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AWK(1) General Commands Manual AWK(1)

awk - pattern-directed scanning and processing language

awk [ -F fs ] [ -v var=value ] [ 'prog' | -f progfile ] [ file ... ]

Awk scans each input file for lines that match any of a set of patterns specified literally in prog or in one or more files specified as -f progfile. With each pattern there can be an associated action that will be performed when a line of a file matches the pattern. Each line is matched against the pattern portion of every pattern-action statement; the associated action is performed for each matched pattern. The file name - means the standard input. Any file of the form var=value is treated as an assignment, not a filename, and is executed at the time it would have been opened if it were a filename. The option -v followed by var=value is an assignment to be done before prog is executed; any number of -v options may be present. The -F fs option defines the input field separator to be the regular expression fs.

An input line is normally made up of fields separated by white space, or by the regular expression FS. The fields are denoted $1, $2, ..., while $0 refers to the entire line. If FS is null, the input line is split into one field per character.

A pattern-action statement has the form:

pattern { action }

A missing { action } means print the line; a missing pattern always matches. Pattern-action statements are separated by newlines or semicolons.

An action is a sequence of statements. A statement can be one of the following:





if( expression ) statement [ else statement ]
while( expression ) statement
for( expression ; expression ; expression ) statement
for( var in array ) statement
do statement while( expression )
break
continue
{ [ statement ... ] }
expression	# commonly var = expression
print [ expression-list ] [ > expression ]
printf format [ , expression-list ] [ > expression ]
return [ expression ]
next	# skip remaining patterns on this input line
nextfile	# skip rest of this file, open next, start at top
delete array[ expression ]	# delete an array element
delete array	# delete all elements of array
exit [ expression ]	# exit immediately; status is expression




Statements are terminated by semicolons, newlines or right braces.
    An empty expression-list stands for $0. String constants are
    quoted " ", with the usual C
    escapes recognized within. Expressions take on string or numeric values as
    appropriate, and are built using the operators + - * / % ^
    (exponentiation), and concatenation (indicated by white space). The
    operators ! ++ -- += -= *= /= %= ^= > >= < <= == != ?:
    are also available in expressions. Variables may be scalars, array elements
    (denoted x[i]) or fields. Variables are
    initialized to the null string. Array subscripts may be any string, not
    necessarily numeric; this allows for a form of associative memory. Multiple
    subscripts such as [i,j,k] are permitted; the constituents are
    concatenated, separated by the value of SUBSEP.


The print statement prints its arguments on the standard
    output (or on a file if > file  or >> file
     is present or on a pipe if | cmd  is present), separated
    by the current output field separator, and terminated by the output record
    separator. file and cmd may be literal names or parenthesized
    expressions; identical string values in different statements denote the same
    open file. The printf statement formats its expression list according
    to the format (see printf(3)). The built-in function
    close(expr) closes the file or pipe expr. The
    built-in function fflush(expr) flushes any buffered
    output for the file or pipe expr.


The mathematical functions atan2, cos, exp,
    log, sin, and sqrt are built in. Other built-in
    functions:



  

  

  





  

  
the length of its argument taken as a string, number of elements in an
      array for an array argument, or length of $0 if no argument.

  

  
random number on [0,1).

  

  
sets seed for rand and returns the previous seed.

  

  
truncates to an integer value.

  

  
the n-character substring of s that begins at position
      m counted from 1. If no n, use the rest of the string.

  

  
the position in s where the string t occurs, or 0 if it does
      not.

  

  
the position in s where the regular expression r occurs, or
      0 if it does not. The variables RSTART and RLENGTH are set
      to the position and length of the matched string.

  

  
splits the string s into array elements a[1],
      a[2], ..., a[n],
      and returns n. The separation is done with the regular expression
      fs or with the field separator FS if fs is not given.
      An empty string as field separator splits the string into one array
      element per character.

  

  
substitutes t for the first occurrence of the regular expression
      r in the string s. If s is not given, $0 is
      used.

  

  
same as sub except that all occurrences of the regular expression
      are replaced; sub and gsub return the number of
      replacements.

  

  
the string resulting from formatting expr ... according to the
      printf(3) format fmt.

  

  
executes cmd and returns its exit status. This will be -1 upon
      error, cmd's exit status upon a normal exit, 256 + sig upon
      death-by-signal, where sig is the number of the murdering signal,
      or 512 + sig if there was a core dump.

  

  
returns a copy of str with all upper-case characters translated to
      their corresponding lower-case equivalents.

  

  
returns a copy of str with all lower-case characters translated to
      their corresponding upper-case equivalents.




The ``function'' getline sets $0 to the next input
    record from the current input file; getline < file 
    sets $0 to the next record from file. getline x
    sets variable x instead. Finally, cmd | getline  pipes
    the output of cmd into getline; each call of getline
    returns the next line of output from cmd. In all cases,
    getline returns 1 for a successful input, 0 for end of file, and -1
    for an error.


Patterns are arbitrary Boolean combinations (with ! ||
    &&) of regular expressions and relational expressions. Regular
    expressions are as defined in re_format(7). Isolated regular
    expressions in a pattern apply to the entire line. Regular expressions may
    also occur in relational expressions, using the operators ~ and
    !~. /re/ is a constant regular expression; any
    string (constant or variable) may be used as a regular expression, except in
    the position of an isolated regular expression in a pattern.


A pattern may consist of two patterns separated by a comma; in
    this case, the action is performed for all lines from an occurrence of the
    first pattern though an occurrence of the second.


A relational expression is one of the following:



  

  
expression matchop regular-expression
    

    expression relop expression
    

    expression in array-name
    

    (expr,expr,...) in array-name




where a relop is any of the six relational operators in C,
    and a matchop is either ~ (matches) or !~ (does not
    match). A conditional is an arithmetic expression, a relational expression,
    or a Boolean combination of these.


The special patterns BEGIN and END may be used to
    capture control before the first input line is read and after the last.
    BEGIN and END do not combine with other patterns. They may
    appear multiple times in a program and execute in the order they are read by
    awk.


Variable names with special meanings:



  

  

  





  

  
argument count, assignable.

  

  
argument array, assignable; non-null members are taken as filenames.

  

  
conversion format used when converting numbers (default %.6g).

  

  
array of environment variables; subscripts are names.

  

  
the name of the current input file.

  

  
ordinal number of the current record in the current file.

  

  
regular expression used to separate fields; also settable by option
      -Ffs.

  

  
number of fields in the current record.

  

  
ordinal number of the current record.

  

  
output format for numbers (default %.6g).

  

  
output field separator (default space).

  

  
output record separator (default newline).

  

  
the length of a string matched by match.

  

  
input record separator (default newline). If empty, blank lines separate
      records. If more than one character long, RS is treated as a
      regular expression, and records are separated by text matching the
      expression.

  

  
the start position of a string matched by match.

  

  
separates multiple subscripts (default 034).




Functions may be defined (at the position of a pattern-action
    statement) thus:



  

  
function foo(a, b, c) { ...; return x }




Parameters are passed by value if scalar and by reference if array
    name; functions may be called recursively. Parameters are local to the
    function; all other variables are global. Thus local variables may be
    created by providing excess parameters in the function definition.








If POSIXLY_CORRECT is set in the environment, then
    awk follows the POSIX rules for sub and gsub with
    respect to consecutive backslashes and ampersands.








length($0) > 72



Print lines longer than 72 characters.


{ print $2, $1 }



Print first two fields in opposite order.




BEGIN { FS = ",[ \t]*|[ \t]+" }


      { print $2, $1 }




  

  
Same, with input fields separated by comma and/or spaces and tabs.






	{ s += $1 }
END	{ print "sum is", s, " average is", s/NR }



  

  
Add up first column, print sum and average.
    
/start/, /stop/
    

    Print all lines between start/stop pairs.






BEGIN	{	# Simulate echo(1)
	for (i = 1; i < ARGC; i++) printf "%s ", ARGV[i]
	printf "\n"
	exit }








grep(1), lex(1), sed(1)
  

  A. V. Aho, B. W. Kernighan, P. J. Weinberger, The AWK Programming
    Language, Addison-Wesley, 1988. ISBN 0-201-07981-X.








There are no explicit conversions between numbers and strings. To
    force an expression to be treated as a number add 0 to it; to force it to be
    treated as a string concatenate "" to
  it.


The scope rules for variables in functions are a botch; the syntax
    is worse.


Only eight-bit characters sets are handled correctly.






2020-11-24
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