(ARGV[Optind], 2) + 0

    for (i = 1; i < Optind; i++)
        ARGV[i] = ""

    if (repeated_only == 0 && non_repeated_only == 0)
        repeated_only = non_repeated_only = 1

    if (ARGC - Optind == 2) {
        outputfile = ARGV[ARGC - 1]
        ARGV[ARGC - 1] = ""

The following function, are_equal, compares the current line, $0, to the previous line, last. It handles skipping fields and characters. If no field count and no character count are specified, are_equal simply returns one or zero depending upon the result of a simple string comparison of last and $0. Otherwise, things get more complicated. If fields have to be skipped, each line is broken into an array using split (see section String Manipulation Functions); the desired fields are then joined back into a line using join. The joined lines are stored in clast and cline. If no fields are skipped, clast and cline are set to last and $0, respectively. Finally, if characters are skipped, substr is used to strip off the leading charcount characters in clast and cline. The two strings are then compared and are_equal returns the result:

function are_equal(    n, m, clast, cline, alast, aline)
    if (fcount == 0 && charcount == 0)
        return (last == $0)

    if (fcount > 0) {
        n = split(last, alast)
        m = split($0, aline)
        clast = join(alast, fcount+1, n)
        cline = join(aline, fcount+1, m)
    } else {
        clast = last
        cline = $0
    if (charcount) {
        clast = substr(clast, charcount + 1)
        cline = substr(cline, charcount + 1)

    return (clast == cline)

The following two rules are the body of the program. The first one is executed only for the very first line of data. It sets last equal to $0, so that subsequent lines of text have something to be compared to.

The second rule does the work. The variable equal is one or zero, depending upon the results of are_equal's comparison. If uniq is counting repeated lines, and the lines are equal, then it increments the count variable. Otherwise, it prints the line and resets count, since the two lines are not equal.

If uniq is not counting, and if the lines are equal, count is incremented. Nothing is printed, since the point is to remove duplicates. Otherwise, if uniq is counting repeated lines and more than one line is seen, or if uniq is counting nonrepeated lines and only one line is seen, then the line is printed, and count is reset.

Finally, similar logic is used in the END rule to print the final line of input data:

NR == 1 {
    last = $0

    equal = are_equal()

    if (do_count) {    # overrides -d and -u
        if (equal)
        else {
            printf("%4d %s\n", count, last) > outputfile
            last = $0
            count = 1    # reset

    if (equal)
    else {
        if ((repeated_only && count > 1) ||
            (non_repeated_only && count == 1))
                print last > outputfile
        last = $0
        count = 1

    if (do_count)
        printf("%4d %s\n", count, last) > outputfile
    else if ((repeated_only && count > 1) ||
            (non_repeated_only && count == 1))
        print last > outputfile

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