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33.11 Quick Reference: awk

This article also covers nawk and gawk (33.12). With the exception of array subscripts, values in [brackets] are optional; don't type the [ or ].

33.11.1 Command-line Syntax

awk can be invoked in two ways:

awk [options] 'script' [var=value] [file(s)]
awk [options] -f scriptfile [var=value] [file(s)]

You can specify a script directly on the command line, or you can store a script in a scriptfile and specify it with -f. In most versions, the -f option can be used multiple times. The variable var can be assigned a value on the command line. The value can be a literal, a shell variable ($name), or a command substitution (`cmd`), but the value is available only after a line of input is read (i.e., after the BEGIN statement). awk operates on one or more file(s). If none are specified (or if - is specified), awk reads from the standard input (13.1).

The other recognized options are:

-Fc

Set the field separator to character c. This is the same as setting the system variable FS. nawk allows c to be a regular expression (26.4). Each record (by default, one input line) is divided into fields by white space (blanks or tabs) or by some other user-definable field separator. Fields are referred to by the variables $1, $2,...$n. $0 refers to the entire record. For example, to print the first three (colon-separated) fields on separate lines:

    % awk -F: '{print $1; print $2; print $3}' /etc/passwd

-v var=value

Assign a value to variable var. This allows assignment before the script begins execution. (Available in nawk only.)

33.11.2 Patterns and Procedures

awk scripts consist of patterns and procedures:

pattern {procedure}

Both are optional. If pattern is missing, {procedure} is applied to all records. If {procedure} is missing, the matched record is written to the standard output.

33.11.2.1 Patterns

pattern can be any of the following:

/regular expression/
relational expression
pattern-matching expression
BEGIN
END
  • Expressions can be composed of quoted strings, numbers, operators, functions, defined variables, or any of the predefined variables described later under the section "awk System Variables."

  • Regular expressions use the extended set of metacharacters as described in article 26.4. In addition, ^ and $ can be used to refer to the beginning and end of a field, respectively, rather than the beginning and end of a record (line).

  • Relational expressions use the relational operators listed under the section "Operators" later in this article. Comparisons can be either string or numeric. For example, $2 > $1 selects records for which the second field is greater than the first.

  • Pattern-matching expressions use the operators ~ (match) and !~ (don't match). See the section "Operators" later in this article.

  • The BEGIN pattern lets you specify procedures that will take place before the first input record is processed. (Generally, you set global variables here.)

  • The END pattern lets you specify procedures that will take place after the last input record is read.

Except for BEGIN and END, patterns can be combined with the Boolean operators || (OR), && (AND), and ! (NOT). A range of lines can also be specified using comma-separated patterns:

pattern,pattern

33.11.2.2 Procedures

procedure can consist of one or more commands, functions, or variable assignments, separated by newlines or semicolons (;), and contained within curly braces ({}). Commands fall into four groups:

  • Variable or array assignments

  • Printing commands

  • Built-in functions

  • Control-flow commands

33.11.2.3 Simple Pattern-Procedure Examples

  1. Print first field of each line:

    { print $1 }

  2. Print all lines that contain pattern:

    /pattern/

  3. Print first field of lines that contain pattern:

    /pattern/{ print $1 }

  4. Print records containing more than two fields:

    NF > 2

  5. Interpret input records as a group of lines up to a blank line:

    BEGIN { FS = "\n"; RS = "" }
    { ...process records... }

  6. Print fields 2 and 3 in switched order, but only on lines whose first field matches the string URGENT:

    $1 ~ /URGENT/ { print $3, $2 }

  7. Count and print the number of pattern found:

    /pattern/ { ++x }
    END { print x }

  8. Add numbers in second column and print total:

    {total += $2 }; 
    END { print "column total is", total}

  9. Print lines that contain less than 20 characters:

    length($0) < 20

  10. Print each line that begins with Name: and that contains exactly seven fields:

    NF == 7 && /^Name:/

33.11.3 awk System Variables

nawk supports all awk variables. gawk supports both nawk and awk.
VersionVariableDescription
awkFILENAMECurrent filename
FSField separator (default is whitespace)
NFNumber of fields in current record
NRNumber of the current record
OFMTOutput format for numbers (default is %.6g)
OFSOutput field separator (default is a blank)
ORSOutput record separator (default is a newline)
RSRecord separator (default is a newline)
$0Entire input record
$nnth field in current record; fields are separated by FS
nawkARGCNumber of arguments on command line
ARGVAn array containing the command-line arguments
ENVIRONAn associative array of environment variables
FNRLike NR, but relative to the current file
RSTARTFirst position in the string matched by match function
RLENGTHLength of the string matched by match function
SUBSEPSeparator character for array subscripts (default is \034)

33.11.4 Operators

The table below lists the operators, in order of increasing precedence, that are available in awk:
SymbolMeaning
= += -= *= /= %= ^=Assignment (^= only in nawk and gawk)
?:C conditional expression (nawk and gawk)
||Logical OR
&&Logical AND
~ !~Match regular expression and negation
< <= > >= != ==Relational operators
(blank)Concatenation
+ -Addition, subtraction
* / %Multiplication, division, and modulus
+ - !Unary plus and minus, and logical negation
^Exponentiation (nawk and gawk)
++ -- Increment and decrement, either prefix or postfix
$Field reference

33.11.5 Variables and Array Assignments

Variables can be assigned a value with an equal sign (=). For example:

FS = ","

Expressions using the operators +, -, *, /, and % (modulo) can be assigned to variables.

Arrays can be created with the split function (see below), or they can simply be named in an assignment statement. Array elements can be subscripted with numbers (array[1],...array[n]) or with names. For example, to count the number of occurrences of a pattern, you could use the following script:

/pattern/ { array["pattern"]++ }
END { print array["pattern"] }

33.11.6 Group Listing of awk Commands

awk commands may be classified as follows:
ArithmeticStringControl FlowInput/Output
FunctionsFunctionsStatementsProcessing
atan2*gsub*breakclose*
cos*indexcontinuedelete*
explengthdo/while*getline*
intmatch*exitnext
logsplitforprint
rand*sub*ifprintf
sin*substrreturn*sprintf
sqrttolower*whilesystem*
srand*toupper*
*Not in original awk

33.11.7 Alphabetical Summary of Commands

The following alphabetical list of statements and functions includes all that are available in awk, nawk, or gawk. Unless otherwise mentioned, the statement or function is found in all versions. New statements and functions introduced with nawk are also found in gawk.

atan2

atan2(y,x) Returns the arctangent of y/x in radians. (nawk)

break

Exit from a while, for, or do loop.

close

close(filename-expr)
close(command-expr)
In some implementations of awk, you can have only ten files open simultaneously and one pipe; modern versions allow more than one pipe open. Therefore, nawk provides a close statement that allows you to close a file or a pipe. close takes as an argument the same expression that opened the pipe or file. (nawk)

continue

Begin next iteration of while, for, or do loop immediately.

cos

cos(x) Return cosine of x (in radians). (nawk)

delete

delete array[element] Delete element of array. (nawk)

do

do
     bodywhile (expr)
Looping statement. Execute statements in body, then evaluate expr. If expr is true, execute body again. More than one command must be put inside braces ({}). (nawk)

exit

exit[expr] Do not execute remaining instructions and do not read new input. END procedure, if any, will be executed. The expr, if any, becomes awk's exit status (44.7).

exp

exp(arg) Return the natural exponent of arg.

for

for ([init-expr]; [test-expr]; [incr-expr]) command C-language-style looping construct. Typically, init-expr assigns the initial value of a counter variable. test-expr is a relational expression that is evaluated each time before executing the command. When test-expr is false, the loop is exited. incr-expr is used to increment the counter variable after each pass. A series of commands must be put within braces ({}). Example:

for (i = 1; i <= 10; i++)
     printf "Element %d is %s.\n", i, array[i]

for

for (item in array) command For each item in an associative array, do command. More than one command must be put inside braces ({}). Refer to each element of the array as array[item].

getline

getline [var][<file] or command | getline [var] Read next line of input. Original awk does not support the syntax to open multiple input streams. The first form reads input from file, and the second form reads the standard output of a UNIX command. Both forms read one line at a time, and each time the statement is executed it gets the next line of input. The line of input is assigned to $0, and it is parsed into fields, setting NF, NR, and FNR. If var is specified, the result is assigned to var and the $0 is not changed. Thus, if the result is assigned to a variable, the current line does not change. getline is actually a function and it returns 1 if it reads a record successfully, 0 if end-of-file is encountered, and -1 if for some reason it is otherwise unsuccessful. (nawk)

gsub

gsub(r,s[,t]) Globally substitute s for each match of the regular expression r in the string t. Return the number of substitutions. If t is not supplied, defaults to $0. (nawk)

if

if (condition)

      command
[else
      command]

If condition is true, do command(s), otherwise do command(s) in else clause (if any). condition can be an expression that uses any of the relational operators <, <=, ==, !=, >=, or >, as well as the pattern-matching operators ~ or !~ (e.g., if ($1 ~ /[Aa].*[Zz]/)). A series of commands must be put within braces ({}).

index

index(str,substr) Return position of first substring substr in string str or 0 if not found.

int

int(arg) Return integer value of arg.

length

length(arg) Return the length of arg.

log

log(arg) Return the natural logarithm of arg.

match

match(s,r) Function that matches the pattern, specified by the regular expression r, in the string s and returns either the position in s where the match begins or 0 if no occurrences are found. Sets the values of RSTART and RLENGTH. (nawk)

next

Read next input line and start new cycle through pattern/procedures statements.

print

print [args] [destination] Print args on output, followed by a newline. args is usually one or more fields, but may also be one or more of the predefined variables - or arbitrary expressions. If no args are given, prints $0 (the current input line). Literal strings must be quoted. Fields are printed in the order they are listed. If separated by commas (,) in the argument list, they are separated in the output by the OFS character. If separated by spaces, they are concatenated in the output. destination is a UNIX redirection or pipe expression (e.g., file) that redirects the default standard output.

printf

format [, expression(s)] [destination] Formatted print statement. Fields or variables can be formatted according to instructions in the format argument. The number of expressions must correspond to the number specified in the format sections. format follows the conventions of the C-language printf statement. Here are a few of the most common formats:

%s

A string.

%d

A decimal number.

%n.mf

A floating-point number, where n is the total number of digits and m is the number of digits after the decimal point.

%[-]nc

n specifies minimum field length for format type c, while - left justifies value in field; otherwise value is right justified. format can also contain embedded escape sequences: \n (newline) or \t (tab) are the most common. destination is a UNIX redirection or pipe expression (e.g., file) that redirects the default standard output. Example: Using the script:

{printf "The sum on line %s is %d.\n", NR, $1+$2}

The following input line:

5   5

produces this output, followed by a newline:

The sum on line 1 is 10.

rand

rand() Generate a random number between 0 and 1. This function returns the same series of numbers each time the script is executed, unless the random number generator is seeded using the srand( ) function. (nawk)

return

return [expr] Used at end of user-defined functions to exit the function, returning value of expression expr, if any. (nawk)

sin

sin(x) Return sine of x (in radians). (nawk)

split

split(string,array[,sep]) Split string into elements of array array[1],... array[n]. string is split at each occurrence of separator sep. (In nawk, the separator may be a regular expression.) If sep is not specified, FS is used. The number of array elements created is returned.

sprintf

sprintf (format [, expression(s)]) Return the value of expression(s), using the specified format (see printf). Data is formatted but not printed.

sqrt

sqrt(arg) Return square root of arg.

srand

srand(expr) Use expr to set a new seed for random number generator. Default is time of day. Returns the old seed. (nawk)

sub

sub(r,s[,t]) Substitute s for first match of the regular expression r in the string t. Return 1 if successful; 0 otherwise. If t is not supplied, defaults to $0. (nawk)

substr

substr(string,m[,n]) Return substring of string beginning at character position m and consisting of the next n characters. If n is omitted, include all characters to the end of string.

system

system(command) Function that executes the specified UNIX command and returns its status (44.7). The status of the command that is executed typically indicates its success (0) or failure (non-zero). The output of the command is not available for processing within the nawk script. Use command | getline to read the output of the command into the script. (nawk)

tolower

tolower(str) Translate all uppercase characters in str to lowercase and return the new string. (nawk)

toupper

toupper(str) Translate all lowercase characters in str to uppercase and return the new string. (nawk)

while

while (condition)
      command
Do command while condition is true (see if for a description of allowable conditions). A series of commands must be put within braces ({}).

- DG from O'Reilly & Associates' UNIX in a Nutshell (SVR4/Solaris)


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