man/cat1p/awk.1p.txt
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AWK(1P) POSIX Programmer's Manual AWK(1P)
PROLOG This manual page is part of the POSIX Programmer's Man- ual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux. NAME awk - pattern scanning and processing language SYNOPSIS awk [-F ERE][-v assignment] ... program [argument ...] awk [-F ERE] -f progfile ... [-v assignment] ...[argu- ment ...] DESCRIPTION The awk utility shall execute programs written in the awk programming language, which is specialized for tex- tual data manipulation. An awk program is a sequence of patterns and corresponding actions. When input is read that matches a pattern, the action associated with that pattern is carried out. Input shall be interpreted as a sequence of records. By default, a record is a line, less its terminating <new- line>, but this can be changed by using the RS built-in variable. Each record of input shall be matched in turn against each pattern in the program. For each pattern matched, the associated action shall be executed. The awk utility shall interpret each input record as a sequence of fields where, by default, a field is a string of non- <blank>s. This default white-space field delimiter can be changed by using the FS built-in vari- able or -F ERE. The awk utility shall denote the first field in a record $1, the second $2, and so on. The sym- bol $0 shall refer to the entire record; setting any other field causes the re-evaluation of $0. Assigning to $0 shall reset the values of all other fields and the NF built-in variable. OPTIONS The awk utility shall conform to the Base Definitions volume of IEEE Std 1003.1-2001, Section 12.2, Utility Syntax Guidelines. The following options shall be supported: -F ERE Define the input field separator to be the extended regular expression ERE, before any input is read; see Regular Expressions . -f progfile Specify the pathname of the file progfile con- taining an awk program. If multiple instances of this option are specified, the concatenation of the files specified as progfile in the order specified shall be the awk program. The awk pro- gram can alternatively be specified in the com- mand line as a single argument. -v assignment The application shall ensure that the assignment argument is in the same form as an assignment operand. The specified variable assignment shall occur prior to executing the awk program, includ- ing the actions associated with BEGIN patterns (if any). Multiple occurrences of this option can be specified. OPERANDS The following operands shall be supported: program If no -f option is specified, the first operand to awk shall be the text of the awk program. The application shall supply the program operand as a single argument to awk. If the text does not end in a <newline>, awk shall interpret the text as if it did. argument Either of the following two types of argument can be intermixed: file A pathname of a file that contains the input to be read, which is matched against the set of pat- terns in the program. If no file operands are specified, or if a file operand is '-', the stan- dard input shall be used. assignment An operand that begins with an underscore or alphabetic character from the portable character set (see the table in the Base Definitions volume of IEEE Std 1003.1-2001, Section 6.1, Portable Character Set), followed by a sequence of under- scores, digits, and alphabetics from the portable character set, followed by the '=' character, shall specify a variable assignment rather than a pathname. The characters before the '=' represent the name of an awk variable; if that name is an awk reserved word (see Grammar ) the behavior is undefined. The characters following the equal sign shall be interpreted as if they appeared in the awk program preceded and followed by a dou- ble-quote ( ' )' character, as a STRING token (see Grammar ), except that if the last character is an unescaped backslash, it shall be inter- preted as a literal backslash rather than as the first character of the sequence "\"" . The vari- able shall be assigned the value of that STRING token and, if appropriate, shall be considered a numeric string (see Expressions in awk ), the variable shall also be assigned its numeric value. Each such variable assignment shall occur just prior to the processing of the following file, if any. Thus, an assignment before the first file argument shall be executed after the BEGIN actions (if any), while an assignment after the last file argument shall occur before the END actions (if any). If there are no file arguments, assignments shall be executed before processing the standard input. STDIN The standard input shall be used only if no file operands are specified, or if a file operand is '-' ; see the INPUT FILES section. If the awk program contains no actions and no patterns, but is otherwise a valid awk program, standard input and any file operands shall not be read and awk shall exit with a return status of zero. INPUT FILES Input files to the awk program from any of the following sources shall be text files: * Any file operands or their equivalents, achieved by modifying the awk variables ARGV and ARGC * Standard input in the absence of any file operands * Arguments to the getline function Whether the variable RS is set to a value other than a <newline> or not, for these files, implementations shall support records terminated with the specified separator up to {LINE_MAX} bytes and may support longer records. If -f progfile is specified, the application shall ensure that the files named by each of the progfile option-arguments are text files and their concatenation, in the same order as they appear in the arguments, is an awk program. ENVIRONMENT VARIABLES The following environment variables shall affect the execution of awk: LANG Provide a default value for the internationaliza- tion variables that are unset or null. (See the Base Definitions volume of IEEE Std 1003.1-2001, Section 8.2, Internationalization Variables for the precedence of internationalization variables used to determine the values of locale cate- gories.) LC_ALL If set to a non-empty string value, override the values of all the other internationalization variables. LC_COLLATE Determine the locale for the behavior of ranges, equivalence classes, and multi-character collat- ing elements within regular expressions and in comparisons of string values. LC_CTYPE Determine the locale for the interpretation of sequences of bytes of text data as characters (for example, single-byte as opposed to multi- byte characters in arguments and input files), the behavior of character classes within regular expressions, the identification of characters as letters, and the mapping of uppercase and lower- case characters for the toupper and tolower func- tions. LC_MESSAGES Determine the locale that should be used to affect the format and contents of diagnostic mes- sages written to standard error. LC_NUMERIC Determine the radix character used when inter- preting numeric input, performing conversions between numeric and string values, and formatting numeric output. Regardless of locale, the period character (the decimal-point character of the POSIX locale) is the decimal-point character rec- ognized in processing awk programs (including assignments in command line arguments). NLSPATH Determine the location of message catalogs for the processing of LC_MESSAGES . PATH Determine the search path when looking for com- mands executed by system(expr), or input and out- put pipes; see the Base Definitions volume of IEEE Std 1003.1-2001, Chapter 8, Environment Variables. In addition, all environment variables shall be visible via the awk variable ENVIRON. ASYNCHRONOUS EVENTS Default. STDOUT The nature of the output files depends on the awk pro- gram. STDERR The standard error shall be used only for diagnostic messages. OUTPUT FILES The nature of the output files depends on the awk pro- gram. EXTENDED DESCRIPTION Overall Program Structure An awk program is composed of pairs of the form: pattern { action } Either the pattern or the action (including the enclos- ing brace characters) can be omitted. A missing pattern shall match any record of input, and a missing action shall be equivalent to: { print } Execution of the awk program shall start by first exe- cuting the actions associated with all BEGIN patterns in the order they occur in the program. Then each file operand (or standard input if no files were specified) shall be processed in turn by reading data from the file until a record separator is seen ( <newline> by default). Before the first reference to a field in the record is evaluated, the record shall be split into fields, according to the rules in Regular Expressions, using the value of FS that was current at the time the record was read. Each pattern in the program then shall be evaluated in the order of occurrence, and the action associated with each pattern that matches the current record executed. The action for a matching pattern shall be executed before evaluating subsequent patterns. Finally, the actions associated with all END patterns shall be executed in the order they occur in the pro- gram. Expressions in awk Expressions describe computations used in patterns and actions. In the following table, valid expression operations are given in groups from highest precedence first to lowest precedence last, with equal-precedence operators grouped between horizontal lines. In expres- sion evaluation, where the grammar is formally ambigu- ous, higher precedence operators shall be evaluated before lower precedence operators. In this table expr, expr1, expr2, and expr3 represent any expression, while lvalue represents any entity that can be assigned to (that is, on the left side of an assignment operator). The precise syntax of expressions is given in Grammar . Table: Expressions in Decreasing Precedence in awk IEEE/The Open Group 2003 AWK(1P) |