http://www.tutorialspoint.com/unix_commands/awk.htm

gawk - pattern scanning and processing language

gawk?[?POSIX?or?GNU?style options ]?-f?program-file?[?--?] file ...?gawk?[?POSIX?or?GNU?style options ] [?--?]?program-text?file ...

pgawk?[?POSIX?or?GNU?style options ]?-f?program-file?[?--?] file ...?pgawk?[?POSIX?or?GNU?style options ] [?--?]?program-text?file ...

Gawk?is the?GNU?Project’s implementation of the?AWK?programming language. It conforms to the definition of the language in the?POSIX?1003.2 Command Language And Utilities Standard. This version in turn is based on the description in?The AWK Programming Language,by Aho,Kernighan,and Weinberger,with the additional features found in the System V Release 4 version of?UNIX?awk.?Gawk?also provides more recent Bell Laboratories?awk?extensions,and a number of?GNU-specific extensions.

Pgawk?is the profiling version of?gawk. It is identical in every way to?gawk,except that programs run more slowly,and it automatically produces an execution profile in the fileawkprof.out?when done. See the?--profile?option,below.

The command line consists of options to?gawk?itself,the?AWK?program text (if not supplied via the?-f?or?--file?options),and values to be made available in the?ARGC?andARGV?pre-defined?AWK?variables.

Gawk?options may be either traditional?POSIX?one letter options,or?GNU?style long options.?POSIX?options start with a single ‘‘-’’,while long options start with ‘‘--’’. Long options are provided for both?GNU-specific features and for?POSIX-mandated features.

Following the?POSIX?standard,?gawk-specific options are supplied via arguments to the?-W?option. Multiple?-W?options may be supplied Each?-W?option has a corresponding long option,as detailed below. Arguments to long options are either joined with the option by an?=?sign,with no intervening spaces,or they may be provided in the next command line argument. Long options may be abbreviated,as long as the abbreviation remains unique.

Gawk?accepts the following options,listed alphabetically.

-F?fs?--field-separator?fsfs?for the input field separator (the value of the?FSpredefined variable).</tr>
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-v?var=val?--assign?var=val
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<td width="6%">?</td>
<td>Assign the value?val?to the variable?var,before execution of the program begins. Such variable values are available to theBEGIN?block of an?AWK?program.</td>

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-f?program-file?--file?program-file
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<td width="6%">?</td>
<td>Read the?AWK?program source from the file?program-file,instead of from the first command line argument. Multiple?-f?(or?--file) options may be used.</td>

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-mf?NNN?-mr?NNN
</td>
<td valign="bottom">Set various memory limits to the value?NNN. The?f?flag sets the maximum number of fields,and the?r?flag sets the maximum record size. These two flags and the?-m?option are from the Bell Laboratories research version of?UNIX?awk. They are ignored bygawk,since?gawk?has no pre-defined limits.</td>

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-W compat?-W traditional?--compat?--traditional
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<td width="6%">?</td>
<td>Run in?compatibility?mode. In compatibility mode,?gawk?behaves identically to?UNIX?awk; none of the?GNU-specific extensions are recognized. The use of?--traditional?is preferred over the other forms of this option. See?<a href="http://www.tutorialspoint.com/#34"&gt;GNU EXTENSIONS,below,for more information.</td>

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-W copyleft?-W copyright?--copyleft?--copyright
</td>

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<td width="6%">?</td>
<td>Print the short version of the?GNU?copyright information message on the standard output and exit successfully.</td>

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-W dump-variables[=file]?--dump-variables[=file]</td>

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<td width="6%">?</td>
<td>Print a sorted list of global variables,their types and final values to?file. If no?file?is provided,?gawk?uses a file named?awkvars.outin the current directory.

Having a list of all the global variables is a good way to look for typographical errors in your programs. You would also use this option if you have a large program with a lot of functions,and you want to be sure that your functions don’t inadvertently use global variables that you meant to be local. (This is a particularly easy mistake to make with simple variable names like?i,?j,and so on.)

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-W exec?file?--exec?file
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<td width="6%">?</td>
<td>Similar to?-f,however,this is option is the last one processed. This should be used with?#!?scripts,particularly for CGI applications,to avoid passing in options or source code (!) on the command line from a URL. This option disables command-line variable assignments.</td>

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-W gen-po?--gen-po
</td>
<td valign="bottom">Scan and parse the?AWK?program,and generate a?GNU?.poformat file on standard output with entries for all localizable strings in the program. The program itself is not executed. See the?GNU?gettext?distribution for more information on?.po?files.</td>

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-W help?-W usage?--help?--usage
</td>
<td valign="bottom">Print a relatively short summary of the available options on the standard output. (Per the?GNU Coding Standards,these options cause an immediate,successful exit.)</td>

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-W lint[=value]?--lint[=value]</td>

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<td width="6%">?</td>
<td>Provide warnings about constructs that are dubious or non-portable to other?AWK?implementations. With an optional argument of?fatal,lint warnings become fatal errors. This may be drastic,but its use will certainly encourage the development of cleaner?AWK?programs. With an optional argument of?invalid,only warnings about things that are actually invalid are issued. (This is not fully implemented yet.)</td>

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-W lint-old?--lint-old
</td>

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<td width="6%">?</td>
<td>Provide warnings about constructs that are not portable to the original version of Unix?awk.</td>

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<td colspan="2">
-W non-decimal-data?--non-decimal-data
</td>

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<td width="6%">?</td>
<td>Recognize octal and hexadecimal values in input data.?Use this option with great caution!
</td>

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-W posix?--posix
</td>

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<td width="6%">?</td>
<td>This turns on?compatibility?mode,with the following additional restrictions:
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x?escape sequences are not recognized.</tr>
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<td>o</td>
<td valign="bottom">Only space and tab act as field separators when?FS?is set to a single space,newline does not.</td>

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<td>o</td>
<td valign="bottom">You cannot continue lines after???and?:.</td>

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<td>o</td>
<td valign="bottom">The synonym?func?for the keyword?function?is not recognized.</td>

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<td>o</td>
<td valign="bottom">The operators??and?=?cannot be used in place of?^?and?^=.</td>

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<td>o</td>
<td valign="bottom">The?fflush()?function is not available.</td>

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-W profile[=prof_file]?--profile[=prof_file]</td>

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<td width="1%">?</td>
<td>Send profiling data to?prof_file. The default is?awkprof.out. When run with?gawk,the profile is just a ‘‘pretty printed’’ version of the program. When run with?pgawk,the profile contains execution counts of each statement in the program in the left margin and function call counts for each user-defined function.</td>

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-W re-interval?--re-interval
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<td width="1%">?</td>
<td>Enable the use of?interval expressions?in regular expression matching (see?<a href="http://www.tutorialspoint.com/#16"&gt;Regular Expressions,below). Interval expressions were not traditionally available in the?AWKlanguage. The?POSIX?standard added them,to make?awk?andegrep?consistent with each other. However,their use is likely to break old?AWK?programs,so?gawk?only provides them if they are requested with this option,or when?--posix?is specified.</td>

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-W source?program-text?--source?program-text
</td>

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<td width="1%">?</td>
<td>Use?program-text?as?AWK?program source code. This option allows the easy intermixing of library functions (used via the?-fand?--file?options) with source code entered on the command line. It is intended primarily for medium to large?AWK?programs used in shell scripts.</td>

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-W version?--version
</td>

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<td width="1%">?</td>
<td>Print version information for this particular copy of?gawk?on the standard output. This is useful mainly for knowing if the current copy of?gawk?on your system is up to date with respect to whatever the Free Software Foundation is distributing. This is also useful when reporting bugs. (Per the?GNU Coding Standards,successful exit.)</td>

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<td>--</td>
<td valign="bottom">Signal the end of options. This is useful to allow further arguments to the?AWK?program itself to start with a ‘‘-’’. This is mainly for consistency with the argument parsing convention used by most other?POSIX?programs.</td>

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In compatibility mode,any other options are flagged as invalid,but are otherwise ignored. In normal operation,as long as program text has been supplied,unknown options are passed on to the?AWK?program in the?ARGV?array for processing. This is particularly useful for running?AWK?programs via the ‘‘#!’’ executable interpreter mechanism.

An?AWK?program consists of a sequence of pattern-action statements and optional function definitions.

pattern?{?action statements?}?function?name(parameter list) {?statements?}

Gawk?first reads the program source from the?program-file(s) if specified,from arguments to?--source,or from the first non-option argument on the command line. The?-f?and?--source?options may be used multiple times on the command line.?Gawk?reads the program text as if all the?program-files and command line source texts had been concatenated together. This is useful for building libraries of?AWK?functions,without having to include them in each new?AWK?program that uses them. It also provides the ability to mix library functions with command line programs.

The environment variable?AWKPATH?specifies a search path to use when finding source files named with the?-f?option. If this variable does not exist,the default path is".:/usr/local/share/awk". (The actual directory may vary,depending upon how?gawkwas built and installed.) If a file name given to the?-f?option contains a ‘‘/’’ character,no path search is performed.

Gawk?executes?AWK?programs in the following order. First,all variable assignments specified via the?-v?option are performed. Next,?gawk?compiles the program into an internal form. Then,?gawk?executes the code in the?BEGIN?block(s) (if any),and then proceeds to read each file named in the?ARGV?array. If there are no files named on the command line,?gawk?reads the standard input.

If a filename on the command line has the form?var=val?it is treated as a variable assignment. The variable?var?will be assigned the value?val. (This happens after anyBEGIN?block(s) have been run.) Command line variable assignment is most useful for dynamically assigning values to the variables?AWK?uses to control how input is broken into fields and records. It is also useful for controlling state if multiple passes are needed over a single data file.

If the value of a particular element of?ARGV?is empty (""),?gawk?skips over it.

For each record in the input,?gawk?tests to see if it matches any?pattern?in the?AWKprogram. For each pattern that the record matches,the associated?action?is executed. The patterns are tested in the order they occur in the program.

Finally,after all the input is exhausted,?gawk?executes the code in the?END?block(s) (if any).

AWK?variables are dynamic; they come into existence when they are first used. Their values are either floating-point numbers or strings,or both,depending upon how they are used.?AWK?also has one dimensional arrays; arrays with multiple dimensions may be simulated. Several pre-defined variables are set as a program runs; these will be described as needed and summarized below.

Records

Normally,records are separated by newline characters. You can control how records are separated by assigning values to the built-in variable?RS. If?RS?is any single character,that character separates records. Otherwise,?RS?is a regular expression. Text in the input that matches this regular expression separates the record. However,in compatibility mode,only the first character of its string value is used for separating records. If?RS?is set to the null string,then records are separated by blank lines. WhenRS?is set to the null string,the newline character always acts as a field separator,in addition to whatever value?FS?may have.

Fields

As each input record is read,?gawk?splits the record into?fields,using the value of the?FSvariable as the field separator. If?FS?is a single character,fields are separated by that character. If?FS?is the null string,then each individual character becomes a separate field. Otherwise,?FS?is expected to be a full regular expression. In the special case thatFS?is a single space,fields are separated by runs of spaces and/or tabs and/or newlines. (But see the discussion of?--posix,below).?NOTE:?The value of?IGNORECASE(see below) also affects how fields are split when?FS?is a regular expression,and how records are separated when?RS?is a regular expression.

If the?FIELDWIDTHS?variable is set to a space separated list of numbers,each field is expected to have fixed width,and?gawk?splits up the record using the specified widths. The value of?FS?is ignored. Assigning a new value to?FS?overrides the use ofFIELDWIDTHS,and restores the default behavior.

Each field in the input record may be referenced by its position,?$1,?$2,and so on.?$0?is the whole record. Fields need not be referenced by constants:

n = 5?print $n

prints the fifth field in the input record.

The variable?NF?is set to the total number of fields in the input record.

References to non-existent fields (i.e. fields after?$NF) produce the null-string. However,assigning to a non-existent field (e.g.,?$(NF+2) = 5) increases the value of?NF,creates any intervening fields with the null string as their value,and causes the value of?$0?to be recomputed,with the fields being separated by the value of?OFS. References to negative numbered fields cause a fatal error. Decrementing?NF?causes the values of fields past the new value to be lost,and the value of?$0?to be recomputed,with the fields being separated by the value of?OFS.

Assigning a value to an existing field causes the whole record to be rebuilt when?$0?is referenced. Similarly,assigning a value to?$0?causes the record to be resplit,creating new values for the fields.

Built-in Variables

Gawk’s built-in variables are:

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ARGCgawk,or the program source).</tr>
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<td>ARGIND</td>
<td valign="bottom">The index in?ARGV?of the current file being processed.</td>

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<td>ARGV</td>
<td valign="bottom">Array of command line arguments. The array is indexed from 0 to?ARGC?- 1. Dynamically changing the contents of?ARGV?can control the files used for data.</td>

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<td>BINMODE</td>
<td valign="bottom">On non-POSIX systems,specifies use of ‘‘binary’’ mode for all file I/O. Numeric values of 1,2,or 3,specify that input files,output files,or all files,respectively,should use binary I/O. String values of?"r",or?"w"?specify that input files,or output files,should use binary I/O. String values of?"rw"?or"wr"?specify that all files should use binary I/O. Any other string value is treated as?"rw",but generates a warning message.</td>

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<td>CONVFMT</td>
<td valign="bottom">The conversion format for numbers,?"%.6g",by default.</td>

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<td>ENVIRON</td>
<td valign="bottom">An array containing the values of the current environment. The array is indexed by the environment variables,each element being the value of that variable (e.g.,?ENVIRON["HOME"]?might be?/home/arnold). Changing this array does not affect the environment seen by programs which?gawk?spawns via redirection or the?system()?function.</td>

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<td>ERRNO</td>
<td valign="bottom">If a system error occurs either doing a redirection for?getline,during a read for?getline,or during a?close(),then?ERRNO?will contain a string describing the error. The value is subject to translation in non-English locales.</td>

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<td>FIELDWIDTHS</td>
<td valign="bottom">A white-space separated list of fieldwidths. When set,?gawkparses the input into fields of fixed width,instead of using the value of the?FS?variable as the field separator.</td>

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<td>FILENAME</td>
<td valign="bottom">The name of the current input file. If no files are specified on the command line,the value of?FILENAME?is ‘‘-’’. However,FILENAME?is undefined inside the?BEGIN?block (unless set bygetline).</td>

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<td>FNR</td>
<td valign="bottom">The input record number in the current input file.</td>

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<td>FS</td>
<td valign="bottom">The input field separator,a space by default. See?<a href="http://www.tutorialspoint.com/#8"&gt;Fields,above.</td>

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<td>IGNORECASE</td>
<td valign="bottom">Controls the case-sensitivity of all regular expression and string operations. If?IGNORECASE?has a non-zero value,then string comparisons and pattern matching in rules,field splitting withFS,record separating with?RS,regular expression matching with?~?and?!~,and the?gensub(),?gsub(),?index(),?match(),split(),and?sub()?built-in functions all ignore case when doing regular expression operations.?NOTE:?Array subscripting is?notaffected. However,the?asort()?and?asorti()?functions are affected.

Thus,if?IGNORECASE?is not equal to zero,?/aB/?matches all of the strings?"ab",?"aB",?"Ab",and?"AB". As with all?AWKvariables,the initial value of?IGNORECASE?is zero,so all regular expression and string operations are normally case-sensitive. Under Unix,the full ISO 8859-1 Latin-1 character set is used when ignoring case. As of?gawk?3.1.4,the case equivalencies are fully locale-aware,based on the C??facilities such as?isalpha(),and?tolupper().

</td>

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<td>LINT</td>
<td valign="bottom">Provides dynamic control of the?--lint?option from within an?AWKprogram. When true,?gawk?prints lint warnings. When false,it does not. When assigned the string value?"fatal",lint warnings become fatal errors,exactly like?--lint=fatal. Any other true value just prints warnings.</td>

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<td>NF</td>
<td valign="bottom">The number of fields in the current input record.</td>

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<td>NR</td>
<td valign="bottom">The total number of input records seen so far.</td>

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<td>OFMT</td>
<td valign="bottom">The output format for numbers,by default.</td>

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<td>OFS</td>
<td valign="bottom">The output field separator,a space by default.</td>

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<td>ORS</td>
<td valign="bottom">The output record separator,by default a newline.</td>

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<td>PROCINFO</td>
<td valign="bottom">The elements of this array provide access to information about the running?AWK?program. On some systems,there may be elements in the array,?"group1"?through?"groupn"?for some?n,which is the number of supplementary groups that the process has. Use the?in?operator to test for these elements. The following elements are guaranteed to be available:
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PROCINFO["egid"]getegid(2) system call.</tr>
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<td>PROCINFO["euid"]</td>
<td valign="bottom">the value of the?geteuid(2) system call.</td>

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<td>PROCINFO["FS"]</td>
<td valign="bottom">
"FS"?if field splitting with?FS?is in effect,or?"FIELDWIDTHS"?if field splitting with?FIELDWIDTHS?is in effect.</td>

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<td>PROCINFO["gid"]</td>
<td valign="bottom">the value of the?getgid(2) system call.</td>

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<td>PROCINFO["pgrpid"]</td>
<td valign="bottom">the process group ID of the current process.</td>

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<td>PROCINFO["pid"]</td>
<td valign="bottom">the process ID of the current process.</td>

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<td>PROCINFO["ppid"]</td>
<td valign="bottom">the parent process ID of the current process.</td>

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<td>PROCINFO["uid"]</td>
<td valign="bottom">the value of the?getuid(2) system call.</td>

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<td colspan="2">PROCINFO["version"]</td>

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<td width="17%">?</td>
<td>The version of?gawk. This is available from version 3.1.4 and later.</td>

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<td>RS</td>
<td valign="bottom">The input record separator,by default a newline.</td>

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<td>RT</td>
<td valign="bottom">The record terminator.?Gawk?sets?RT?to the input text that matched the character or regular expression specified by?RS.</td>

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<td>RSTART</td>
<td valign="bottom">The index of the first character matched by?match(); 0 if no match. (This implies that character indices start at one.)</td>

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<td>RLENGTH</td>
<td valign="bottom">The length of the string matched by?match(); -1 if no match.</td>

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<td>SUBSEP</td>
<td valign="bottom">The character used to separate multiple subscripts in array elements,by default?"34".</td>

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<td>TEXTDOMAIN</td>
<td valign="bottom">The text domain of the?AWK?program; used to find the localized translations for the program’s strings.</td>

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Arrays

Arrays are subscripted with an expression between square brackets ([?and?]). If the expression is an expression list (expr,?expr?...) then the array subscript is a string consisting of the concatenation of the (string) value of each expression,separated by the value of the?SUBSEP?variable. This facility is used to simulate multiply dimensioned arrays. For example:

i = "A"; j = "B"; k = "C"?x[i,j,k] = "hello,worldn"

assigns the string?"hello,worldn"?to the element of the array?x?which is indexed by the string?"A34B34C". All arrays in?AWK?are associative,i.e. indexed by string values.

The special operator?in?may be used in an?if?or?while?statement to see if an array has an index consisting of a particular value.

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if (val in array)
????????print array[val]

If the array has multiple subscripts,use?(i,j) in array.

The?in?construct may also be used in a?for?loop to iterate over all the elements of an array.

An element may be deleted from an array using the?delete?statement. The?deletestatement may also be used to delete the entire contents of an array,just by specifying the array name without a subscript.

Variable Typing And Conversion

Variables and fields may be (floating point) numbers,or strings,or both. How the value of a variable is interpreted depends upon its context. If used in a numeric expression,it will be treated as a number,if used as a string it will be treated as a string.

To force a variable to be treated as a number,add 0 to it; to force it to be treated as a string,concatenate it with the null string.

When a string must be converted to a number,the conversion is accomplished usingstrtod(3). A number is converted to a string by using the value of?CONVFMT?as a format string for?sprintf(3),with the numeric value of the variable as the argument. However,even though all numbers in?AWK?are floating-point,integral values are?always?converted as integers. Thus,given

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CONVFMT = "%2.2f"
a = 12
b = a ""

the variable?b?has a string value of?"12"?and not?"12.00".

Gawk?performs comparisons as follows: If two variables are numeric,they are compared numerically. If one value is numeric and the other has a string value that is a ‘‘numeric string,’’ then comparisons are also done numerically. Otherwise,the numeric value is converted to a string and a string comparison is performed. Two strings are compared,of course,as strings. Note that the POSIX standard applies the concept of ‘‘numeric string’’ everywhere,even to string constants. However,this is clearly incorrect,andgawk?does not do this. (Fortunately,this is fixed in the next version of the standard.)

Note that string constants,such as?"57",are?not?numeric strings,they are string constants. The idea of ‘‘numeric string’’ only applies to fields,?getline?input,?FILENAME,ARGV?elements,?ENVIRON?elements and the elements of an array created by?split()that are numeric strings. The basic idea is that?user input,and only user input,that looks numeric,should be treated that way.

Uninitialized variables have the numeric value 0 and the string value "" (the null,or empty,string).

Octal and Hexadecimal Constants

Starting with version 3.1 of?gawk,?you may use C-style octal and hexadecimal constants in your AWK program source code. For example,the octal value?011?is equal to decimal9,and the hexadecimal value?0x11?is equal to decimal 17.

String Constants

String constants in?AWK?are sequences of characters enclosed between double quotes ("). Within strings,certain?escape sequences?are recognized,as in C. These are:

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<tr>
<th width="25%">Tag

aASCII?BEL?character.bfnrtvxhex digitsx. As in?ANSI?C,all following hexadecimal digits are considered part of the escape sequence. (This feature should tell us something about language design by committee.) E.g.,?"x1B"?is the?ASCII?ESC?(escape) character.ddd"33"?is the?ASCII?ESC?(escape) character.cc.The escape sequences may also be used inside constant regular expressions (e.g.,?/[ tfnrv]/?matches whitespace characters).

In compatibility mode,the characters represented by octal and hexadecimal escape sequences are treated literally when used in regular expression constants. Thus,/a52b/?is equivalent to?/a*b/.

AWK?is a line-oriented language. The pattern comes first,and then the action. Action statements are enclosed in?{?and?}. Either the pattern may be missing,or the action may be missing,but,not both. If the pattern is missing,the action is executed for every single record of input. A missing action is equivalent to

{ print }

which prints the entire record.

Comments begin with the ‘‘#’’ character,and continue until the end of the line. Blank lines may be used to separate statements. Normally,a statement ends with a newline,this is not the case for lines ending in a ‘‘,’’,?{,??,?:,?&&,or?||. Lines ending indo?or?else?also have their statements automatically continued on the following line. In other cases,a line can be continued by ending it with a ‘‘’’,in which case the newline will be ignored.

Multiple statements may be put on one line by separating them with a ‘‘;’’. This applies to both the statements within the action part of a pattern-action pair (the usual case),and to the pattern-action statements themselves.

Patterns

AWK?patterns may be one of the following:

<table class="src" cellspacing="5" cellpadding="5">

<tr>
<td>

BEGIN 
END 
/regular expression/ 
relational expression 
pattern && pattern 
pattern || pattern 
pattern ? pattern : pattern 
(pattern) 
! pattern 
pattern1,pattern2 

BEGIN?and?END?are two special kinds of patterns which are not tested against the input. The action parts of all?BEGIN?patterns are merged as if all the statements had been written in a single?BEGIN?block. They are executed before any of the input is read. Similarly,all the?END?blocks are merged,and executed when all the input is exhausted (or when an?exit?statement is executed).?BEGIN?and?END?patterns cannot be combined with other patterns in pattern expressions.?BEGIN?and?END?patterns cannot have missing action parts.

For?/regular expression/?patterns,the associated statement is executed for each input record that matches the regular expression. Regular expressions are the same as those in?egrep(1),and are summarized below.

A?relational expression?may use any of the operators defined below in the section on actions. These generally test whether certain fields match certain regular expressions.

The?&&,?||,and?!?operators are logical AND,logical OR,and logical NOT,as in C. They do short-circuit evaluation,also as in C,and are used for combining more primitive pattern expressions. As in most languages,parentheses may be used to change the order of evaluation.

The??:?operator is like the same operator in C. If the first pattern is true then the pattern used for testing is the second pattern,otherwise it is the third. Only one of the second and third patterns is evaluated.

The?pattern1,?pattern2?form of an expression is called a?range pattern. It matches all input records starting with a record that matches?pattern1,and continuing until a record that matches?pattern2,inclusive. It does not combine with any other sort of pattern expression.

Regular Expressions

Regular expressions are the extended kind found in?egrep. They are composed of characters as follows:

Description
Description
Description
Description
Description

cc.cc..including?newline.^$[abc...]abc....[^abc...]abc....r1|r2r1?or?r2.r1r2r1,and then?r2.r+r’s.r*r’s.r?r’s.(r)r.r{n}?r{n,}?r{n,m}interval expression. If there is one number in the braces,the preceding regular expression?r?is repeated?n?times. If there are two numbers separated by a comma,?r?is repeated?n?to?m?times. If there is one number followed by a comma,then?r?is repeated at least?n?times.

Interval expressions are only available if either?--posix?or?--re-interval?is specified on the command line.

</td>

</tr>
<tr valign="top">
<td>y</td>
<td valign="bottom">matches the empty string at either the beginning or the end of a word.</td>

</tr>
<tr valign="top">
<td>B</td>
<td valign="bottom">matches the empty string within a word.</td>

</tr>
<tr valign="top">
<td>&;</td>
<td valign="bottom">matches the empty string at the beginning of a word.</td>

</tr>
<tr valign="top">
<td>></td>
<td valign="bottom">matches the empty string at the end of a word.</td>

</tr>
<tr valign="top">
<td>w</td>
<td valign="bottom">matches any word-constituent character (letter,digit,or underscore).</td>

</tr>
<tr valign="top">
<td>W</td>
<td valign="bottom">matches any character that is not word-constituent.</td>

</tr>
<tr valign="top">
<td>‘</td>
<td valign="bottom">matches the empty string at the beginning of a buffer (string).</td>

</tr>
<tr valign="top">
<td>’</td>
<td valign="bottom">matches the empty string at the end of a buffer.</td>

</tr>

The escape sequences that are valid in string constants (see below) are also valid in regular expressions.

Character classes?are a new feature introduced in the?POSIX?standard. A character class is a special notation for describing lists of characters that have a specific attribute,but where the actual characters themselves can vary from country to country and/or from character set to character set. For example,the notion of what is an alphabetic character differs in the USA and in France.

A character class is only valid in a regular expression?inside?the brackets of a character list. Character classes consist of?[:,a keyword denoting the class,and?:]. The character classes defined by the?POSIX?standard are:

Description

[:alnum:]</tr>
<tr valign="top">
<td>[:alpha:]</td>
<td valign="bottom">Alphabetic characters.</td>

</tr>
<tr valign="top">
<td>[:blank:]</td>
<td valign="bottom">Space or tab characters.</td>

</tr>
<tr valign="top">
<td>[:cntrl:]</td>
<td valign="bottom">Control characters.</td>

</tr>
<tr valign="top">
<td>[:digit:]</td>
<td valign="bottom">Numeric characters.</td>

</tr>
<tr valign="top">
<td>[:graph:]</td>
<td valign="bottom">Characters that are both printable and visible. (A space is printable,but not visible,while an?a?is both.)</td>

</tr>
<tr valign="top">
<td>[:lower:]</td>
<td valign="bottom">Lower-case alphabetic characters.</td>

</tr>
<tr valign="top">
<td>[:print:]</td>
<td valign="bottom">Printable characters (characters that are not control characters.)</td>

</tr>
<tr valign="top">
<td>[:punct:]</td>
<td valign="bottom">Punctuation characters (characters that are not letter,digits,control characters,or space characters).</td>

</tr>
<tr valign="top">
<td>[:space:]</td>
<td valign="bottom">Space characters (such as space,tab,and formfeed,to name a few).</td>

</tr>
<tr valign="top">
<td>[:upper:]</td>
<td valign="bottom">Upper-case alphabetic characters.</td>

</tr>
<tr valign="top">
<td colspan="2">[:xdigit:]</td>

</tr>
<tr valign="top">
<td width="8%">?</td>
<td>Characters that are hexadecimal digits.</td>

</tr>

For example,before the?POSIX?standard,to match alphanumeric characters,you would have had to write?/[A-Za-z0-9]/. If your character set had other alphabetic characters in it,this would not match them,and if your character set collated differently from?ASCII,this might not even match the?ASCII?alphanumeric characters. With the?POSIX?character classes,you can write?/[[:alnum:]]/,and this matches the alphabetic and numeric characters in your character set.

Two additional special sequences can appear in character lists. These apply to non-ASCII?character sets,which can have single symbols (called?collating elements) that are represented with more than one character,as well as several characters that are equivalent for?collating,or sorting,purposes. (E.g.,in French,a plain ‘‘e’’ and a grave-accented e` are equivalent.)

Description

</tr>
<tr valign="top">
<td width="6%">?</td>
<td>A collating symbol is a multi-character collating element enclosed in?[.?and?.]. For example,if?ch?is a collating element,then[[.ch.]]?is a regular expression that matches this collating element,while?[ch]?is a regular expression that matches eitherc?or?h.</td>

</tr>
<tr valign="top">
<td colspan="2">Equivalence Classes</td>

</tr>
<tr valign="top">
<td width="6%">?</td>
<td>An equivalence class is a locale-specific name for a list of characters that are equivalent. The name is enclosed in?[=?and=]. For example,the name?e?might be used to represent all of ‘‘e,’’ ‘‘e',’’ and ‘‘e.’’ In this case,?<strong>[[=e=]]</strong>?is a regular expression that matches any of?<strong>e</strong>,?<strong>e'</strong>,or?<strong>e.</td>

</tr>
<tr>
<td colspan="2">These features are very valuable in non-English speaking locales. The library functions that?gawk?uses for regular expression matching currently only recognize?POSIXcharacter classes; they do not recognize collating symbols or equivalence classes.</td>

</tr>
<tr>
<td colspan="2">The?y,?B,?&;,?>,?w,?W,?‘,and?’?operators are specific to?gawk; they are extensions based on facilities in the?GNU?regular expression libraries.</td>

</tr>
<tr>
<td colspan="2">The various command line options control how?gawk?interprets characters in regular expressions.</td>

</tr>
<tr valign="top">
<td colspan="2">No options</td>

</tr>
<tr valign="top">
<td width="6%">?</td>
<td>In the default case,?gawk?provide all the facilities of?POSIXregular expressions and the?GNU?regular expression operators described above. However,interval expressions are not supported.</td>

</tr>
<tr valign="top">
<td>--posix</td>
<td valign="bottom">Only?POSIX?regular expressions are supported,the?GNUoperators are not special. (E.g.,?w?matches a literal?w). Interval expressions are allowed.</td>

</tr>
<tr valign="top">
<td colspan="2">--traditional</td>

</tr>
<tr valign="top">
<td width="6%">?</td>
<td>Traditional Unix?awk?regular expressions are matched. The?GNUoperators are not special,interval expressions are not available,and neither are the?POSIX?character classes ([[:alnum:]]?and so on). Characters described by octal and hexadecimal escape sequences are treated literally,even if they represent regular expression metacharacters.</td>

</tr>
<tr valign="top">
<td colspan="2">--re-interval</td>

</tr>
<tr valign="top">
<td width="6%">?</td>
<td>Allow interval expressions in regular expressions,even if?--traditional?has been provided.</td>

</tr>

Actions

Action statements are enclosed in braces,?{?and?}. Action statements consist of the usual assignment,conditional,and looping statements found in most languages. The operators,control statements,and input/output statements available are patterned after those in C.

Operators

The operators in?AWK,in order of decreasing precedence,are

<table class="src" border="1" cellspacing="0" cellpadding="5">

<tr>
<th width="25%">Tag

(...)</tr>
<tr valign="top">
<td>$</td>
<td valign="bottom">Field reference.</td>

</tr>
<tr valign="top">
<td>++ --</td>
<td valign="bottom">Increment and decrement,both prefix and postfix.</td>

</tr>
<tr valign="top">
<td>^</td>
<td valign="bottom">Exponentiation (?may also be used,and?=?for the assignment operator).</td>

</tr>
<tr valign="top">
<td>+ - !</td>
<td valign="bottom">Unary plus,unary minus,and logical negation.</td>

</tr>
<tr valign="top">
<td>* / %</td>
<td valign="bottom">Multiplication,division,and modulus.</td>

</tr>
<tr valign="top">
<td>+ -</td>
<td valign="bottom">Addition and subtraction.</td>

</tr>
<tr valign="top">
<td>space</td>
<td valign="bottom">String concatenation.</td>

</tr>
<tr valign="top">
<td>
< >?<= >=?!= ==
</td>
<td valign="bottom">The regular relational operators.</td>

</tr>
<tr valign="top">
<td>~ !~</td>
<td valign="bottom">Regular expression match,negated match.?NOTE:?Do not use a constant regular expression (/foo/) on the left-hand side of a?~or?!~. Only use one on the right-hand side. The expression/foo/ ~?exp?has the same meaning as?(($0 ~ /foo/) ~?exp). This is usually?not?what was intended.</td>

</tr>
<tr valign="top">
<td>in</td>
<td valign="bottom">Array membership.</td>

</tr>
<tr valign="top">
<td>&&</td>
<td valign="bottom">Logical AND.</td>

</tr>
<tr valign="top">
<td>||</td>
<td valign="bottom">Logical OR.</td>

</tr>
<tr valign="top">
<td>?:</td>
<td valign="bottom">The C conditional expression. This has the form?expr1???expr2?:expr3?. If?expr1?is true,the value of the expression is?expr2,otherwise it is?expr3. Only one of?expr2?and?expr3?is evaluated.</td>

</tr>
<tr valign="top">
<td>
= += -=?*= /= %= ^=
</td>
<td valign="bottom">Assignment. Both absolute assignment?(var?=?value)?and operator-assignment (the other forms) are supported.</td>

</tr>

Control Statements

The control statements are as follows:

<table class="src" cellspacing="5" cellpadding="5">

<tr>
<td>

if (condition) statement [ else statement ]
while (condition) statement 
do statement while (condition)
for (expr1; expr2; expr3) statement
for (var in array) statement
break
continue
delete array[index]
delete array
exit [ expression ]
{ statements }

I/O Statements

The input/output statements are as follows:

<table class="src" border="1" cellspacing="0" cellpadding="5">

<tr>
<th width="25%">Tag

close(file?[,?how])how?should only be used when closing one end of a two-way pipe to a co-process. It must be a string value,either?"to"?or?"from".getline$0?from next input record; set?NF,?NR,?FNR.getline <file$0?from next record of?file; set?NF.getline?varvar?from next input record; set?NR,?FNR.getline?var?<filevar?from next record of?file.command?| getline?[var]command?piping the output either into?$0?or?var,as above.command?|& getline?[var]command?as a co-process piping the output either into?$0or?var,as above. Co-processes are a?gawk?extension.nextAWK?program. If the end of the input data is reached,the?ENDblock(s),if any,are executed.nextfileFILENAME?and?ARGIND?are updated,?FNR?is reset to 1,and processing starts over with the first pattern in the?AWK?program. If the end of the input data is reached,the?END?block(s),are executed.printORS?variable.print?expr-listOFS?variable. The output record is terminated with the value of the?ORS?variable.print?expr-list?>filefile. Each expression is separated by the value of the?OFS?variable. The output record is terminated with the value of the?ORS?variable.printf?fmt,expr-listprintf?fmt,expr-list?>filefile.system(cmd-line)cmd-line,and return the exit status. (This may not be available on non-POSIX?systems.)fflush([file])file. If?file?is missing,then standard output is flushed. If?file?is the null string,then all open output files and pipes have their buffers flushed.Additional output redirections are allowed for?print?and?printf.

Description
Description
Description

print ... >>?filefile.print ... |?commandprint ... |&?commandThe?getline?command returns 0 on end of file and -1 on an error. Upon an error,?ERRNOcontains a string describing the problem.

NOTE:?If using a pipe or co-process to?getline,or from?print?or?printf?within a loop,youmust?use?close()?to create new instances of the command.?AWK?does not automatically close pipes or co-processes when they return EOF.

The printf Statement

The?AWK?versions of the?printf?statement and?sprintf()?function (see below) accept the following conversion specification formats:

Description

%cASCII?character. If the argument used for?%c?is numeric,it is treated as a character and printed. Otherwise,the argument is assumed to be a string,and the only first character of that string is printed.%d,?%i%e,%E[-]d.dddddde[+-]dd. The%E?format uses?E?instead of?e.%f[-]ddd.dddddd.%g,%G%e?or?%f?conversion,whichever is shorter,with nonsignificant zeros suppressed. The?%G?format uses?%Einstead of?%e.%o%u%s%x,%X%X?format uses?ABCDEF?instead of?abcdef.%%%?character; no argument is converted.NOTE: When using the integer format-control letters for values that are outside the range of a C?long?integer,?gawk?switches to the?%g?format specifier. If?--lint?is provided on the command line?gawk?warns about this. Other versions of?awk?may print invalid values or do something else entirely.

Optional,additional parameters may lie between the?%?and the control letter:

Description

count$count’th argument at this point in the formatting. This is called a?positional specifier?and is intended primarily for use in translated versions of format strings,not in the original text of an AWK program. It is a?gawk?extension.-space++?overrides the space modifier.#%o,supply a leading zero. For?%x,and?%X,supply a leading?0x?or0X?for a nonzero result. For?%e,?%E,and?%f,the result always contains a decimal point. For?%g,and?%G,trailing zeros are not removed from the result.00?(zero) acts as a flag,that indicates output should be padded with zeroes instead of spaces. This applies even to non-numeric output formats. This flag only has an effect when the field width is wider than the value to be printed.width0?flag has been used,it is padded with zeroes..prec%e,and?%f?formats,this specifies the number of digits you want printed to the right of the decimal point. For the?%g,and?%G?formats,it specifies the maximum number of significant digits. For the?%d,?%o,?%i,?%u,?%x,and?%X?formats,it specifies the minimum number of digits to print. For?%s,it specifies the maximum number of characters from the string that should be printed.The dynamic?width?and?prec?capabilities of the?ANSI?C?printf()?routines are supported. A*?in place of either the?width?or?prec?specifications causes their values to be taken from the argument list to?printf?or?sprintf(). To use a positional specifier with a dynamic width or precision,supply the?count$?after the?*?in the format string. For example,"%3$*2$.*1$s".

Special File Names

When doing I/O redirection from either?print?or?printf?into a file,or via?getline?from a file,?gawk?recognizes certain special filenames internally. These filenames allow access to open file descriptors inherited from?gawk’s parent process (usually the shell). These file names may also be used on the command line to name data files. The filenames are:

Description

/dev/stdin/dev/stdout/dev/stderr/dev/fd/nn.These are particularly useful for error messages. For example:

print "You blew it!" > "/dev/stderr"

whereas you would otherwise have to use

print "You blew it!" | "cat 1>&2"

The following special filenames may be used with the?|&?co-process operator for creating TCP/IP network connections.

Description

/inet/tcp/lport/rhost/rportlport?to remote hostrhost?on remote port?rport. Use a port of?0?to have the system pick a port./inet/udp/lport/rhost/rport/inet/raw/lport/rhost/rportOther special filenames provide access to information about the running?gawk?process.These filenames are now obsolete.?Use the?PROCINFO?array to obtain the information they provide. The filenames are:

Description

/dev/pid/dev/ppid/dev/pgrpid/dev/user$1?is the value of the?getuid(2) system call,?$2?is the value of the?geteuid(2) system call,?$3?is the value of the?getgid(2) system call,and?$4is the value of the?getegid(2) system call. If there are any additional fields,they are the group IDs returned bygetgroups(2). Multiple groups may not be supported on all systems.Numeric Functions

AWK?has the following built-in arithmetic functions:

<table class="src" border="1" cellspacing="0" cellpadding="5">

<tr>
<th width="25%">Tag

atan2(y,?x)y/x?in radians.cos(expr)expr,which is in radians.exp(expr)int(expr)log(expr)rand()N,between 0 and 1,such that 0 <=N?< 1.sin(expr)expr,which is in radians.sqrt(expr)srand([expr])expr?as a new seed for the random number generator. If no?expr?is provided,the time of day is used. The return value is the previous seed for the random number generator.String Functions

Gawk?has the following built-in string functions:

<table class="src" border="1" cellspacing="0" cellpadding="5">

<tr>
<th width="25%">Tag

asort(s?[,?d])s. The contents of?s?are sorted using?gawk’s normal rules for comparing values,and the indexes of the sorted values of?s?are replaced with sequential integers starting with 1. If the optional destination array?d?is specified,then?s?is first duplicated into?d,and then?d?is sorted,leaving the indexes of the source array?sunchanged.asorti(s?[,?d])s. The behavior is the same as that of?asort(),except that the arrayindices?are used for sorting,not the array values. When done,the array is indexed numerically,and the values are those of the original indices. The original values are lost; thus provide a second array if you wish to preserve the original.gensub(r,?s,?h?[,?t])t?for matches of the regular expressionr. If?h?is a string beginning with?g?or?G,then replace all matches of?r?with?s. Otherwise,?h?is a number indicating which match of?rto replace. If?t?is not supplied,?$0?is used instead. Within the replacement text?s,the sequence?n,where?n?is a digit from 1 to 9,may be used to indicate just the text that matched the?n’th parenthesized subexpression. The sequence??represents the entire matched text,as does the character?&. Unlike?sub()?andgsub(),the modified string is returned as the result of the function,and the original target string is?not?changed.gsub(r,?s?[,?t])r?in the string?t,substitute the string?s,and return the number of substitutions. If?t?is not supplied,use?$0. An?&?in the replacement text is replaced with the text that was actually matched. Use?&;?to get a literal?&. (This must be typed as"&"; see?GAWK: Effective AWK Programming?for a fuller discussion of the rules for?&’s?and backslashes in the replacement text of?sub(),and?gensub().)index(s,?t)t?in the string?s,or 0 if?t?is not present. (This implies that character indices start at one.)length([s])s,or the length of?$0?if?s?is not supplied. Starting with version 3.1.5,as a non-standard extension,with an array argument,?length()?returns the number of elements in the array.match(s,?r?[,?a])s?where the regular expression?r?occurs,or 0 if?r?is not present,and sets the values of?RSTART?andRLENGTH. Note that the argument order is the same as for the~?operator:?str?~?re. If array?a?is provided,?a?is cleared and then elements 1 through?n?are filled with the portions of?s?that match the corresponding parenthesized subexpression in?r. The 0’th element of?a?contains the portion of?s?matched by the entire regular expression?r. Subscripts?a[n,"start"],and?a[n,"length"]?provide the starting index in the string and length respectively,of each matching substring.split(s,?a?[,?r])s?into the array?a?on the regular expression?r,and returns the number of fields. If?r?is omitted,?FS?is used instead. The array?a?is cleared first. Splitting behaves identically to field splitting,described above.sprintf(fmt,?expr-list)expr-list?according to?fmt,and returns the resulting string.strtonum(str)str,and returns its numeric value. If?str?begins with a leading?0,?strtonum()?assumes that?str?is an octal number. Ifstr?begins with a leading?0x?or?0X,?strtonum()?assumes thatstr?is a hexadecimal number.sub(r,?t])gsub(),but only the first matching substring is replaced.substr(s,?i?[,?n])n-character substring of?s?starting at?i. If?nis omitted,the rest of?s?is used.tolower(str)str,with all the upper-case characters in?str?translated to their corresponding lower-case counterparts. Non-alphabetic characters are left unchanged.toupper(str)str,with all the lower-case characters in?str?translated to their corresponding upper-case counterparts. Non-alphabetic characters are left unchanged.Time Functions

Since one of the primary uses of?AWK?programs is processing log files that contain time stamp information,?gawk?provides the following functions for obtaining time stamps and formatting them.

<table class="src" border="1" cellspacing="0" cellpadding="5">

<tr>
<th width="25%">Tag

mktime(datespec)datespec?into a time stamp of the same form as returned by?systime(). The?datespec?is a string of the form?YYYY MM DD HH MM SS[ DST]. The contents of the string are six or seven numbers representing respectively the full year including century,the month from 1 to 12,the day of the month from 1 to 31,the hour of the day from 0 to 23,the minute from 0 to 59,and the second from 0 to 60,and an optional daylight saving flag. The values of these numbers need not be within the ranges specified; for example,an hour of -1 means 1 hour before midnight. The origin-zero Gregorian calendar is assumed,with year 0 preceding year 1 and year -1 preceding year 0. The time is assumed to be in the local timezone. If the daylight saving flag is positive,the time is assumed to be daylight saving time; if zero,the time is assumed to be standard time; and if negative (the default),?mktime()?attempts to determine whether daylight saving time is in effect for the specified time. Ifdatespec?does not contain enough elements or if the resulting time is out of range,?mktime()?returns -1.strftime([format?[,?timestamp]])timestamp?according to the specification in?format.?Thetimestamp?should be of the same form as returned bysystime(). If?timestamp?is missing,the current time of day is used. If?format?is missing,a default format equivalent to the output of?date(1) is used. See the specification for thestrftime()?function in?ANSI?C for the format conversions that are guaranteed to be available. A public-domain version ofstrftime(3) and a man page for it come with?gawk; if that version was used to build?gawk,then all of the conversions described in that man page are available to?gawk.systime()POSIX?systems).Bit Manipulations Functions

Starting with version 3.1 of?gawk,the following bit manipulation functions are available. They work by converting double-precision floating point values to?unsigned longintegers,doing the operation,and then converting the result back to floating point. The functions are:

Description
Description
Description
Description

and(v1,?v2)v1?and?v2.compl(val)val.lshift(val,?count)val,shifted left by?count?bits.or(v1,?v2)v1?and?v2.rshift(val,shifted right by?count?bits.xor(v1,?v2)v1?and?v2.Internationalization Functions

Starting with version 3.1 of?gawk,the following functions may be used from within your AWK program for translating strings at run-time. For full details,see?GAWK: Effective AWK Programming.

Description

bindtextdomain(directory?[,?domain])gawk?looks for the?.mo?files,in case they will not or cannot be placed in the ‘‘standard’’ locations (e.g.,during testing). It returns the directory wheredomain?is ‘‘bound.’’

The default?domain?is the value of?TEXTDOMAIN. If?directory?is the null string (""),then?bindtextdomain()?returns the current binding for the given?domain.

dcgettext(string?[,?domain?[,?category]])string?in text domain?domain?for locale category?category. The default value for?domain?is the current value of?TEXTDOMAIN. The default value for?category?is"LC_MESSAGES".

If you supply a value for?category,it must be a string equal to one of the known locale categories described in?GAWK: Effective AWK Programming. You must also supply a text domain. UseTEXTDOMAIN?if you want to use the current domain.

dcngettext(string1?,?string2?,?number?[,?category]])number?of the translation ofstring1?and?string2?in text domain?domain?for locale categorycategory. The default value for?domain?is the current value ofTEXTDOMAIN. The default value for?category?is"LC_MESSAGES".

If you supply a value for?category,it must be a string equal to one of the known locale categories described in?GAWK: Effective AWK Programming. You must also supply a text domain. UseTEXTDOMAIN?if you want to use the current domain.

Functions in?AWK?are defined as follows:

function?name(parameter list) {?statements?}

Functions are executed when they are called from within expressions in either patterns or actions. Actual parameters supplied in the function call are used to instantiate the formal parameters declared in the function. Arrays are passed by reference,other variables are passed by value.

Since functions were not originally part of the?AWK?language,the provision for local variables is rather clumsy: They are declared as extra parameters in the parameter list. The convention is to separate local variables from real parameters by extra spaces in the parameter list. For example:

<table class="src" cellspacing="5" cellpadding="5">

<tr>
<td>

function  f(p,q,a,b)?????# a and b are local
{
????????...
}

/abc/???{ ... ; f(1,2) ; ... }

The left parenthesis in a function call is required to immediately follow the function name,without any intervening white space. This is to avoid a syntactic ambiguity with the concatenation operator. This restriction does not apply to the built-in functions listed above.

Functions may call each other and may be recursive. Function parameters used as local variables are initialized to the null string and the number zero upon function invocation.

Use?return?expr?to return a value from a function. The return value is undefined if no value is provided,or if the function returns by ‘‘falling off’’ the end.

If?--lint?has been provided,?gawk?warns about calls to undefined functions at parse time,instead of at run time. Calling an undefined function at run time is a fatal error.

The word?func?may be used in place of?function.

Beginning with version 3.1 of?gawk,you can dynamically add new built-in functions to the running?gawk?interpreter. The full details are beyond the scope of this manual page; see?GAWK: Effective AWK Programming?for the details.

<table class="src" border="1" cellspacing="0" cellpadding="5">

<tr>
<th width="25%">Tag

extension(object,?function)object,and invoke?function?in that object,to perform initialization. These should both be provided as strings. Returns the value returned by?function.This function is provided and documented in?GAWK: Effective AWK Programming,but everything about this feature is likely to change in the next release. We STRONGLY recommend that you do not use this feature for anything that you aren’t willing to redo.

pgawk?accepts two signals.?SIGUSR1?causes it to dump a profile and function call stack to the profile file,which is either?awkprof.out,or whatever file was named with the?--profile?option. It then continues to run.?SIGHUP?causes it to dump the profile and function call stack and then exit.

Description
Description

Print and sort the login names of all users:

????????BEGIN???{ FS = ":" } ????????????????{ print $1 | "sort" }

Count lines in a file:

????????????????{ nlines++ } ????????END?????{ print nlines }

Precede each line by its number in the file:

????????{ print FNR,$0 }

Concatenate and line number (a variation on a theme):

????????{ print NR,$0 } Run an external command for particular lines of data:

????????tail -f access_log | ????????awk ’/myhome.html/ { system("nmap " $1 ">> logdir/myhome.html") }’

String constants are sequences of characters enclosed in double quotes. In non-English speaking environments,it is possible to mark strings in the?AWK?program as requiring translation to the native natural language. Such strings are marked in the?AWK?program with a leading underscore (‘‘_’’). For example,

gawk ’BEGIN { print "hello,world" }’

always prints?hello,world. But,

gawk ’BEGIN { print _"hello,world" }’

might print?bonjour,monde?in France.

There are several steps involved in producing and running a localizable?AWK?program.

BEGIN?action to assign a value to the?TEXTDOMAINvariable to set the text domain to a name associated with your program.

? ?BEGIN { TEXTDOMAIN = "myprog" }

This allows?gawk?to find the?.mo?file associated with your program. Without this step,?gawk?uses the?messages?text domain,which likely does not contain translations for your program.

dcgettext()?and/or?bindtextdomain()functions in your program,as appropriate.gawk --gen-po -f myprog.awk > myprog.po?to generate a.po?file for your program..mo?file.The internationalization features are described in full detail in?GAWK: Effective AWK Programming.

A primary goal for?gawk?is compatibility with the?POSIX?standard,as well as with the latest version of?UNIX?awk. To this end,?gawk?incorporates the following user visible features which are not described in the?AWK?book,but are part of the Bell Laboratories version of?awk,and are in the?POSIX?standard.

The book indicates that command line variable assignment happens when?awk?would otherwise open the argument as a file,which is after the?BEGIN?block is executed. However,in earlier implementations,when such an assignment appeared before any file names,the assignment would happen?before?the?BEGIN?block was run. Applications came to depend on this ‘‘feature.’’ When?awk?was changed to match its documentation,the?-v?option for assigning variables before program execution was added to accommodate applications that depended upon the old behavior. (This feature was agreed upon by both the Bell Laboratories and the?GNU?developers.)

The?-W?option for implementation specific features is from the?POSIX?standard.

When processing arguments,?gawk?uses the special option ‘‘--’’ to signal the end of arguments. In compatibility mode,it warns about but otherwise ignores undefined options. In normal operation,such arguments are passed on to the?AWK?program for it to process.

The?AWK?book does not define the return value of?srand(). The?POSIX?standard has it return the seed it was using,to allow keeping track of random number sequences. Therefore?srand()?in?gawk?also returns its current seed.

Other new features are: The use of multiple?-f?options (from MKS?awk); the?ENVIRONarray; the?a,and?v?escape sequences (done originally in?gawk?and fed back into the Bell Laboratories version); the?tolower()?and?toupper()?built-in functions (from the Bell Laboratories version); and the?ANSI?C conversion specifications in?printf?(done first in the Bell Laboratories version).

There are two features of historical?AWK?implementations that?gawk?supports. First,it is possible to call the?length()?built-in function not only with no argument,but even without parentheses! Thus,

a = length??????# Holy Algol 60,Batman!

is the same as either of

a = length()?a = length($0)

This feature is marked as ‘‘deprecated’’ in the?POSIX?standard,and?gawk?issues a warning about its use if?--lint?is specified on the command line.

The other feature is the use of either the?continue?or the?break?statements outside the body of a?while,?for,or?do?loop. Traditional?AWK?implementations have treated such usage as equivalent to the?next?statement.?Gawk?supports this usage if?--traditionalhas been specified.

Gawk?has a number of extensions to?POSIX?awk. They are described in this section. All the extensions described here can be disabled by invoking?gawk?with the?--traditionaloption.

The following features of?gawk?are not available in?POSIX?awk.

Description

-f?option. Therefore the?AWKPATH?environment variable is not special.</tr>
<tr valign="top">
<td>o</td>
<td valign="bottom">The?x?escape sequence. (Disabled with?--posix.)</td>

</tr>
<tr valign="top">
<td>o</td>
<td valign="bottom">The?fflush()?function. (Disabled with?--posix.)</td>

</tr>
<tr valign="top">
<td>o</td>
<td valign="bottom">The ability to continue lines after???and?:. (Disabled with?--posix.)</td>

</tr>
<tr valign="top">
<td>o</td>
<td valign="bottom">Octal and hexadecimal constants in AWK programs.</td>

</tr>
<tr valign="top">
<td>o</td>
<td valign="bottom">The?ARGIND,?BINMODE,?ERRNO,?LINT,?RT?and?TEXTDOMAINvariables are not special.</td>

</tr>
<tr valign="top">
<td>o</td>
<td valign="bottom">The?IGNORECASE?variable and its side-effects are not available.</td>

</tr>
<tr valign="top">
<td>o</td>
<td valign="bottom">The?FIELDWIDTHS?variable and fixed-width field splitting.</td>

</tr>
<tr valign="top">
<td>o</td>
<td valign="bottom">The?PROCINFO?array is not available.</td>

</tr>
<tr valign="top">
<td>o</td>
<td valign="bottom">The use of?RS?as a regular expression.</td>

</tr>
<tr valign="top">
<td>o</td>
<td valign="bottom">The special file names available for I/O redirection are not recognized.</td>

</tr>
<tr valign="top">
<td>o</td>
<td valign="bottom">The?|&?operator for creating co-processes.</td>

</tr>
<tr valign="top">
<td>o</td>
<td valign="bottom">The ability to split out individual characters using the null string as the value of?FS,and as the third argument to?split().</td>

</tr>
<tr valign="top">
<td>o</td>
<td valign="bottom">The optional second argument to the?close()?function.</td>

</tr>
<tr valign="top">
<td>o</td>
<td valign="bottom">The optional third argument to the?match()?function.</td>

</tr>
<tr valign="top">
<td>o</td>
<td valign="bottom">The ability to use positional specifiers with?printf?and?sprintf().</td>

</tr>
<tr valign="top">
<td>o</td>
<td valign="bottom">The use of?delete?array?to delete the entire contents of an array.</td>

</tr>
<tr valign="top">
<td>o</td>
<td valign="bottom">The use of?nextfile?to abandon processing of the current input file.</td>

</tr>
<tr valign="top">
<td>o</td>
<td valign="bottom">The?and(),?asort(),?asorti(),?bindtextdomain(),?compl(),dcgettext(),?dcngettext(),?gensub(),?lshift(),?mktime(),?or(),rshift(),?strftime(),?strtonum(),?systime()?and?xor()functions.</td>

</tr>
<tr valign="top">
<td>o</td>
<td valign="bottom">Localizable strings.</td>

</tr>
<tr valign="top">
<td>o</td>
<td valign="bottom">Adding new built-in functions dynamically with the?extension()function.</td>

</tr>

The?AWK?book does not define the return value of the?close()?function.?Gawk’s?close()returns the value from?fclose(3),or?pclose(3),when closing an output file or pipe,respectively. It returns the process’s exit status when closing an input pipe. The return value is -1 if the named file,pipe or co-process was not opened with a redirection.

When?gawk?is invoked with the?--traditional?option,if the?fs?argument to the?-F?option is ‘‘t’’,then?FS?is set to the tab character. Note that typing?gawk -Ft ...?simply causes the shell to quote the ‘‘t,and does not pass ‘‘t’’ to the?-F?option. Since this is a rather ugly special case,it is not the default behavior. This behavior also does not occur if?--posix?has been specified. To really get a tab character as the field separator,it is best to use single quotes:?gawk -F’t’ ....

If?gawk?is?configured?with the?--enable-switch?option to the?configure?command,then it accepts an additional control-flow statement:

Description

switch (expression) { case value|regex : statement ... [ default: statement ] }

The?AWKPATH?environment variable can be used to provide a list of directories thatgawk?searches when looking for files named via the?-f?and?--file?options.

If?POSIXLY_CORRECT?exists in the environment,then?gawk?behaves exactly as if?--posix?had been specified on the command line. If?--lint?has been specified,?gawk?issues a warning message to this effect.

getpid(2),?getppid(2),?getpgrp(2),?getuid(2),?geteuid(2),?getgid(2),?getegid(2),getgroups(2)

The AWK Programming Language,Alfred V. Aho,Brian W. Kernighan,Peter J. Weinberger,Addison-Wesley,1988. ISBN 0-201-07981-X.

GAWK: Effective AWK Programming,Edition 3.0,published by the Free Software Foundation,2001.

The?-F?option is not necessary given the command line variable assignment feature; it remains only for backwards compatibility.

Syntactically invalid single character programs tend to overflow the parse stack,generating a rather unhelpful message. Such programs are surprisingly difficult to diagnose in the completely general case,and the effort to do so really is not worth it.

The original version of?UNIX?awk?was designed and implemented by Alfred Aho,Peter Weinberger,and Brian Kernighan of Bell Laboratories. Brian Kernighan continues to maintain and enhance it.

Paul Rubin and Jay Fenlason,of the Free Software Foundation,wrote?gawk,to be compatible with the original version of?awk?distributed in Seventh Edition?UNIX. John Woods contributed a number of bug fixes. David Trueman,with contributions from Arnold Robbins,made?gawk?compatible with the new version of?UNIX?awk. Arnold Robbins is the current maintainer.

The initial DOS port was done by Conrad Kwok and Scott Garfinkle. Scott Deifik is the current DOS maintainer. Pat Rankin did the port to VMS,and Michal Jaegermann did the port to the Atari ST. The port to OS/2 was done by Kai Uwe Rommel,with contributions and help from Darrel Hankerson. Fred Fish supplied support for the Amiga,Stephen Davies provided the Tandem port,and Martin Brown provided the BeOS port.

This man page documents?gawk,version 3.1.5.

If you find a bug in?gawk,please send electronic mail to?. Please include your operating system and its revision,the version of?gawk?(from?gawk --version),what C compiler you used to compile it,and a test program and data that are as small as possible for reproducing the problem.

Before sending a bug report,please do two things. First,verify that you have the latest version of?gawk. Many bugs (usually subtle ones) are fixed at each release,and if yours is out of date,the problem may already have been solved. Second,please read this man page and the reference manual carefully to be sure that what you think is a bug really is,instead of just a quirk in the language.

Whatever you do,do?NOT?post a bug report in?comp.lang.awk. While the?gawkdevelopers occasionally read this newsgroup,posting bug reports there is an unreliable way to report bugs. Instead,please use the electronic mail addresses given above.

If you’re using a GNU/Linux system or BSD-based system,you may wish to submit a bug report to the vendor of your distribution. That’s fine,but please send a copy to the official email address as well,since there’s no guarantee that the bug will be forwarded to the?gawk?maintainer.

Brian Kernighan of Bell Laboratories provided valuable assistance during testing and debugging. We thank him.

Copyright ? 1989,1991,1992,1993,1994,1995,1996,1997,1998,1999,2001,2002,2003,2004,2005 Free Software Foundation,Inc.

Permission is granted to make and distribute verbatim copies of this manual page provided the copyright notice and this permission notice are preserved on all copies.

Permission is granted to copy and distribute modified versions of this manual page under the conditions for verbatim copying,provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual page into another language,under the above conditions for modified versions,except that this permission notice may be stated in a translation approved by the Foundation.?

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