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GAWK(1) Utility Commands GAWK(1) NAME gawk - pattern scanning and processing language SYNOPSIS gawk [ -W gawk-options ] [ -Ffs ] [ -v var=val ] -f program-file [ -- ] file ... gawk [ -W gawk-options ] [ -Ffs ] [ -v var=val ] [ -- ] program-text file ... DESCRIPTION Gawk is the GNU Project's implementation of the AWK program- ming language. It conforms to the definition of the language in the POSIX P1003.2 Command Language And Utilities Standard (draft 11). This version in turn is based on the description in The AWK Programming Language, by Aho, Ker- nighan, and Weinberger, with the additional features defined in the System V Release 4 version of UNIX awk. Gawk also provides some GNU-specific extensions. The command line consists of options to gawk itself, the AWK program text (if not supplied via the -f option), and values to be made available in the ARGC and ARGV pre-defined AWK variables. Gawk accepts the following options, which should be avail- able on any implementation of the AWK language. -Ffs Use fs for the input field separator (the value of the FS predefined variable). -v var=val Assign the value val, to the variable var, before exe- cution of the program begins. Such variable values are available to the BEGIN block of an AWK program. -f program-file Read the AWK program source from the file program-file, instead of from the first command line argument. Mul- tiple -f options may be used. -- 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. Following the POSIX standard, gawk-specific options are sup- plied via arguments to the -W option. Multiple -W options may be supplied, or multiple arguments may be supplied together if they are separated by commas, or enclosed in quotes and separated by white space. Case is ignored in arguments to the -W option. Gawk recognizes the following arguments to -W: compat Run in compatibility mode. In compatibility mode, gawk behaves identically to UNIX awk; none of the GNU-specific extensions are recognized. copyleft copyright Print the short version of the GNU copyright information message on the error output. version Print version information for this particular copy of gawk on the error 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. posix This turns on compatibility mode, with the follow- ing additional restrictions: o \x escape sequences are not recognized. o The synonym func for the keyword function is not recognized. o The operators ** and **= can not be used in place of ^ and ^=. lint Provide warnings about constructs that are dubious or non-portable to other AWK implementations. Any other options are flagged as illegal, but are otherwise ignored. AWK PROGRAM EXECUTION 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, or from the first non-option argument on the command line. The -f option may be used multiple times on the command line. Gawk will read the program text as if all the program-files had been concatenated together. This is useful for building libraries of AWK functions, without hav- ing to include them in each new AWK program that uses them. To use a library function in a file from a program typed in on the command line, specify /dev/tty as one of the program-files, type your program, and end it with a ^D (control-d). 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/lib/awk:/usr/local/lib/awk". 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, gawk compiles the program into an internal form. Next, all variable assignments specified via the -v option are per- formed. 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 ``file'' named 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 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 line in the input, gawk tests to see if it matches any pattern in the AWK program. For each pattern that the line matches, the associated action is executed. The pat- terns 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). VARIABLES AND FIELDS 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 dimension arrays; multiply dimen- sioned arrays may be simulated. There are several pre- defined variables that AWK sets as a program runs; these will be described as needed and summarized below. Fields As each input line is read, gawk splits the line into fields, using the value of the FS variable as the field separator. If FS is a single character, fields are separated by that character. Otherwise, FS is expected to be a full regular expression. In the special case that FS is a single blank, fields are separated by runs of blanks and/or tabs. Note that the value of IGNORECASE (see below) will also affect how fields are split when FS 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 will split up the record using the specified widths. The value of FS is ignored. Each field in the input line may be referenced by its posi- tion, $1, $2, and so on. $0 is the whole line. The value of a field may be assigned to as well. Fields need not be referenced by constants: n = 5 print $n prints the fifth field in the input line. The variable NF is set to the total number of fields in the input line. 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) will increase the value of NF, create any intervening fields with the null string as their value, and cause the value of $0 to be recomputed, with the fields being separated by the value of OFS. Built-in Variables AWK's built-in variables are: ARGC The number of command line arguments (does not include options to gawk, or the program source). ARGV 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. CONVFMT The conversion format for numbers, "%.6g", by default. ENVIRON 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 /u/arnold). Changing this array does not affect the environment seen by programs which gawk spawns via redirection or the sys- tem() function. (This may change in a future version of gawk.) FIELDWIDTHS A white-space separated list of fieldwidths. When set, gawk parses the input into fields of fixed width, instead of using the value of the FS variable. FILENAME The name of the current input file. If no files are specified on the command line, the value of FILENAME is ``-''. FNR The input record number in the current input file. FS The input field separator, a blank by default. IGNORECASE Controls the case-sensitivity of all regular expression operations. If IGNORECASE has a non- zero value, then pattern matching in rules, field splitting with FS, regular expression matching with ~ and !~, and the gsub(), index(), match(), split(), and sub() pre-defined func- tions will all ignore case when doing regular expression operations. Thus, if IGNORECASE is not equal to zero, /aB/ matches all of the strings "ab", "aB", "Ab", and "AB". As with all AWK variables, the initial value of IGNORECASE is zero, so all regular expression operations are normally case-sensitive. NF The number of fields in the current input record. NR The total number of input records seen so far. OFMT The output format for numbers, "%.6g", by default. OFS The output field separator, a blank by default. ORS The output record separator, by default a new- line. RS The input record separator, by default a new- line. RS is exceptional in that 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. When RS is set to the null string, then the newline character always acts as a field separator, in addition to whatever value FS may have. RSTART The index of the first character matched by match(); 0 if no match. RLENGTH The length of the string matched by match(); -1 if no match. SUBSEP The character used to separate multiple sub- scripts in array elements, by default "\034". 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 con- sisting 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, world\n" assigns the string "hello, world\n" to the element of the array x which is indexed by the string "A\034B\034C". All arrays in AWK are associative, i.e. indexed by string values. The special operator in may be used in an if or while state- ment to see if an array has an index consisting of a partic- ular value. 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. Variable Typing 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. 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. According to the POSIX standard (draft 11), even if two strings are numeric strings, a numeric comparison is performed. However, this is clearly incorrect, and gawk does not do this. Uninitialized variables have the numeric value 0 and the string value "" (the null, or empty, string). PATTERNS AND ACTIONS 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, of course, not both. If the pattern is miss- ing, the action will be executed for every single line of input. A missing action is equivalent to { print } which prints the entire line. 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, how- ever, this is not the case for lines ending in a ``,'', ``{'', ``?'', ``:'', ``&&'', or ``||''. Lines ending in do or else also have their statements automatically continued on the following line. In other cases, a line can be con- tinued 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: 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 writ- ten 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 line 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, respectively, 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 line 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: c matches the non-metacharacter c. \c matches the literal character c. . matches any character except newline. ^ matches the beginning of a line or a string. $ matches the end of a line or a string. [abc...] character class, matches any of the characters abc.... [^abc...] negated character class, matches any character except abc... and newline. r1|r2 alternation: matches either r1 or r2. r1r2 concatenation: matches r1, and then r2. r+ matches one or more r's. r* matches zero or more r's. r? matches zero or one r's. (r) grouping: matches r. The escape sequences that are valid in string constants (see below) are also legal in regular expressions. 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 increasing precedence, are = += -= *= /= %= ^= Assignment. Both absolute assignment (var = value) and operator-assignment (the other forms) are supported. ?: 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. || Logical OR. && Logical AND. ~ !~ 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. < <= > >= != == The regular relational operators. blank String concatenation. + - Addition and subtraction. * / % Multiplication, division, and modulus. + - ! Unary plus, unary minus, and logical negation. ^ Exponentiation (** may also be used, and **= for the assignment operator). ++ -- Increment and decrement, both prefix and postfix. $ Field reference. Control Statements The control statements are as follows: 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] exit [ expression ] { statements } I/O Statements The input/output statements are as follows: close(filename) Close file (or pipe, see below). getline Set $0 from next input record; set NF, NR, FNR. getline <file Set $0 from next record of file; set NF. getline var Set var from next input record; set NF, FNR. getline var <file Set var from next record of file. next Stop processing the current input record. The next input record is read 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), if any, are exe- cuted. print Prints the current record. print expr-list Prints expressions. print expr-list >file Prints expressions on file. printf fmt, expr-list Format and print. printf fmt, expr-list >file Format and print on file. system(cmd-line) Execute the command cmd-line, and return the exit status. (This may not be available on non-POSIX systems.) Other input/output redirections are also allowed. For print and printf, >>file appends output to the file, while | com- mand writes on a pipe. In a similar fashion, command | get- line pipes into getline. Getline will return 0 on end of file, and -1 on an error. The printf Statement The AWK versions of the printf and sprintf (see below) func- tions accept the following conversion specification formats: %c An ASCII 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 A decimal number (the integer part). %i Just like %d. %e A floating point number of the form [-]d.ddddddE[+-]dd. %f A floating point number of the form [-]ddd.dddddd. %g Use e or f conversion, whichever is shorter, with non- significant zeros suppressed. %o An unsigned octal number (again, an integer). %s A character string. %x An unsigned hexadecimal number (an integer). %X Like %x, but using ABCDEF instead of abcdef. %% A single % character; no argument is converted. There are optional, additional parameters that may lie between the % and the control letter: - The expression should be left-justified within its field. width The field should be padded to this width. If the number has a leading zero, then the field will be padded with zeros. Otherwise it is padded with blanks. .prec A number indicating the maximum width of strings or digits to the right of the decimal point. The dynamic width and prec capabilities of the ANSI C printf routines are supported. A * in place of either the width or prec specifications will cause their values to be taken from the argument list to printf or sprintf. 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 pro- cess (usually the shell). The filenames are: /dev/stdin The standard input. /dev/stdout The standard output. /dev/stderr The standard error output. /dev/fd/n The file denoted by the open file descriptor n. These are particularly useful for error messages. For exam- ple: print "You blew it!" > "/dev/stderr" whereas you would otherwise have to use print "You blew it!" | "cat 1>&2" These file names may also be used on the command line to name data files. Numeric Functions AWK has the following pre-defined arithmetic functions: atan2(y, x) returns the arctangent of y/x in radians. cos(expr) returns the cosine in radians. exp(expr) the exponential function. int(expr) truncates to integer. log(expr) the natural logarithm function. rand() returns a random number between 0 and 1. sin(expr) returns the sine in radians. sqrt(expr) the square root function. srand(expr) use expr as a new seed for the random number generator. If no expr is provided, the time of day will be used. The return value is the pre- vious seed for the random number generator. String Functions AWK has the following pre-defined string functions: gsub(r, s, t) for each substring matching the reg- ular expression r in the string t, substitute the string s, and return the number of substitutions. If t is not supplied, use $0. index(s, t) returns the index of the string t in the string s, or 0 if t is not present. length(s) returns the length of the string s, or the length of $0 if s is not sup- plied. match(s, r) returns the position in s where the regular expression r occurs, or 0 if r is not present, and sets the values of RSTART and RLENGTH. split(s, a, r) splits the string s into the array a on the regular expression r, and returns the number of fields. If r is omitted, FS is used instead. sprintf(fmt, expr-list) prints expr-list according to fmt, and returns the resulting string. sub(r, s, t) just like gsub, but only the first matching substring is replaced. substr(s, i, n) returns the n-character substring of s starting at i. If n is omitted, the rest of s is used. tolower(str) returns a copy of the string str, with all the upper-case characters in str translated to their corresponding lower-case counter- parts. Non-alphabetic characters are left unchanged. toupper(str) returns a copy of the string str, with all the lower-case characters in str translated to their corresponding upper-case counter- parts. Non-alphabetic characters are left unchanged. Time Functions Since one of the primary uses of AWK programs in processing log files that contain time stamp information, gawk provides the following two functions for obtaining time stamps and formatting them. systime() returns the current time of day as the number of seconds since the Epoch (Midnight UTC, January 1, 1970 on POSIX systems). strftime(format, timestamp) formats timestamp according to the specification in format. The timestamp should be of the same form as returned by sys- time(). If timestamp is miss- ing, the current time of day is used. See the specification for the strftime function in ANSI C for the format conversions that are guaranteed to be available. A public-domain version of strftime(3) and a man page for it are shipped with gawk; if that version was used to build gawk, then all of the conver- sions described in that man page are available to gawk. 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: \\ A literal backslash. \a The ``alert'' character; usually the ASCII BEL charac- ter. \b backspace. \f form-feed. \n new line. \r carriage return. \t horizontal tab. \v vertical tab. \xhex digits The character represented by the string of hexadecimal digits following the \x. 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 The character represented by the 1-, 2-, or 3-digit sequence of octal digits. E.g. "\033" is the ASCII ESC (escape) character. \c The literal character c. The escape sequences may also be used inside constant regu- lar expressions (e.g., /[ \t\f\n\r\v]/ matches whitespace characters). FUNCTIONS Functions in AWK are defined as follows: function name(parameter list) { statements } Functions are executed when called from within the action parts of regular pattern-action statements. Actual parame- ters 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 parame- ter list. The convention is to separate local variables from real parameters by extra spaces in the parameter list. For example: function f(p, q, a, b) { # a & b are local ..... } /abc/ { ... ; f(1, 2) ; ... } The left parenthesis in a function call is required to immediately follow the function name, without any interven- ing 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. Func- tion parameters used as local variables are initialized to the null string and the number zero upon function invoca- tion. The word func may be used in place of function. EXAMPLES 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 } SEE ALSO egrep(1) The AWK Programming Language, Alfred V. Aho, Brian W. Ker- nighan, Peter J. Weinberger, Addison-Wesley, 1988. ISBN 0- 201-07981-X. The GAWK Manual, published by the Free Software Foundation, 1991. POSIX COMPATIBILITY 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 awk in System V Release 4, and are in the POSIX standard. The -v option for assigning variables before program execu- tion starts is new. 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, this option was added to accomodate applications that depended upon the old behaviour. (This feature was agreed upon by both the AT&T and GNU develop- ers.) 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, and warns about, but otherwise ignores, undefined options. The AWK book does not define the return value of srand(). The System V Release 4 version of UNIX awk (and 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 ENVIRON array; the \a, and \v escape sequences (done originally in gawk and fed back into AT&T's); the tolower and toupper built-in functions (from AT&T); and the ANSI C conversion specifications in printf (done first in AT&T's version). GNU EXTENSIONS Gawk has some extensions to POSIX awk. They are described in this section. All the extensions described here can be disabled by invoking gawk with the -W compat option. The following features of gawk are not available in POSIX awk. o The \x escape sequence. o The systime() and strftime(format, secs) functions. o The special file names available for I/O redirection are not recognized. o The IGNORECASE variable and its side-effects are not available. o The FIELDWIDTHS variable and fixed width field split- ting. o No path search is performed for files named via the -f option. Therefore the AWKPATH environment vari- able is not special. 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 a file or pipe, respectively. When gawk is invoked with the -W compat option, if the fs argument to the -F option is ``t'', then FS will be set to the tab character. Since this is a rather ugly special case, it is not the default behavior. BUGS The -F option is not necessary given the command line vari- able assignment feature; it remains only for backwards com- patibility. VERSION INFORMATION This man page documents gawk, version 2.13. For the 2.13 version of gawk, the -c, -v, -V, -a, and -e options of the 2.11 version are recognized. However, gawk will print a warning message, and these options will go away in the 2.14 version. AUTHORS The original version of UNIX awk was designed and imple- mented by Alfred Aho, Peter Weinberger, and Brian Kernighan of AT&T Bell Labs. Brian Kernighan continues to maintain and enhance it. Paul Rubin and Jay Fenlason, of the Free Software Founda- tion, wrote gawk, to be compatible with the original version of awk distributed in Seventh Edition UNIX. John Woods con- tributed a number of bug fixes. David Trueman of Dalhousie University, with contributions from Arnold Robbins at Emory University and AudioFAX, made gawk compatible with the new version of UNIX awk. ACKNOWLEDGEMENTS Brian Kernighan of Bell Labs provided valuable assistance during testing and debugging. We thank him.