Practical awk Programs

Practical awk Programs

This chapter presents a potpourri of awk programs for your reading enjoyment. There are two sections. The first presents awk versions of several common POSIX utilities. The second is a grab-bag of interesting programs.

Many of these programs use the library functions presented in section A Library of awk Functions.

Re-inventing Wheels for Fun and Profit

This section presents a number of POSIX utilities that are implemented in awk. Re-inventing these programs in awk is often enjoyable, since the algorithms can be very clearly expressed, and usually the code is very concise and simple. This is true because awk does so much for you.

It should be noted that these programs are not necessarily intended to replace the installed versions on your system. Instead, their purpose is to illustrate awk language programming for "real world" tasks.

The programs are presented in alphabetical order.

Cutting Out Fields and Columns

The cut utility selects, or "cuts," either characters or fields from its standard input and sends them to its standard output. cut can cut out either a list of characters, or a list of fields. By default, fields are separated by tabs, but you may supply a command line option to change the field delimiter, i.e. the field separator character. cut's definition of fields is less general than awk's.

A common use of cut might be to pull out just the login name of logged-on users from the output of who. For example, the following pipeline generates a sorted, unique list of the logged on users: 

who | cut -c1-8 | sort | uniq

The options for cut are:

-c list
Use list as the list of characters to cut out. Items within the list may be separated by commas, and ranges of characters can be separated with dashes. The list `1-8,15,22-35' specifies characters one through eight, 15, and 22 through 35.
-f list
Use list as the list of fields to cut out.
-d delim
Use delim as the field separator character instead of the tab character.
-s
Suppress printing of lines that do not contain the field delimiter.

The awk implementation of cut 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 comment describing the options and a usage function which prints out a usage message and exits. usage is called if invalid arguments are supplied. 

# cut.awk -- implement cut in awk
# Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain
# May 1993

# Options:
#    -f list        Cut fields
#    -d c           Field delimiter character
#    -c list        Cut characters
#
#    -s        Suppress lines without the delimiter character

function usage(    e1, e2)
{
    e1 = "usage: cut [-f list] [-d c] [-s] [files...]"
    e2 = "usage: cut [-c list] [files...]"
    print e1 > "/dev/stderr"
    print e2 > "/dev/stderr"
    exit 1
}

The variables e1 and e2 are used so that the function fits nicely on the page.

Next comes a BEGIN rule that parses the command line options. It sets FS to a single tab character, since that is cut's default field separator. The output field separator is also set to be the same as the input field separator. Then getopt is used to step through the command line options. One or the other of the variables by_fields or by_chars is set to true, to indicate that processing should be done by fields or by characters respectively. When cutting by characters, the output field separator is set to the null string. 

BEGIN    \ 
{
    FS = "\t"    # default
    OFS = FS
    while ((c = getopt(ARGC, ARGV, "sf:c:d:")) != -1) {
        if (c == "f") {
            by_fields = 1
            fieldlist = Optarg
        } else if (c == "c") {
            by_chars = 1
            fieldlist = Optarg
            OFS = ""
        } else if (c == "d") {
            if (length(Optarg) > 1) {
                printf("Using first character of %s" \ 
                " for delimiter\n", Optarg) > "/dev/stderr"
                Optarg = substr(Optarg, 1, 1)
            }
            FS = Optarg
            OFS = FS
            if (FS == " ")    # defeat awk semantics
                FS = "[ ]"
        } else if (c == "s")
            suppress++
        else
            usage()
    }

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

Special care is taken when the field delimiter is a space. Using " " (a single space) for the value of FS is incorrect---awk would separate fields with runs of spaces and/or tabs, and we want them to be separated with individual spaces. Also, note that after getopt is through, we have to clear out all the elements of ARGV from one to Optind, so that awk will not try to process the command line options as file names.

After dealing with the command line options, the program verifies that the options make sense. Only one or the other of `-c' and `-f' should be used, and both require a field list. Then either set_fieldlist or set_charlist is called to pull apart the list of fields or characters. 

    if (by_fields && by_chars)
        usage()

    if (by_fields == 0 && by_chars == 0)
        by_fields = 1    # default

    if (fieldlist == "") {
        print "cut: needs list for -c or -f" > "/dev/stderr"
        exit 1
    }

    if (by_fields)
        set_fieldlist()
    else
        set_charlist()
}

Here is set_fieldlist. It first splits the field list apart at the commas, into an array. Then, for each element of the array, it looks to see if it is actually a range, and if so splits it apart. The range is verified to make sure the first number is smaller than the second. Each number in the list is added to the flist array, which simply lists the fields that will be printed. Normal field splitting is used. The program lets awk handle the job of doing the field splitting. 

function set_fieldlist(        n, m, i, j, k, f, g)
{
    n = split(fieldlist, f, ",")
    j = 1    # index in flist
    for (i = 1; i <= n; i++) {
        if (index(f[i], "-") != 0) { # a range
            m = split(f[i], g, "-")
            if (m != 2 || g[1] >= g[2]) {
                printf("bad field list: %s\n",
                                  f[i]) > "/dev/stderr"
                exit 1
            }
            for (k = g[1]; k <= g[2]; k++)
                flist[j++] = k
        } else
            flist[j++] = f[i]
    }
    nfields = j - 1
}

The set_charlist function is more complicated than set_fieldlist. The idea here is to use gawk's FIELDWIDTHS variable (see section Reading Fixed-width Data), which describes constant width input. When using a character list, that is exactly what we have.

Setting up FIELDWIDTHS is more complicated than simply listing the fields that need to be printed. We have to keep track of the fields to be printed, and also the intervening characters that have to be skipped. For example, suppose you wanted characters one through eight, 15, and 22 through 35. You would use `-c 1-8,15,22-35'. The necessary value for FIELDWIDTHS would be "8 6 1 6 14". This gives us five fields, and what should be printed are $1, $3, and $5. The intermediate fields are "filler," stuff in between the desired data.

flist lists the fields to be printed, and t tracks the complete field list, including filler fields. 

function set_charlist(    field, i, j, f, g, t,
                          filler, last, len)
{
    field = 1   # count total fields
    n = split(fieldlist, f, ",")
    j = 1       # index in flist
    for (i = 1; i <= n; i++) {
        if (index(f[i], "-") != 0) { # range
            m = split(f[i], g, "-")
            if (m != 2 || g[1] >= g[2]) {
                printf(bad character list: %s\n",
                               f[i]) > "/dev/stderr"
                exit 1
            }
            len = g[2] - g[1] + 1
            if (g[1] > 1)  # compute length of filler
                filler = g[1] - last - 1
            else
                filler = 0
            if (filler)
                t[field++] = filler
            t[field++] = len  # length of field
            last = g[2]
            flist[j++] = field - 1
        } else {
            if (f[i] > 1)
                filler = f[i] - last - 1
            else
                filler = 0
            if (filler)
                t[field++] = filler
            t[field++] = 1
            last = f[i]
            flist[j++] = field - 1
        }
    }
    FIELDWIDTHS = join(t, 1, field - 1)
    nfields = j - 1
}

Here is the rule that actually processes the data. If the `-s' option was given, then suppress will be true. The first if statement makes sure that the input record does have the field separator. If cut is processing fields, suppress is true, and the field separator character is not in the record, then the record is skipped.

If the record is valid, then at this point, gawk has split the data into fields, either using the character in FS or using fixed-length fields and FIELDWIDTHS. The loop goes through the list of fields that should be printed. If the corresponding field has data in it, it is printed. If the next field also has data, then the separator character is written out in between the fields. 

{
    if (by_fields && suppress && $0 !~ FS)
        next

    for (i = 1; i <= nfields; i++) {
        if ($flist[i] != "") {
            printf "%s", $flist[i]
            if (i < nfields && $flist[i+1] != "")
                printf "%s", OFS
        }
    }
    print ""
}

This version of cut relies on gawk's FIELDWIDTHS variable to do the character-based cutting. While it would be possible in other awk implementations to use substr (see section Built-in Functions for String Manipulation), it would also be extremely painful to do so. The FIELDWIDTHS variable supplies an elegant solution to the problem of picking the input line apart by characters.

Searching for Regular Expressions in Files

The egrep utility searches files for patterns. It uses regular expressions that are almost identical to those available in awk (see section Regular Expression Constants). It is used this way: 

egrep [ options ] 'pattern' files ...

The pattern is a regexp. In typical usage, the regexp is quoted to prevent the shell from expanding any of the special characters as file name wildcards. Normally, egrep prints the lines that matched. If multiple file names are provided on the command line, each output line is preceded by the name of the file and a colon.

The options are:

-c
Print out a count of the lines that matched the pattern, instead of the lines themselves.
-s
Be silent. No output is produced, and the exit value indicates whether or not the pattern was matched.
-v
Invert the sense of the test. egrep prints the lines that do not match the pattern, and exits successfully if the pattern was not matched.
-i
Ignore case distinctions in both the pattern and the input data.
-l
Only print the names of the files that matched, not the lines that matched.
-e pattern
Use pattern as the regexp to match. The purpose of the `-e' option is to allow patterns that start with a `-'.

This version uses the getopt library function (see section Processing Command Line Options), and the file transition library program (see section Noting Data File Boundaries).

The program begins with a descriptive comment, and then a BEGIN rule that processes the command line arguments with getopt. The `-i' (ignore case) option is particularly easy with gawk; we just use the IGNORECASE built in variable (see section Built-in Variables). 

# egrep.awk -- simulate egrep in awk
# Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain
# May 1993

# Options:
#    -c    count of lines
#    -s    silent - use exit value
#    -v    invert test, success if no match
#    -i    ignore case
#    -l    print filenames only
#    -e    argument is pattern

BEGIN {
    while ((c = getopt(ARGC, ARGV, "ce:svil")) != -1) {
        if (c == "c")
            count_only++
        else if (c == "s")
            no_print++
        else if (c == "v")
            invert++
        else if (c == "i")
            IGNORECASE = 1
        else if (c == "l")
            filenames_only++
        else if (c == "e")
            pattern = Optarg
        else
            usage()
    }

Next comes the code that handles the egrep specific behavior. If no pattern was supplied with `-e', the first non-option on the command line is used. The awk command line arguments up to ARGV[Optind] are cleared, so that awk won't try to process them as files. If no files were specified, the standard input is used, and if multiple files were specified, we make sure to note this so that the file names can precede the matched lines in the output.

The last two lines are commented out, since they are not needed in gawk. They should be uncommented if you have to use another version of awk. 

    if (pattern == "")
        pattern = ARGV[Optind++]

    for (i = 1; i < Optind; i++)
        ARGV[i] = ""
    if (Optind >= ARGC) {
        ARGV[1] = "-"
        ARGC = 2
    } else if (ARGC - Optind > 1)
        do_filenames++

#    if (IGNORECASE)
#        pattern = tolower(pattern)
}

The next set of lines should be uncommented if you are not using gawk. This rule translates all the characters in the input line into lower-case if the `-i' option was specified. The rule is commented out since it is not necessary with gawk. 

#{
#    if (IGNORECASE)
#        $0 = tolower($0)
#}

The beginfile function is called by the rule in `ftrans.awk' when each new file is processed. In this case, it is very simple; all it does is initialize a variable fcount to zero. fcount tracks how many lines in the current file matched the pattern. 

function beginfile(junk)
{
    fcount = 0
}

The endfile function is called after each file has been processed. It is used only when the user wants a count of the number of lines that matched. no_print will be true only if the exit status is desired. count_only will be true if line counts are desired. egrep will therefore only print line counts if printing and counting are enabled. The output format must be adjusted depending upon the number of files to be processed. Finally, fcount is added to total, so that we know how many lines altogether matched the pattern. 

function endfile(file)
{
    if (! no_print && count_only)
        if (do_filenames)
            print file ":" fcount
        else
            print fcount

    total += fcount
}

This rule does most of the work of matching lines. The variable matches will be true if the line matched the pattern. If the user wants lines that did not match, the sense of the matches is inverted using the `!' operator. fcount is incremented with the value of matches, which will be either one or zero, depending upon a successful or unsuccessful match. If the line did not match, the next statement just moves on to the next record.

There are several optimizations for performance in the following few lines of code. If the user only wants exit status (no_print is true), and we don't have to count lines, then it is enough to know that one line in this file matched, and we can skip on to the next file with nextfile. Along similar lines, if we are only printing file names, and we don't need to count lines, we can print the file name, and then skip to the next file with nextfile.

Finally, each line is printed, with a leading filename and colon if necessary. 

{
    matches = ($0 ~ pattern)
    if (invert)
        matches = ! matches

    fcount += matches    # 1 or 0

    if (! matches)
        next

    if (no_print && ! count_only)
        nextfile

    if (filenames_only && ! count_only) {
        print FILENAME
        nextfile
    }

    if (do_filenames && ! count_only)
        print FILENAME ":" $0
    else if (! count_only)
        print
}

The END rule takes care of producing the correct exit status. If there were no matches, the exit status is one, otherwise it is zero. 

END    {
    if (total == 0)
        exit 1
    exit 0
}

The usage function prints a usage message in case of invalid options and then exits. 

function usage(    e)
{
    e = "Usage: egrep [-csvil] [-e pat] [files ...]"
    print e > "/dev/stderr"
    exit 1
}

The variable e is used so that the function fits nicely on the printed page.

Printing Out User Information

The id utility lists a user's real and effective user-id numbers, real and effective group-id numbers, and the user's group set, if any. id will only print the effective user-id and group-id if they are different from the real ones. If possible, id will also supply the corresponding user and group names. The output might look like this: 

$ id
-| uid=2076(arnold) gid=10(staff) groups=10(staff),4(tty)

This information is exactly what is provided by gawk's `/dev/user' special file (see section Special File Names in gawk). However, the id utility provides a more palatable output than just a string of numbers.

Here is a simple version of id written in awk. It uses the user database library functions (see section Reading the User Database), and the group database library functions (see section Reading the Group Database).

The program is fairly straightforward. All the work is done in the BEGIN rule. The user and group id numbers are obtained from `/dev/user'. If there is no support for `/dev/user', the program gives up.

The code is repetitive. The entry in the user database for the real user-id number is split into parts at the `:'. The name is the first field. Similar code is used for the effective user-id number, and the group numbers. 

# id.awk -- implement id in awk
# Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain
# May 1993

# output is:
# uid=12(foo) euid=34(bar) gid=3(baz) \ 
#             egid=5(blat) groups=9(nine),2(two),1(one)

BEGIN    {
    if ((getline < "/dev/user") < 0) {
        err = "id: no /dev/user support - cannot run"
        print err > "/dev/stderr"
        exit 1
    }
    close("/dev/user")

    uid = $1
    euid = $2
    gid = $3
    egid = $4

    printf("uid=%d", uid)
    pw = getpwuid(uid)
    if (pw != "") {
        split(pw, a, ":")
        printf("(%s)", a[1])
    }

    if (euid != uid) {
        printf(" euid=%d", euid)
        pw = getpwuid(euid)
        if (pw != "") {
            split(pw, a, ":")
            printf("(%s)", a[1])
        }
    }

    printf(" gid=%d", gid)
    pw = getgrgid(gid)
    if (pw != "") {
        split(pw, a, ":")
        printf("(%s)", a[1])
    }

    if (egid != gid) {
        printf(" egid=%d", egid)
        pw = getgrgid(egid)
        if (pw != "") {
            split(pw, a, ":")
            printf("(%s)", a[1])
        }
    }

    if (NF > 4) {
        printf(" groups=");
        for (i = 5; i <= NF; i++) {
            printf("%d", $i)
            pw = getgrgid($i)
            if (pw != "") {
                split(pw, a, ":")
                printf("(%s)", a[1])
            }
            if (i < NF)
                printf(",")
        }
    }
    print ""
}

Splitting a Large File Into Pieces

The split program splits large text files into smaller pieces. By default, the output files are named `xaa', `xab', and so on. Each file has 1000 lines in it, with the likely exception of the last file. To change the number of lines in each file, you supply a number on the command line preceded with a minus, e.g., `-500' for files with 500 lines in them instead of 1000. To change the name of the output files to something like `myfileaa', `myfileab', and so on, you supply an additional argument that specifies the filename.

Here is a version of split in awk. It uses the ord and chr functions presented in section Translating Between Characters and Numbers.

The program first sets its defaults, and then tests to make sure there are not too many arguments. It then looks at each argument in turn. The first argument could be a minus followed by a number. If it is, this happens to look like a negative number, so it is made positive, and that is the count of lines. The data file name is skipped over, and the final argument is used as the prefix for the output file names. 

# split.awk -- do split in awk
# Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain
# May 1993

# usage: split [-num] [file] [outname]

BEGIN    {
    outfile = "x"    # default
    count = 1000
    if (ARGC > 4)
        usage()

    i = 1
    if (ARGV[i] ~ /^-[0-9]+$/) {
        count = -ARGV[i]
        ARGV[i] = ""
        i++
    }
    # test argv in case reading from stdin instead of file
    if (i in ARGV)
        i++    # skip data file name
    if (i in ARGV) {
        outfile = ARGV[i]
        ARGV[i] = ""
    }

    s1 = s2 = "a"
    out = (outfile s1 s2)
}

The next rule does most of the work. tcount (temporary count) tracks how many lines have been printed to the output file so far. If it is greater than count, it is time to close the current file and start a new one. s1 and s2 track the current suffixes for the file name. If they are both `z', the file is just too big. Otherwise, s1 moves to the next letter in the alphabet and s2 starts over again at `a'. 

{
    if (++tcount > count) {
        close(out)
        if (s2 == "z") {
            if (s1 == "z") {
                printf("split: %s is too large to split\n", \  
                       FILENAME) > "/dev/stderr"
                exit 1
            }
            s1 = chr(ord(s1) + 1)
            s2 = "a"
        } else
            s2 = chr(ord(s2) + 1)
        out = (outfile s1 s2)
        tcount = 1
    }
    print > out
}

The usage function simply prints an error message and exits. 

function usage(   e)
{
    e = "usage: split [-num] [file] [outname]"
    print e > "/dev/stderr"
    exit 1
}

The variable e is used so that the function fits nicely on the page.

This program is a bit sloppy; it relies on awk to close the last file for it automatically, instead of doing it in an END rule.

Duplicating Output Into Multiple Files

The tee program is known as a "pipe fitting." tee copies its standard input to its standard output, and also duplicates it to the files named on the command line. Its usage is: 

tee [-a] file ...

The `-a' option tells tee to append to the named files, instead of truncating them and starting over.

The BEGIN rule first makes a copy of all the command line arguments, into an array named copy. ARGV[0] is not copied, since it is not needed. tee cannot use ARGV directly, since awk will attempt to process each file named in ARGV as input data.

If the first argument is `-a', then the flag variable append is set to true, and both ARGV[1] and copy[1] are deleted. If ARGC is less than two, then no file names were supplied, and tee prints a usage message and exits. Finally, awk is forced to read the standard input by setting ARGV[1] to "-", and ARGC to two. 

# tee.awk -- tee in awk
# Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain
# May 1993
# Revised December 1995

BEGIN    {
    for (i = 1; i < ARGC; i++)
        copy[i] = ARGV[i]

    if (ARGV[1] == "-a") {
        append = 1
        delete ARGV[1]
        delete copy[1]
        ARGC--
    }
    if (ARGC < 2) {
        print "usage: tee [-a] file ..." > "/dev/stderr"
        exit 1
    }
    ARGV[1] = "-"
    ARGC = 2
}

The single rule does all the work. Since there is no pattern, it is executed for each line of input. The body of the rule simply prints the line into each file on the command line, and then to the standard output. 

{
    # moving the if outside the loop makes it run faster
    if (append)
        for (i in copy)
            print >> copy[i]
    else
        for (i in copy)
            print > copy[i]
    print
}

It would have been possible to code the loop this way: 

for (i in copy)
    if (append)
        print >> copy[i]
    else
        print > copy[i]

This is more concise, but it is also less efficient. The `if' is tested for each record and for each output file. By duplicating the loop body, the `if' is only tested once for each input record. If there are N input records and M input files, the first method only executes N `if' statements, while the second would execute N*M `if' statements.

Finally, the END rule cleans up, by closing all the output files. 

END    {
    for (i in copy)
        close(copy[i])
}

Printing Non-duplicated Lines of Text

The uniq utility reads sorted lines of data on its standard input, and (by default) removes duplicate lines. In other words, only unique lines are printed, hence the name. uniq has a number of options. The usage is: 

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

The option meanings are:

-d
Only print repeated lines.
-u
Only print non-repeated lines.
-c
Count lines. This option overrides `-d' and `-u'. Both repeated and non-repeated lines are counted.
-n
Skip n fields before comparing lines. The definition of fields is the same as awk's default: non-whitespace characters separated by runs of spaces and/or tabs.
+n
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 had been provided.

Here is an awk implementation of uniq. It 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 were supplied (Optarg looks like a number), Optarg is concatenated with the option digit, and then 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, and Optind must be decremented so that getopt will process it next time. This code is admittedly a bit tricky.

If no options were supplied, then the default is taken, to print both repeated and non-repeated lines. The output file, if provided, is assigned to outputfile. Earlier, outputfile was initialized to the standard output, `/dev/stdout'. 

# uniq.awk -- do uniq in awk
# Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain
# May 1993

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

BEGIN    {
    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")
            non_repeated_only++
        else if (c == "d")
            repeated_only++
        else if (c == "c")
            do_count++
        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
                Optind--
            }
        } else
            usage()
    }

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

    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 were 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 Built-in Functions for String Manipulation), and then the desired fields are 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 will be one or zero depending upon the results of are_equal's comparison. If uniq is counting repeated lines, then the count variable is incremented if the lines are equal. Otherwise the line is printed and count is reset, since the two lines are not equal.

If uniq is not counting, count is incremented if the lines are equal. Otherwise, if uniq is counting repeated lines, and more than one line has been seen, or if uniq is counting non-repeated lines, and only one line has been 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
    next
}
    
{
    equal = are_equal()

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

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

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

Counting Things

The wc (word count) utility counts lines, words, and characters in one or more input files. Its usage is: 

wc [-lwc] [ files ... ]

If no files are specified on the command line, wc reads its standard input. If there are multiple files, it will also print total counts for all the files. The options and their meanings are:

-l
Only count lines.
-w
Only count words. A "word" is a contiguous sequence of non-whitespace characters, separated by spaces and/or tabs. Happily, this is the normal way awk separates fields in its input data.
-c
Only count characters.

Implementing wc in awk is particularly elegant, since awk does a lot of the work for us; it splits lines into words (i.e. fields) and counts them, it counts lines (i.e. records) for us, and it can easily tell us how long a line is.

This version uses the getopt library function (see section Processing Command Line Options), and the file transition functions (see section Noting Data File Boundaries).

This version has one major difference from traditional versions of wc. Our version always prints the counts in the order lines, words, and characters. Traditional versions note the order of the `-l', `-w', and `-c' options on the command line, and print the counts in that order.

The BEGIN rule does the argument processing. The variable print_total will be true if more than one file was named on the command line. 

# wc.awk -- count lines, words, characters
# Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain
# May 1993

# Options:
#    -l    only count lines
#    -w    only count words
#    -c    only count characters
#
# Default is to count lines, words, characters

BEGIN {
    # let getopt print a message about
    # invalid options. we ignore them
    while ((c = getopt(ARGC, ARGV, "lwc")) != -1) {
        if (c == "l")
            do_lines = 1
        else if (c == "w")
            do_words = 1
        else if (c == "c")
            do_chars = 1
    }
    for (i = 1; i < Optind; i++)
        ARGV[i] = ""

    # if no options, do all
    if (! do_lines && ! do_words && ! do_chars)
        do_lines = do_words = do_chars = 1

    print_total = (ARC - i > 2)
}

The beginfile function is simple; it just resets the counts of lines, words, and characters to zero, and saves the current file name in fname.

The endfile function adds the current file's numbers to the running totals of lines, words, and characters. It then prints out those numbers for the file that was just read. It relies on beginfile to reset the numbers for the following data file. 

function beginfile(file)
{
    chars = lines = words = 0
    fname = FILENAME
}

function endfile(file)
{
    tchars += chars
    tlines += lines
    twords += words
    if (do_lines)
        printf "\t%d", lines
    if (do_words)
        printf "\t%d", words
    if (do_chars)
        printf "\t%d", chars
    printf "\t%s\n", fname
}

There is one rule that is executed for each line. It adds the length of the record to chars. It has to add one, since the newline character separating records (the value of RS) is not part of the record itself. lines is incremented for each line read, and words is incremented by the value of NF, the number of "words" on this line.(19)

Finally, the END rule simply prints the totals for all the files. 

# do per line
{
    chars += length($0) + 1    # get newline
    lines++
    words += NF
}

END {
    if (print_total) {
        if (do_lines)
            printf "\t%d", tlines
        if (do_words)
            printf "\t%d", twords
        if (do_chars)
            printf "\t%d", tchars
        print "\ttotal"
    }
}

A Grab Bag of awk Programs

This section is a large "grab bag" of miscellaneous programs. We hope you find them both interesting and enjoyable.

Finding Duplicated Words in a Document

A common error when writing large amounts of prose is to accidentally duplicate words. Often you will see this in text as something like "the the program does the following ...." When the text is on-line, often the duplicated words occur at the end of one line and the beginning of another, making them very difficult to spot.

This program, `dupword.awk', scans through a file one line at a time, and looks for adjacent occurrences of the same word. It also saves the last word on a line (in the variable prev) for comparison with the first word on the next line.

The first two statements make sure that the line is all lower-case, so that, for example, "The" and "the" compare equal to each other. The second statement removes all non-alphanumeric and non-whitespace characters from the line, so that punctuation does not affect the comparison either. This sometimes leads to reports of duplicated words that really are different, but this is unusual. 

# dupword -- find duplicate words in text
# Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain
# December 1991

{
    $0 = tolower($0)
    gsub(/[^A-Za-z0-9 \t]/, "");
    if ($1 == prev)
        printf("%s:%d: duplicate %s\n",
            FILENAME, FNR, $1)
    for (i = 2; i <= NF; i++)
        if ($i == $(i-1))
            printf("%s:%d: duplicate %s\n",
                FILENAME, FNR, $i)
    prev = $NF
}

An Alarm Clock Program

The following program is a simple "alarm clock" program. You give it a time of day, and an optional message. At the given time, it prints the message on the standard output. In addition, you can give it the number of times to repeat the message, and also a delay between repetitions.

This program uses the gettimeofday function from section Managing the Time of Day.

All the work is done in the BEGIN rule. The first part is argument checking and setting of defaults; the delay, the count, and the message to print. If the user supplied a message, but it does not contain the ASCII BEL character (known as the "alert" character, `\a'), then it is added to the message. (On many systems, printing the ASCII BEL generates some sort of audible alert. Thus, when the alarm goes off, the system calls attention to itself, in case the user is not looking at their computer or terminal.) 

# alarm -- set an alarm
# Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain
# May 1993

# usage: alarm time [ "message" [ count [ delay ] ] ]

BEGIN    {
    # Initial argument sanity checking
    usage1 = "usage: alarm time ['message' [count [delay]]]"
    usage2 = sprintf("\t(%s) time ::= hh:mm", ARGV[1])

    if (ARGC < 2) {
        print usage > "/dev/stderr"
        exit 1
    } else if (ARGC == 5) {
        delay = ARGV[4] + 0
        count = ARGV[3] + 0
        message = ARGV[2]
    } else if (ARGC == 4) {
        count = ARGV[3] + 0
        message = ARGV[2]
    } else if (ARGC == 3) {
        message = ARGV[2]
    } else if (ARGV[1] !~ /[0-9]?[0-9]:[0-9][0-9]/) {
        print usage1 > "/dev/stderr"
        print usage2 > "/dev/stderr"
        exit 1
    }

    # set defaults for once we reach the desired time
    if (delay == 0)
        delay = 180    # 3 minutes
    if (count == 0)
        count = 5
    if (message == "")
        message = sprintf("\aIt is now %s!\a", ARGV[1])
    else if (index(message, "\a") == 0)
        message = "\a" message "\a"

The next section of code turns the alarm time into hours and minutes, and converts it if necessary to a 24-hour clock. Then it turns that time into a count of the seconds since midnight. Next it turns the current time into a count of seconds since midnight. The difference between the two is how long to wait before setting off the alarm. 

    # split up dest time
    split(ARGV[1], atime, ":")
    hour = atime[1] + 0    # force numeric
    minute = atime[2] + 0  # force numeric

    # get current broken down time
    gettimeofday(now)

    # if time given is 12-hour hours and it's after that
    # hour, e.g., `alarm 5:30' at 9 a.m. means 5:30 p.m.,
    # then add 12 to real hour
    if (hour < 12 && now["hour"] > hour)
        hour += 12

    # set target time in seconds since midnight
    target = (hour * 60 * 60) + (minute * 60)

    # get current time in seconds since midnight
    current = (now["hour"] * 60 * 60) + \ 
               (now["minute"] * 60) + now["second"]

    # how long to sleep for
    naptime = target - current
    if (naptime <= 0) {
        print "time is in the past!" > "/dev/stderr"
        exit 1
    }

Finally, the program uses the system function (see section Built-in Functions for Input/Output) to call the sleep utility. The sleep utility simply pauses for the given number of seconds. If the exit status is not zero, the program assumes that sleep was interrupted, and exits. If sleep exited with an OK status (zero), then the program prints the message in a loop, again using sleep to delay for however many seconds are necessary. 

    # zzzzzz..... go away if interrupted
    if (system(sprintf("sleep %d", naptime)) != 0)
        exit 1

    # time to notify!
    command = sprintf("sleep %d", delay)
    for (i = 1; i <= count; i++) {
        print message
        # if sleep command interrupted, go away
        if (system(command) != 0)
            break
    }

    exit 0
}

Transliterating Characters

The system tr utility transliterates characters. For example, it is often used to map upper-case letters into lower-case, for further processing. 

generate data | tr '[A-Z]' '[a-z]' | process data ...

You give tr two lists of characters enclosed in square brackets. Usually, the lists are quoted to keep the shell from attempting to do a filename expansion.(20) When processing the input, the first character in the first list is replaced with the first character in the second list, the second character in the first list is replaced with the second character in the second list, and so on. If there are more characters in the "from" list than in the "to" list, the last character of the "to" list is used for the remaining characters in the "from" list.

Some time ago, a user proposed to us that we add a transliteration function to gawk. Being opposed to "creeping featurism," I wrote the following program to prove that character transliteration could be done with a user-level function. This program is not as complete as the system tr utility, but it will do most of the job.

The translate program demonstrates one of the few weaknesses of standard awk: dealing with individual characters is very painful, requiring repeated use of the substr, index, and gsub built-in functions (see section Built-in Functions for String Manipulation).(21)

There are two functions. The first, stranslate, takes three arguments.

from
A list of characters to translate from.
to
A list of characters to translate to.
target
The string to do the translation on.

Associative arrays make the translation part fairly easy. t_ar holds the "to" characters, indexed by the "from" characters. Then a simple loop goes through from, one character at a time. For each character in from, if the character appears in target, gsub is used to change it to the corresponding to character.

The translate function simply calls stranslate using $0 as the target. The main program sets two global variables, FROM and TO, from the command line, and then changes ARGV so that awk will read from the standard input.

Finally, the processing rule simply calls translate for each record. 

# translate -- do tr like stuff
# Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain
# August 1989

# bugs: does not handle things like: tr A-Z a-z, it has
# to be spelled out. However, if `to' is shorter than `from',
# the last character in `to' is used for the rest of `from'.

function stranslate(from, to, target,     lf, lt, t_ar, i, c)
{
    lf = length(from)
    lt = length(to)
    for (i = 1; i <= lt; i++)
        t_ar[substr(from, i, 1)] = substr(to, i, 1)
    if (lt < lf)
        for (; i <= lf; i++)
            t_ar[substr(from, i, 1)] = substr(to, lt, 1)
    for (i = 1; i <= lf; i++) {
        c = substr(from, i, 1)
        if (index(target, c) > 0)
            gsub(c, t_ar[c], target)
    }
    return target
}

function translate(from, to)
{
    return $0 = stranslate(from, to, $0)
}

# main program
BEGIN {
    if (ARGC < 3) {
        print "usage: translate from to" > "/dev/stderr"
        exit
    }
    FROM = ARGV[1]
    TO = ARGV[2]
    ARGC = 2
    ARGV[1] = "-"
}

{
    translate(FROM, TO)
    print
}

While it is possible to do character transliteration in a user-level function, it is not necessarily efficient, and we started to consider adding a built-in function. However, shortly after writing this program, we learned that the System V Release 4 awk had added the toupper and tolower functions. These functions handle the vast majority of the cases where character transliteration is necessary, and so we chose to simply add those functions to gawk as well, and then leave well enough alone.

An obvious improvement to this program would be to set up the t_ar array only once, in a BEGIN rule. However, this assumes that the "from" and "to" lists will never change throughout the lifetime of the program.

Printing Mailing Labels

Here is a "real world"(22) program. This script reads lists of names and addresses, and generates mailing labels. Each page of labels has 20 labels on it, two across and ten down. The addresses are guaranteed to be no more than five lines of data. Each address is separated from the next by a blank line.

The basic idea is to read 20 labels worth of data. Each line of each label is stored in the line array. The single rule takes care of filling the line array and printing the page when 20 labels have been read.

The BEGIN rule simply sets RS to the empty string, so that awk will split records at blank lines (see section How Input is Split into Records). It sets MAXLINES to 100, since MAXLINE is the maximum number of lines on the page (20 * 5 = 100).

Most of the work is done in the printpage function. The label lines are stored sequentially in the line array. But they have to be printed horizontally; line[1] next to line[6], line[2] next to line[7], and so on. Two loops are used to accomplish this. The outer loop, controlled by i, steps through every 10 lines of data; this is each row of labels. The inner loop, controlled by j, goes through the lines within the row. As j goes from zero to four, `i+j' is the j'th line in the row, and `i+j+5' is the entry next to it. The output ends up looking something like this: 

line 1          line 6
line 2          line 7
line 3          line 8
line 4          line 9
line 5          line 10

As a final note, at lines 21 and 61, an extra blank line is printed, to keep the output lined up on the labels. This is dependent on the particular brand of labels in use when the program was written. You will also note that there are two blank lines at the top and two blank lines at the bottom.

The END rule arranges to flush the final page of labels; there may not have been an even multiple of 20 labels in the data. 

# labels.awk
# Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain
# June 1992

# Program to print labels.  Each label is 5 lines of data
# that may have blank lines.  The label sheets have 2
# blank lines at the top and 2 at the bottom.

BEGIN    { RS = "" ; MAXLINES = 100 }

function printpage(    i, j)
{
    if (Nlines <= 0)
        return

    printf "\n\n"        # header

    for (i = 1; i <= Nlines; i += 10) {
        if (i == 21 || i == 61)
            print ""
        for (j = 0; j < 5; j++) {
            if (i + j > MAXLINES)
                break
            printf "   %-41s %s\n", line[i+j], line[i+j+5]
        }
        print ""
    }

    printf "\n\n"        # footer

    for (i in line)
        line[i] = ""
}

# main rule
{
    if (Count >= 20) {
        printpage()
        Count = 0
        Nlines = 0
    }
    n = split($0, a, "\n")
    for (i = 1; i <= n; i++)
        line[++Nlines] = a[i]
    for (; i <= 5; i++)
        line[++Nlines] = ""
    Count++
}

END    {
    printpage()
}

Generating Word Usage Counts

The following awk program prints the number of occurrences of each word in its input. It illustrates the associative nature of awk arrays by using strings as subscripts. It also demonstrates the `for x in array' construction. Finally, it shows how awk can be used in conjunction with other utility programs to do a useful task of some complexity with a minimum of effort. Some explanations follow the program listing. 

awk '
# Print list of word frequencies
{
    for (i = 1; i <= NF; i++)
        freq[$i]++
}

END {
    for (word in freq)
        printf "%s\t%d\n", word, freq[word]
}'

The first thing to notice about this program is that it has two rules. The first rule, because it has an empty pattern, is executed on every line of the input. It uses awk's field-accessing mechanism (see section Examining Fields) to pick out the individual words from the line, and the built-in variable NF (see section Built-in Variables) to know how many fields are available.

For each input word, an element of the array freq is incremented to reflect that the word has been seen an additional time.

The second rule, because it has the pattern END, is not executed until the input has been exhausted. It prints out the contents of the freq table that has been built up inside the first action.

This program has several problems that would prevent it from being useful by itself on real text files:

  • Words are detected using the awk convention that fields are separated by whitespace and that other characters in the input (except newlines) don't have any special meaning to awk. This means that punctuation characters count as part of words.
  • The awk language considers upper- and lower-case characters to be distinct. Therefore, `bartender' and `Bartender' are not treated as the same word. This is undesirable since, in normal text, words are capitalized if they begin sentences, and a frequency analyzer should not be sensitive to capitalization.
  • The output does not come out in any useful order. You're more likely to be interested in which words occur most frequently, or having an alphabetized table of how frequently each word occurs.

The way to solve these problems is to use some of the more advanced features of the awk language. First, we use tolower to remove case distinctions. Next, we use gsub to remove punctuation characters. Finally, we use the system sort utility to process the output of the awk script. Here is the new version of the program: 

# Print list of word frequencies
{
    $0 = tolower($0)    # remove case distinctions
    gsub(/[^a-z0-9_ \t]/, "", $0)  # remove punctuation
    for (i = 1; i <= NF; i++)
        freq[$i]++
}

END {
    for (word in freq)
        printf "%s\t%d\n", word, freq[word]
}

Assuming we have saved this program in a file named `wordfreq.awk', and that the data is in `file1', the following pipeline 

awk -f wordfreq.awk file1 | sort +1 -nr

produces a table of the words appearing in `file1' in order of decreasing frequency.

The awk program suitably massages the data and produces a word frequency table, which is not ordered.

The awk script's output is then sorted by the sort utility and printed on the terminal. The options given to sort in this example specify to sort using the second field of each input line (skipping one field), that the sort keys should be treated as numeric quantities (otherwise `15' would come before `5'), and that the sorting should be done in descending (reverse) order.

We could have even done the sort from within the program, by changing the END action to: 

END {
    sort = "sort +1 -nr"
    for (word in freq)
        printf "%s\t%d\n", word, freq[word] | sort
    close(sort)
}

You would have to use this way of sorting on systems that do not have true pipes.

See the general operating system documentation for more information on how to use the sort program.

Removing Duplicates from Unsorted Text

The uniq program (see section Printing Non-duplicated Lines of Text), removes duplicate lines from sorted data.

Suppose, however, you need to remove duplicate lines from a data file, but that you wish to preserve the order the lines are in? A good example of this might be a shell history file. The history file keeps a copy of all the commands you have entered, and it is not unusual to repeat a command several times in a row. Occasionally you might wish to compact the history by removing duplicate entries. Yet it is desirable to maintain the order of the original commands.

This simple program does the job. It uses two arrays. The data array is indexed by the text of each line. For each line, data[$0] is incremented.

If a particular line has not been seen before, then data[$0] will be zero. In that case, the text of the line is stored in lines[count]. Each element of lines is a unique command, and the indices of lines indicate the order in which those lines were encountered. The END rule simply prints out the lines, in order. 

# histsort.awk -- compact a shell history file
# Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain
# May 1993

# Thanks to Byron Rakitzis for the general idea
{
    if (data[$0]++ == 0)
        lines[++count] = $0
}

END {
    for (i = 1; i <= count; i++)
        print lines[i]
}

This program also provides a foundation for generating other useful information. For example, using the following print satement in the END rule would indicate how often a particular command was used. 

print data[lines[i]], lines[i]

This works because data[$0] was incremented each time a line was seen.

Extracting Programs from Texinfo Source Files

Both this chapter and the previous chapter (section A Library of awk Functions), present a large number of awk programs. If you wish to experiment with these programs, it is tedious to have to type them in by hand. Here we present a program that can extract parts of a Texinfo input file into separate files.

This book is written in Texinfo, the GNU project's document formatting language. A single Texinfo source file can be used to produce both printed and on-line documentation. Texinfo is fully documented in Texinfo--The GNU Documentation Format, available from the Free Software Foundation.

For our purposes, it is enough to know three things about Texinfo input files.

  • The "at" symbol, `@', is special in Texinfo, much like `\' in C or awk. Literal `@' symbols are represented in Texinfo source files as `@@'.
  • Comments start with either `@c' or `@comment'. The file extraction program will work by using special comments that start at the beginning of a line.
  • Example text that should not be split across a page boundary is bracketed between lines containing `@group' and `@end group' commands.

The following program, `extract.awk', reads through a Texinfo source file, and does two things, based on the special comments. Upon seeing `@c system ...', it runs a command, by extracting the command text from the control line and passing it on to the system function (see section Built-in Functions for Input/Output). Upon seeing `@c file filename', each subsequent line is sent to the file filename, until `@c endfile' is encountered. The rules in `extract.awk' will match either `@c' or `@comment' by letting the `omment' part be optional. Lines containing `@group' and `@end group' are simply removed. `extract.awk' uses the join library function (see section Merging an Array Into a String).

The example programs in the on-line Texinfo source for AWK Language Programming (`gawk.texi') have all been bracketed inside `file', and `endfile' lines. The gawk distribution uses a copy of `extract.awk' to extract the sample programs and install many of them in a standard directory, where gawk can find them.

`extract.awk' begins by setting IGNORECASE to one, so that mixed upper-case and lower-case letters in the directives won't matter.

The first rule handles calling system, checking that a command was given (NF is at least three), and also checking that the command exited with a zero exit status, signifying OK. 

# extract.awk -- extract files and run programs
#                 from texinfo files
# Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain
# May 1993

BEGIN    { IGNORECASE = 1 }

/^@c(omment)?[ \t]+system/    \ 
{
    if (NF < 3) {
        e = (FILENAME ":" FNR)
        e = (e  ": badly formed `system' line")
        print e > "/dev/stderr"
        next
    }
    $1 = ""
    $2 = ""
    stat = system($0)
    if (stat != 0) {
        e = (FILENAME ":" FNR)
        e = (e ": warning: system returned " stat)
        print e > "/dev/stderr"
    }
}

The variable e is used so that the function fits nicely on the page.

The second rule handles moving data into files. It verifies that a file name was given in the directive. If the file named is not the current file, then the current file is closed. This means that an `@c endfile' was not given for that file. (We should probably print a diagnostic in this case, although at the moment we do not.)

The `for' loop does the work. It reads lines using getline (see section Explicit Input with getline). For an unexpected end of file, it calls the unexpected_eof function. If the line is an "endfile" line, then it breaks out of the loop. If the line is an `@group' or `@end group' line, then it ignores it, and goes on to the next line.

Most of the work is in the following few lines. If the line has no `@' symbols, it can be printed directly. Otherwise, each leading `@' must be stripped off.

To remove the `@' symbols, the line is split into separate elements of the array a, using the split function (see section Built-in Functions for String Manipulation). Each element of a that is empty indicates two successive `@' symbols in the original line. For each two empty elements (`@@' in the original file), we have to add back in a single `@' symbol.

When the processing of the array is finished, join is called with the value of SUBSEP, to rejoin the pieces back into a single line. That line is then printed to the output file. 

/^@c(omment)?[ \t]+file/    {
    if (NF != 3) {
        e = (FILENAME ":" FNR ": badly formed `file' line")
        print e > "/dev/stderr"
        next
    }
    if ($3 != curfile) {
        if (curfile != "")
            close(curfile)
        curfile = $3
    }

    for (;;) {
        if ((getline line) <= 0)
            unexpected_eof()
        if (line ~ /^@c(omment)?[ \t]+endfile/)
            break
        else if (line ~ /^@(end[ \t]+)?group/)
            continue
        if (index(line, "@") == 0) {
            print line > curfile
            continue
        }
        n = split(line, a, "@")
        # if a[1] == "", means leading @,
        # don't add one back in.
        for (i = 2; i <= n; i++) {
            if (a[i] == "") { # was an @@
                a[i] = "@"
                if (a[i+1] == "")
                    i++
            }
        }
        print join(a, 1, n, SUBSEP) > curfile
    }
}

An important thing to note is the use of the `>' redirection. Output done with `>' only opens the file once; it stays open and subsequent output is appended to the file (see section Redirecting Output of print and printf). This allows us to easily mix program text and explanatory prose for the same sample source file (as has been done here!) without any hassle. The file is only closed when a new data file name is encountered, or at the end of the input file.

Finally, the function unexpected_eof prints an appropriate error message and then exits.

The END rule handles the final cleanup, closing the open file. 

function unexpected_eof()
{
    printf("%s:%d: unexpected EOF or error\n", \  
        FILENAME, FNR) > "/dev/stderr"
    exit 1
}

END {
    if (curfile)
        close(curfile)
}

A Simple Stream Editor

The sed utility is a "stream editor," a program that reads a stream of data, makes changes to it, and passes the modified data on. It is often used to make global changes to a large file, or to a stream of data generated by a pipeline of commands.

While sed is a complicated program in its own right, its most common use is to perform global substitutions in the middle of a pipeline: 

command1 < orig.data | sed 's/old/new/g' | command2 > result

Here, the `s/old/new/g' tells sed to look for the regexp `old' on each input line, and replace it with the text `new', globally (i.e. all the occurrences on a line). This is similar to awk's gsub function (see section Built-in Functions for String Manipulation).

The following program, `awksed.awk', accepts at least two command line arguments; the pattern to look for and the text to replace it with. Any additional arguments are treated as data file names to process. If none are provided, the standard input is used. 

# awksed.awk -- do s/foo/bar/g using just print
#    Thanks to Michael Brennan for the idea

# Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain
# August 1995

function usage()
{
    print "usage: awksed pat repl [files...]" > "/dev/stderr"
    exit 1
}

BEGIN {
    # validate arguments
    if (ARGC < 3)
        usage()

    RS = ARGV[1]
    ORS = ARGV[2]

    # don't use arguments as files
    ARGV[1] = ARGV[2] = ""
}

# look ma, no hands!
{
    if (RT == "")
        printf "%s", $0
    else
        print
}

The program relies on gawk's ability to have RS be a regexp and on the setting of RT to the actual text that terminated the record (see section How Input is Split into Records).

The idea is to have RS be the pattern to look for. gawk will automatically set $0 to the text between matches of the pattern. This is text that we wish to keep, unmodified. Then, by setting ORS to the replacement text, a simple print statement will output the text we wish to keep, followed by the replacement text.

There is one wrinkle to this scheme, which is what to do if the last record doesn't end with text that matches RS? Using a print statement unconditionally prints the replacement text, which is not correct.

However, if the file did not end in text that matches RS, RT will be set to the null string. In this case, we can print $0 using printf (see section Using printf Statements for Fancier Printing).

The BEGIN rule handles the setup, checking for the right number of arguments, and calling usage if there is a problem. Then it sets RS and ORS from the command line arguments, and sets ARGV[1] and ARGV[2] to the null string, so that they will not be treated as file names (see section Using ARGC and ARGV).

The usage function prints an error message and exits.

Finally, the single rule handles the printing scheme outlined above, using print or printf as appropriate, depending upon the value of RT.

An Easy Way to Use Library Functions

Using library functions in awk can be very beneficial. It encourages code re-use and the writing of general functions. Programs are smaller, and therefore clearer. However, using library functions is only easy when writing awk programs; it is painful when running them, requiring multiple `-f' options. If gawk is unavailable, then so too is the AWKPATH environment variable and the ability to put awk functions into a library directory (see section Command Line Options).

It would be nice to be able to write programs like so: 

# library functions
@include getopt.awk
@include join.awk
...

# main program
BEGIN {
    while ((c = getopt(ARGC, ARGV, "a:b:cde")) != -1)
        ...
    ...
}

The following program, `igawk.sh', provides this service. It simulates gawk's searching of the AWKPATH variable, and also allows nested includes; i.e. a file that has been included with `@include' can contain further `@include' statements. igawk will make an effort to only include files once, so that nested includes don't accidentally include a library function twice.

igawk should behave externally just like gawk. This means it should accept all of gawk's command line arguments, including the ability to have multiple source files specified via `-f', and the ability to mix command line and library source files.

The program is written using the POSIX Shell (sh) command language. The way the program works is as follows:

  1. Loop through the arguments, saving anything that doesn't represent awk source code for later, when the expanded program is run.
  2. For any arguments that do represent awk text, put the arguments into a temporary file that will be expanded. There are two cases.
    1. Literal text, provided with `--source' or `--source='. This text is just echoed directly. The echo program will automatically supply a trailing newline.
    2. File names provided with `-f'. We use a neat trick, and echo `@include filename' into the temporary file. Since the file inclusion program will work the way gawk does, this will get the text of the file included into the program at the correct point.
  3. Run an awk program (naturally) over the temporary file to expand `@include' statements. The expanded program is placed in a second temporary file.
  4. Run the expanded program with gawk and any other original command line arguments that the user supplied (such as the data file names).

The initial part of the program turns on shell tracing if the first argument was `debug'. Otherwise, a shell trap statement arranges to clean up any temporary files on program exit or upon an interrupt.

The next part loops through all the command line arguments. There are several cases of interest.

--
This ends the arguments to igawk. Anything else should be passed on to the user's awk program without being evaluated.
-W
This indicates that the next option is specific to gawk. To make argument processing easier, the `-W' is appended to the front of the remaining arguments and the loop continues. (This is an sh programming trick. Don't worry about it if you are not familiar with sh.)
-v
-F
These are saved and passed on to gawk.
-f
--file
--file=
-Wfile=
The file name is saved to the temporary file `/tmp/ig.s.$$' with an `@include' statement. The sed utility is used to remove the leading option part of the argument (e.g., `--file=').
--source
--source=
-Wsource=
The source text is echoed into `/tmp/ig.s.$$'.
--version
--version
-Wversion
igawk prints its version number, and runs `gawk --version' to get the gawk version information, and then exits.

If none of `-f', `--file', `-Wfile', `--source', or `-Wsource', were supplied, then the first non-option argument should be the awk program. If there are no command line arguments left, igawk prints an error message and exits. Otherwise, the first argument is echoed into `/tmp/ig.s.$$'.

In any case, after the arguments have been processed, `/tmp/ig.s.$$' contains the complete text of the original awk program.

The `$$' in sh represents the current process ID number. It is often used in shell programs to generate unique temporary file names. This allows multiple users to run igawk without worrying that the temporary file names will clash.

Here's the program: 

#! /bin/sh

# igawk -- like gawk but do @include processing
# Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain
# July 1993

if [ "$1" = debug ]
then
    set -x
    shift
else
    # cleanup on exit, hangup, interrupt, quit, termination
    trap 'rm -f /tmp/ig.[se].$$' 0 1 2 3 15
fi

while [ $# -ne 0 ] # loop over arguments
do
    case $1 in
    --)     shift; break;;

    -W)     shift
            set -- -W"$@"
            continue;;

    -[vF])  opts="$opts $1 '$2'"
            shift;;

    -[vF]*) opts="$opts '$1'" ;;

    -f)     echo @include "$2" >> /tmp/ig.s.$$
            shift;;

    -f*)    f=`echo "$1" | sed 's/-f//'`
            echo @include "$f" >> /tmp/ig.s.$$ ;;

    -?file=*)    # -Wfile or --file
            f=`echo "$1" | sed 's/-.file=//'`
            echo @include "$f" >> /tmp/ig.s.$$ ;;

    -?file)    # get arg, $2
            echo @include "$2" >> /tmp/ig.s.$$
            shift;;

    -?source=*)    # -Wsource or --source
            t=`echo "$1" | sed 's/-.source=//'`
            echo "$t" >> /tmp/ig.s.$$ ;;

    -?source)  # get arg, $2
            echo "$2" >> /tmp/ig.s.$$
            shift;;

    -?version)
            echo igawk: version 1.0 1>&2
            gawk --version
            exit 0 ;;

    -[W-]*)    opts="$opts '$1'" ;;

    *)      break;;
    esac
    shift
done

if [ ! -s /tmp/ig.s.$$ ]
then
    if [ -z "$1" ]
    then
         echo igawk: no program! 1>&2
         exit 1
    else
        echo "$1" > /tmp/ig.s.$$
        shift
    fi
fi

# at this point, /tmp/ig.s.$$ has the program

The awk program to process `@include' directives reads through the program, one line at a time using getline (see section Explicit Input with getline). The input file names and `@include' statements are managed using a stack. As each `@include' is encountered, the current file name is "pushed" onto the stack, and the file named in the `@include' directive becomes the current file name. As each file is finished, the stack is "popped," and the previous input file becomes the current input file again. The process is started by making the original file the first one on the stack.

The pathto function does the work of finding the full path to a file. It simulates gawk's behavior when searching the AWKPATH environment variable (see section The AWKPATH Environment Variable). If a file name has a `/' in it, no path search is done. Otherwise, the file name is concatenated with the name of each directory in the path, and an attempt is made to open the generated file name. The only way in awk to test if a file can be read is to go ahead and try to read it with getline; that is what pathto does. If the file can be read, it is closed, and the file name is returned. 

gawk -- '
# process @include directives

function pathto(file,    i, t, junk)
{
    if (index(file, "/") != 0)
        return file

    for (i = 1; i <= ndirs; i++) {
        t = (pathlist[i] "/" file)
        if ((getline junk < t) > 0) {
            # found it
            close(t)
            return t
        }
    }
    return ""
}

The main program is contained inside one BEGIN rule. The first thing it does is set up the pathlist array that pathto uses. After splitting the path on `:', null elements are replaced with ".", which represents the current directory. 

BEGIN {
    path = ENVIRON["AWKPATH"]
    ndirs = split(path, pathlist, ":")
    for (i = 1; i <= ndirs; i++) {
        if (pathlist[i] == "")
            pathlist[i] = "."
    }

The stack is initialized with ARGV[1], which will be `/tmp/ig.s.$$'. The main loop comes next. Input lines are read in succession. Lines that do not start with `@include' are printed verbatim.

If the line does start with `@include', the file name is in $2. pathto is called to generate the full path. If it could not, then we print an error message and continue.

The next thing to check is if the file has been included already. The processed array is indexed by the full file name of each included file, and it tracks this information for us. If the file has been seen, a warning message is printed. Otherwise, the new file name is pushed onto the stack and processing continues.

Finally, when getline encounters the end of the input file, the file is closed and the stack is popped. When stackptr is less than zero, the program is done. 

    stackptr = 0
    input[stackptr] = ARGV[1] # ARGV[1] is first file

    for (; stackptr >= 0; stackptr--) {
        while ((getline < input[stackptr]) > 0) {
            if (tolower($1) != "@include") {
                print
                continue
            }
            fpath = pathto($2)
            if (fpath == "") {
                printf("igawk:%s:%d: cannot find %s\n", 
                    input[stackptr], FNR, $2) > "/dev/stderr"
                continue
            }
            if (! (fpath in processed)) {
                processed[fpath] = input[stackptr]
                input[++stackptr] = fpath
            } else
                print $2, "included in", input[stackptr],\  
                    "already included in", \ 
                    processed[fpath] > "/dev/stderr"
        }
        close(input[stackptr])
    }
}' /tmp/ig.s.$$ > /tmp/ig.e.$$

The last step is to call gawk with the expanded program and the original options and command line arguments that the user supplied. gawk's exit status is passed back on to igawk's calling program. 

eval gawk -f /tmp/ig.e.$$ $opts -- "$@"

exit $?

This version of igawk represents my third attempt at this program. There are three key simplifications that made the program work better.

  1. Using `@include' even for the files named with `-f' makes building the initial collected awk program much simpler; all the `@include' processing can be done once.
  2. The pathto function doesn't try to save the line read with getline when testing for the file's accessibility. Trying to save this line for use with the main program complicates things considerably.
  3. Using a getline loop in the BEGIN rule does it all in one place. It is not necessary to call out to a separate loop for processing nested `@include' statements.

Also, this program illustrates that it is often worthwhile to combine sh and awk programming together. You can usually accomplish quite a lot, without having to resort to low-level programming in C or C++, and it is frequently easier to do certain kinds of string and argument manipulation using the shell than it is in awk.

Finally, igawk shows that it is not always necessary to add new features to a program; they can often be layered on top. With igawk, there is no real reason to build `@include' processing into gawk itself.

As an additional example of this, consider the idea of having two files in a directory in the search path.

`default.awk'
This file would contain a set of default library functions, such as getopt and assert.
`site.awk'
This file would contain library functions that are specific to a site or installation, i.e. locally developed functions. Having a separate file allows `default.awk' to change with new gawk releases, without requiring the system administrator to update it each time by adding the local functions.

One user suggested that gawk be modified to automatically read these files upon startup. Instead, it would be very simple to modify igawk to do this. Since igawk can process nested `@include' directives, `default.awk' could simply contain `@include' statements for the desired library functions.



Mini annuaire : Gawk

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