11.2.1.1. Deleting all elements of an array

delete array

Normally, to delete every element from an array, you have to use a loop, like this.

for (i in data)
	delete data[i]

With the extended version of the delete statement, you can simply use

delete data

This is particularly useful for arrays with lots of subscripts; this version is considerably faster than the one using a loop.

Even though it no longer has any elements, you cannot use the array name as a simple variable. Once an array, always an array.

This extension appeared first in gawk, then in mawk and the Bell Labs awk.

11.2.1.2. Obtaining individual characters

This extension appeared first in mawk, then in gawk and the Bell Labs awk.

11.2.1.3. Flushing buffered output

Most programs buffer their output, storing data to be written to a file or pipe in an internal chunk of memory until there's enough to send on to the destination. Occasionally, it's useful for the programmer to be able to explicitly flush the buffer, that is, force all buffered data to actually be delivered. This is the purpose of the fflush() function.

This function appeared first in the Bell Labs awk, then in gawk and mawk.

11.2.1.4. Special filenames

printf "Enter your name:" >"/dev/tty"

This prints "Enter your name:" directly on the terminal, no matter where the standard output and the standard error are directed.

Table 11.4. Special Filenames

Note that a special filename, like any filename, must be quoted when specified as a string constant.

The /dev/stdin, /dev/stdout, and /dev/stderr special files originated in V8 UNIX. Gawk was the first to build in special recognition of these files, followed by mawk and the Bell Labs awk.

A printerr() function Error messages inform users about problems often related to missing or incorrect input. You can simply inform the user with a print statement. However, if the output of the program is redirected to a file, the user won't see it. Therefore, it is good practice to specify explicitly that the error message be sent to the terminal. The following printerr() function helps to create consistent user error messages. It prints the word "ERROR" followed by a supplied message, the record number, and the current record. The following example directs output to /dev/tty: function printerr (message) { # print message, record number and record printf("ERROR:%s (%d) %s\n", message, NR, $0) > "/dev/tty" } If the output of the program is sent to the terminal screen, then error messages will be mixed in with the output. Outputting "ERROR" will help the user recognize error messages. In UNIX, the standard destination for error messages is standard error. The rationale for writing to standard error is the same as above. To write to standard error explicitly, you must use the convoluted syntax "cat 1>&2" as in the following example: print "ERROR" | "cat 1>&2" This directs the output of the print statement to a pipe which executes the cat command. You can also use the system() function to execute a UNIX command such as cat or echo and direct its output to standard error. When the special file /dev/stderr is available, this gets much simpler: print "ERROR" > "/dev/stderr" # recent awks only

11.2.1.5. The nextfile statement

The nextfile statement originated in gawk, and then was added to the Bell Labs awk. It will be available in mawk, starting with version 1.4.

11.2.1.6. Regular expression record separators (gawk and mawk)

The ability to have RS be a regular expression first appeared in mawk, and was later added to gawk.

11.2.3.1. Command line options

gawk --source 'script' -f mylibs.awk file1 file2

This example runs the program in script, which can use awk functions from the file mylibs.awk. The input data comes from file1 and file2.

11.2.3.2. An awk program search path

For example, if mylibs.awk was a file of awk functions in /usr/local/share/awk, and myprog.awk was a program in the current directory, we run gawk like this:

gawk -f myprog.awk -f mylibs.awk datafile1

Gawk would find each file in the appropriate place. This makes it much easier to have and use awk library functions.

11.2.3.3. Line continuation

$ gawk 'BEGIN { print "hello, \
> world" }'
hello, world

11.2.3.4. Extended regular expressions

Table 11.5. Gawk Extended Regular Expressions

You can think of "\w" as a shorthand for the (POSIX) notation [[:alnum:]_] and "\W" as a shorthand for [^[:alnum:]_]. The following table gives examples of what the middle four operators match, borrowed from Effective AWK Programming.

Table 11.6. Examples of gawk Extended Regular Expression Operators

11.2.3.5. Regular expression record terminators

Here is a simple example, due to Michael Brennan, that shows the power of gawk's RS and RT variables. As we have seen, one of the most common uses of sed is its substitute command (s/old/new/g). By setting RS to the pattern to match, and ORS to the replacement text, a simple print statement can print the unchanged text followed by the replacement text.

$ cat simplesed.awk
# simplesed.awk --- do s/old/new/g using just print
#    Thanks to Michael Brennan for the idea
#
# NOTE! RS and ORS must be set on the command line
{
    if (RT == "")
        printf "%s", $0
    else
        print
}

$ cat simplesed.data
"This OLD house" is a great show.
I like shopping for old things at garage sales.
$ gawk -f simplesed.awk RS="old|OLD" ORS="brand new" simplesed.data
"This brand new house" is a great show.
I like shopping for brand new things at garage sales.

11.2.3.6. Separating fields

First, as mentioned above, if the value of FS is the empty string, then each character of the input record becomes a separate field.

FIELDWIDTHS = "5 6 8 3"

Here, the record has four fields: $1 is five characters wide, $2 is six characters wide, and so on. Assigning a value to FIELDWIDTHS causes gawk to start using it for field splitting. Assigning a value to FS causes gawk to return to the regular field splitting mechanism. Use FS = FS to make this happen without having to save the value of FS in an extra variable.

This facility would be of most use when working with fixed-width field data, where there may not be any whitespace separating fields, or when intermediate fields may be all blank.

11.2.3.7. Additional special files

Table 11.7. Gawk's Special Filenames

The first three were described earlier. The fourth filename provides access to any open file descriptor that may have been inherited from gawk's parent process (usually the shell). You can use file descriptor 0 for standard input, 1 for standard output, and 2 for standard error.

For example, you would use PROCINFO["pid"] to get the current process ID, instead of using getline pid < "/dev/pid". Check the gawk documentation to see if PROCINFO is available and if these filenames are still supported.

11.2.3.8. Additional variables

Table 11.8. Additional gawk System Variables

We have already seen the record terminator variable, RT, so we'll proceed to the other variables that we haven't covered yet.

The default value of IGNORECASE is zero, which means that pattern matching and string comparison are performed the same as in traditional awk. If IGNORECASE is set to a non-zero value, then case distinctions are ignored. This applies to all places where regular expressions are used, including the field separator FS, the record separator RS, and all string comparisons. It does not apply to array subscripting.

11.2.3.9. Additional functions

Table 11.9. Additional gawk Functions

11.2.3.10. A general substitution function

The pattern can have subpatterns delimited by parentheses. For example, it can have "/(part) (one|two|three)/". Within the replacement string, a backslash followed by a digit represents the text that matched the nth subpattern.

$ echo part two | gawk '{ print gensub(/(part) (one|two|three)/, "\\2", "g") }'
two

The flag is either a string beginning with g or G, in which case the substitution happens globally, or it is a number indicating that the nth occurrence should be replaced.

$ echo a b c a b c a b c | gawk '{ print gensub(/a/, "AA", 2) }'
a b c AA b c a b c

The fourth argument is the string in which to make the change. Unlike sub() and gsub(), the target string is not changed. Instead, the new string is the return value from gensub().

$ gawk '
BEGIN { old = "hello, world"
        new = gensub(/hello/, "goodbye", 1, old)
        printf("<%s>, <%s>\n", old, new)
}'
<hello, world>, <goodbye, world>

11.2.3.11. Time management for programmers

{
...
printf("%s:%s:%d\n", User, Host, systime()) >> "/var/log/cgi/querylog"
...
}

Such a record might look like

arnold:some.domain.com:831322007

$ gawk 'BEGIN { print strftime("Today is %A, %B %d, %Y") }'
Today is Sunday, May 05, 1996

The list of available formats is quite long. See your local strftime(3) manpage, and the gawk documentation for the full list. Our hypothetical CGI log file might be processed by this program:

# cgiformat --- process CGI logs
# data format is user:host:timestamp
#1
BEGIN {	FS = ":"; SUBSEP = "@" }

#2
{
# make data more obvious
	user = $1; host = $2; time = $3
# store first contact by this user
	if (! ((user, host) in first))
		first[user, host] = time
# count contacts
	count[user, host]++
# save last contact
	last[user, host] = time
}

#3
END {
# print the results
	for (contact in count) {
		i = strftime("%y-%m-%d %H:%M", first[contact])
		j = strftime("%y-%m-%d %H:%M", last[contact])
		printf "%s -> %d times between %s and %s\n",
			contact, count[contact], i, j
	}
}

The first step is to set FS to ":" to split the field correctly. We also use a neat trick and set the subscript separator to "@", so that the arrays become indexed by "user@host" strings.

In the second step, we look to see if this is the first time we've seen this user. If so (they're not in the first array), we add them. Then we increment the count of how many times they've connected. Finally we store this record's timestamp in the last array. This element keeps getting overwritten each time we see a new connection by the user. That's OK; what we will end up with is the last (most recent) connection stored in the array.

The END procedure formats the data for us. It loops through the count array, formatting the timestamps in the first and last arrays for printing. Consider a log file with the following records in it.

$ cat /var/log/cgi/querylog
arnold:some.domain.com:831322007
mary:another.domain.org:831312546
arnold:some.domain.com:831327215
mary:another.domain.org:831346231
arnold:some.domain.com:831324598

Here's what running the program produces:

$ gawk -f cgiformat.awk /var/log/cgi/querylog
[email protected] -> 2 times between 96-05-05 12:09 and 96-05-05 21:30
[email protected] -> 3 times between 96-05-05 14:46 and 96-05-05 15:29