1.Programstructure
An AWK program is a sequence of pattern{action} pairs and user function definitions.
A pattern can be:
BEGINEND
expression
expression , expression
One, but not both, of pattern{action} can be omitted. If {action} is omitted it is implicitly { print
}. If pattern is omitted, then it is implicitly matched. BEGIN and END patterns require an action.
Statements are terminated by newlines, semi-colons or both. Groups of statements such as actions or loop
bodies are blocked via { ... } as in C. The last statement in a block doesn't need a terminator. Blank
lines have no meaning; an empty statement is terminated with a semi-colon. Long statements can be
continued with a backslash, \. A statement can be broken without a backslash after a comma, left brace,
&&, ||, do, else, the right parenthesis of an if, while or for statement, and the right parenthesis of a
function definition. A comment starts with # and extends to, but does not include the end of line.
The following statements control program flow inside blocks.
if ( expr ) statementif ( expr ) statementelsestatementwhile ( expr ) statementdostatementwhile ( expr )
for ( opt_expr ; opt_expr ; opt_expr ) statementfor ( varinarray ) statementcontinuebreak2.Datatypes,conversionandcomparison
There are two basic data types, numeric and string. Numeric constants can be integer like -2, decimal
like 1.08, or in scientific notation like -1.1e4 or .28E-3. All numbers are represented internally and
all computations are done in floating point arithmetic. So for example, the expression 0.2e2 == 20 is
true and true is represented as 1.0.
String constants are enclosed in double quotes.
"This is a string with a newline at the end.\n"
Strings can be continued across a line by escaping (\) the newline. The following escape sequences are
recognized.
\\ \
\" "
\a alert, ascii 7
\b backspace, ascii 8
\t tab, ascii 9
\n newline, ascii 10
\v vertical tab, ascii 11
\f formfeed, ascii 12
\r carriage return, ascii 13
\ddd 1, 2 or 3 octal digits for ascii ddd
\xhh 1 or 2 hex digits for ascii hh
If you escape any other character \c, you get \c, i.e., lmawk ignores the escape.
There are really three basic data types; the third is numberandstring which has both a numeric value
and a string value at the same time. User defined variables come into existence when first referenced
and are initialized to null, a number and string value which has numeric value 0 and string value "".
Non-trivial number and string typed data come from input and are typically stored in fields. (See
section 4).
The type of an expression is determined by its context and automatic type conversion occurs if needed.
For example, to evaluate the statements
y = x + 2 ; z = x "hello"
The value stored in variable y will be typed numeric. If x is not numeric, the value read from x is
converted to numeric before it is added to 2 and stored in y. The value stored in variable z will be
typed string, and the value of x will be converted to string if necessary and concatenated with "hello".
(Of course, the value and type stored in x is not changed by any conversions.) A string expression is
converted to numeric using its longest numeric prefix as with atof(3). A numeric expression is converted
to string by replacing expr with sprintf(CONVFMT, expr), unless expr can be represented on the host
machine as an exact integer then it is converted to sprintf("%d", expr). Sprintf() is an AWK built-in
that duplicates the functionality of sprintf(3), and CONVFMT is a built-in variable used for internal
conversion from number to string and initialized to "%.6g". Explicit type conversions can be forced,
expr "" is string and expr+0 is numeric.
To evaluate, expr1 rel-opexpr2, if both operands are numeric or number and string then the comparison is
numeric; if both operands are string the comparison is string; if one operand is string, the non-string
operand is converted and the comparison is string. The result is numeric, 1 or 0.
In boolean contexts such as, if ( expr ) statement, a string expression evaluates true if and only if it
is not the empty string ""; numeric values if and only if not numerically zero.
3.Regularexpressions
In the AWK language, records, fields and strings are often tested for matching a regularexpression.
Regular expressions are enclosed in slashes, and
expr ~ /r/
is an AWK expression that evaluates to 1 if expr "matches" r, which means a substring of expr is in the
set of strings defined by r. With no match the expression evaluates to 0; replacing ~ with the "not
match" operator, !~ , reverses the meaning. As pattern-action pairs,
/r/ { action } and $0 ~ /r/ { action }
are the same, and for each input record that matches r, action is executed. In fact, /r/ is an AWK
expression that is equivalent to ($0 ~ /r/) anywhere except when on the right side of a match operator or
passed as an argument to a built-in function that expects a regular expression argument.
AWK uses extended regular expressions as with egrep(1). The regular expression metacharacters, i.e.,
those with special meaning in regular expressions are
^ $ . [ ] | ( ) * + ?
Regular expressions are built up from characters as follows:
c matches any non-metacharacter c.
\c matches a character defined by the same escape sequences used in string constants or
the literal character c if \c is not an escape sequence.
. matches any character (including newline).
^ matches the front of a string.
$ matches the back of a string.
[c1c2c3...] matches any character in the class c1c2c3... . An interval of characters is denoted
c1-c2 inside a class [...].
[^c1c2c3...] matches any character not in the class c1c2c3...
Regular expressions are built up from other regular expressions as follows:
r1r2 matches r1 followed immediately by r2 (concatenation).
r1 | r2 matches r1 or r2 (alternation).
r* matches r repeated zero or more times.
r+ matches r repeated one or more times.
r? matches r zero or once.
(r) matches r, providing grouping.
The increasing precedence of operators is alternation, concatenation and unary (*, + or ?).
For example,
/^[_a-zA-Z][_a-zA-Z0-9]*$/ and
/^[-+]?([0-9]+\.?|\.[0-9])[0-9]*([eE][-+]?[0-9]+)?$/
are matched by AWK identifiers and AWK numeric constants respectively. Note that . has to be escaped to
be recognized as a decimal point, and that metacharacters are not special inside character classes.
Any expression can be used on the right hand side of the ~ or !~ operators or passed to a built-in that
expects a regular expression. If needed, it is converted to string, and then interpreted as a regular
expression. For example,
BEGIN { identifier = "[_a-zA-Z][_a-zA-Z0-9]*" }
$0 ~ "^" identifier
prints all lines that start with an AWK identifier.
lmawk recognizes the empty regular expression, //, which matches the empty string and hence is matched by
any string at the front, back and between every character. For example,
echo abc | lmawk { gsub(//, "X") ; print }
XaXbXcX
4.Recordsandfields
Records are read in one at a time, and stored in the field variable $0. The record is split into fields
which are stored in $1, $2, ..., $NF. The built-in variable NF is set to the number of fields, and NR
and FNR are incremented by 1. Fields above $NF are set to "".
Assignment to $0 causes the fields and NF to be recomputed. Assignment to NF or to a field causes $0 to
be reconstructed by concatenating the $i's separated by OFS. Assignment to a field with index greater
than NF, increases NF and causes $0 to be reconstructed.
Data input stored in fields is string, unless the entire field has numeric form and then the type is
number and string. For example,
echo 24 24E |
lmawk '{ print($1>100, $1>"100", $2>100, $2>"100") }'
0 1 1 1
$0 and $2 are string and $1 is number and string. The first comparison is numeric, the second is string,
the third is string (100 is converted to "100"), and the last is string.
5.Expressionsandoperators
The expression syntax is similar to C. Primary expressions are numeric constants, string constants,
variables, fields, arrays and function calls. The identifier for a variable, array or function can be a
sequence of letters, digits and underscores, that does not start with a digit. Variables are not
declared; they exist when first referenced and are initialized to null.
New expressions are composed with the following operators in order of increasing precedence.
assignment = += -= *= /= %= ^=
conditional ? :
logicalor ||
logicaland &&
arraymembershipinmatching ~ !~
relational < > <= >= == !=
concatenation (no explicit operator)
addops + -
mulops * / %
unary + -
logicalnot !
exponentiation ^
incanddec ++ -- (both post and pre)
field $
Assignment, conditional and exponentiation associate right to left; the other operators associate left to
right. Any expression can be parenthesized.
6.Arrays
Awk provides one-dimensional arrays. Array elements are expressed as array[expr]. Expr is internally
converted to string type, so, for example, A[1] and A["1"] are the same element and the actual index is
"1". Arrays indexed by strings are called associative arrays. Initially an array is empty; elements
exist when first accessed. An expression, exprinarray evaluates to 1 if array[expr] exists, else to 0.
There is a form of the for statement that loops over each index of an array.
for ( varinarray ) statement
sets var to each index of array and executes statement. The order that var transverses the indices of
array is not defined.
The statement, deletearray[expr], causes array[expr] not to exist. lmawk supports an extension, deletearray, which deletes all elements of array.
Multidimensional arrays are synthesized with concatenation using the built-in variable SUBSEP.
array[expr1,expr2] is equivalent to array[expr1 SUBSEPexpr2]. Testing for a multidimensional element
uses a parenthesized index, such as
if ( (i, j) in A ) print A[i, j]
7.Builtin-variables
The following variables are built-in and initialized before program execution.
ARGC number of command line arguments.
ARGV array of command line arguments, 0..ARGC-1.
CONVFMT format for internal conversion of numbers to string, initially = "%.6g".
ENVIRON array indexed by environment variables. An environment string, var=value is stored as
ENVIRON[var] = value.
FILENAME name of the current input file.
FNR current record number in FILENAME.
FS splits records into fields as a regular expression.
NF number of fields in the current record.
NR current record number in the total input stream.
OFMT format for printing numbers; initially = "%.6g".
OFS inserted between fields on output, initially = " ".
ORS terminates each record on output, initially = "\n".
RLENGTH length set by the last call to the built-in function, match().
RS input record separator, initially = "\n".
RSTART index set by the last call to match().
SUBSEP used to build multiple array subscripts, initially = "\034".
ERRNO misc built-in functions (libmawk extensions) use this variable to rerport error. All
extension calls will set this variable before returning, therefor ERRNO holds the result
of the last call. An empty string value means no error. Error messages are formatted in
a way that the first word is an unique integer, followed by a human readable error
message from the second word. int(ERRNO) can be used to acquire the error code, which
then can be used as a secondary output from the extension function. For example, an awk
program can use valueof() to determine if a global symbol exists and is a function or a
variable or anything else.
LIBPATH is a semicolon separated list of search paths. When loading an awk script by file name
(-f command line argument or include from another awk script) these paths are inserted
before the file name, in order, one by one, until the first path that allows opening the
file. An empty path is equivalent to the current working directory. LIBPATH can be
modified from the command line using -v, as arguments are scanned before loading the
scripts. Setting LIBPATH to empty string results in the original behaviour of mawk.
LIBPATH is ignored for script file names starting with slash ('/') as those are assumed
to be absolute paths.
8.Built-infunctions
String functions
gsub(r,s,t) gsub(r,s)
Global substitution, every match of regular expression r in variable t is replaced by
string s. The number of replacements is returned. If t is omitted, $0 is used. An & in
the replacement string s is replaced by the matched substring of t. \& and \\ put literal
& and \, respectively, in the replacement string.
index(s,t)
If t is a substring of s, then the position where t starts is returned, else 0 is returned.
The first character of s is in position 1.
length(s)
Returns the length of string s.
match(s,r)
Returns the index of the first longest match of regular expression r in string s. Returns
0 if no match. As a side effect, RSTART is set to the return value. RLENGTH is set to the
length of the match or -1 if no match. If the empty string is matched, RLENGTH is set to
0, and 1 is returned if the match is at the front, and length(s)+1 is returned if the match
is at the back.
split(s,A,r) split(s,A)
String s is split into fields by regular expression r and the fields are loaded into array
A. The number of fields is returned. See section 11 below for more detail. If r is
omitted, FS is used.
sprintf(format,expr-list)
Returns a string constructed from expr-list according to format. See the description of
printf() below.
sub(r,s,t) sub(r,s)
Single substitution, same as gsub() except at most one substitution.
substr(s,i,n) substr(s,i)
Returns the substring of string s, starting at index i, of length n. If n is omitted, the
suffix of s, starting at i is returned.
tolower(s)
Returns a copy of s with all upper case characters converted to lower case.
toupper(s)
Returns a copy of s with all lower case characters converted to upper case.
Arithmetic functions
atan2(y,x) Arctan of y/x between -PI and PI.
cos(x) Cosine function, x in radians.
exp(x) Exponential function.
int(x) Returns x truncated towards zero.
log(x) Natural logarithm.
rand() Returns a random number between zero and one.
sin(x) Sine function, x in radians.
sqrt(x) Returns square root of x.
srand(expr) srand()
Seeds the random number generator, using the clock if expr is omitted, and returns the
value of the previous seed. lmawk seeds the random number generator from the clock at
startup so there is no real need to call srand(). Srand(expr) is useful for repeating
pseudo random sequences.
Misc functions (libmawk extensions)
call(fname,arg1,arg2,...)
Call awk function fname with the supplied arguments. If the call fails, empty value, else
the return value of the callee is returned. Built-in variable ERRNO is always set.
acall(fname,arrname)
Call awk function fname with arguments supplied in array named arrname (both arguments are
strings naming an existing object). The array should be indexed from 1. Number of
arguments is determined by looking for the first empty (non-existing) index in the array.
If the call fails, empty value, else the return value of the callee is returned. Built-in
variable ERRNO is always set.
valueof(vname[,idx])
Return the value of variable fname; if the variable is an array, return the element indexed
by idx (which must be present in this case). If index is not present or is empty (""), the
variable is expected to be scalar. Built-in variable ERRNO is always set. NOTE: valueof()
has access to the global symbol table only. It will fail to resolve anything else than
global objects; most notably it will fail on local variables, $ arguments and on most of
the built-in variables.
9.Inputandoutput
There are two output statements, print and printf.
print writes $0ORS to standard output.
print expr1, expr2, ..., exprn
writes expr1 OFSexpr2 OFS ... exprn ORS to standard output. Numeric expressions are
converted to string with OFMT.
printf format,expr-list
duplicates the printf C library function writing to standard output. The complete ANSI C
format specifications are recognized with conversions %c, %d, %e, %E, %f, %g, %G, %i, %o,
%s, %u, %x, %X and %%, and conversion qualifiers h and l.
The argument list to print or printf can optionally be enclosed in parentheses. Print formats numbers
using OFMT or "%d" for exact integers. "%c" with a numeric argument prints the corresponding 8 bit
character, with a string argument it prints the first character of the string. The output of print and
printf can be redirected to a file or command by appending > file, >> file or | command to the end of the
print statement. Redirection opens file or command only once, subsequent redirections append to the
already open stream. By convention, lmawk associates the filename "/dev/stderr" with stderr which allows
print and printf to be redirected to stderr. lmawk also associates "-" and "/dev/stdout" with stdin and
stdout which allows these streams to be passed to functions. Opening /dev/fd/N will do an fdopen() on
file descriptor N, where N is an integer - this is a libmawk extension. If any of the /dev heuristics
needs to be bypassed (i.e. the script wants to open the real /dev/stdout or the real /dev/fd/5), the
leading slash should be doubled (e.g. //dev/fd/5).
The input function getline has the following variations.
getline
reads into $0, updates the fields, NF, NR and FNR.
getline < file
reads into $0 from file, updates the fields and NF.
getline var
reads the next record into var, updates NR and FNR.
getline var < file
reads the next record of file into var.
command | getline
pipes a record from command into $0 and updates the fields and NF.
command | getline var
pipes a record from command into var.
Getline returns 0 on end-of-file, -1 on error, otherwise 1.
Commands on the end of pipes are executed by /bin/sh.
The function close(expr) closes the file or pipe associated with expr. Close returns 0 if expr is an
open file, the exit status if expr is a piped command, and -1 otherwise. Close is used to reread a file
or command, make sure the other end of an output pipe is finished or conserve file resources.
The function fflush(expr) flushes the output file or pipe associated with expr. Fflush returns 0 if expr
is an open output stream else -1. Fflush without an argument flushes stdout. Fflush with an empty
argument ("") flushes all open output.
The function system(expr) uses /bin/sh to execute expr and returns the exit status of the command expr.
Changes made to the ENVIRON array are not passed to commands executed with system or pipes.
10.Userdefinedfunctions
The syntax for a user defined function is
function name( args ) { statements }
The function body can contain a return statement
returnopt_expr
A return statement is not required. Function calls may be nested or recursive. Functions are passed
expressions by value and arrays by reference. Extra arguments serve as local variables and are
initialized to null. For example, csplit(s,A) puts each character of s into array A and returns the
length of s.
function csplit(s, A, n, i)
{
n = length(s)
for( i = 1 ; i <= n ; i++ ) A[i] = substr(s, i, 1)
return n
}
Putting extra space between passed arguments and local variables is conventional. Functions can be
referenced before they are defined, but the function name and the '(' of the arguments must touch to
avoid confusion with concatenation.
11.Splittingstrings,recordsandfiles
Awk programs use the same algorithm to split strings into arrays with split(), and records into fields on
FS. lmawk uses essentially the same algorithm to split files into records on RS.
Split(expr,A,sep) works as follows:
(1) If sep is omitted, it is replaced by FS. Sep can be an expression or regular expression.
If it is an expression of non-string type, it is converted to string.
(2) If sep = " " (a single space), then <SPACE> is trimmed from the front and back of expr, and
sep becomes <SPACE>. lmawk defines <SPACE> as the regular expression /[ \t\n]+/.
Otherwise sep is treated as a regular expression, except that meta-characters are ignored
for a string of length 1, e.g., split(x, A, "*") and split(x, A, /\*/) are the same.
(3) If expr is not string, it is converted to string. If expr is then the empty string "",
split() returns 0 and A is set empty. Otherwise, all non-overlapping, non-null and longest
matches of sep in expr, separate expr into fields which are loaded into A. The fields are
placed in A[1], A[2], ..., A[n] and split() returns n, the number of fields which is the
number of matches plus one. Data placed in A that looks numeric is typed number and
string.
Splitting records into fields works the same except the pieces are loaded into $1, $2,..., $NF. If $0 is
empty, NF is set to 0 and all $i to "".
lmawk splits files into records by the same algorithm, but with the slight difference that RS is really a
terminator instead of a separator. (ORS is really a terminator too).
E.g., if FS = ":+" and $0 = "a::b:" , then NF = 3 and $1 = "a", $2 = "b" and $3 = "", but if
"a::b:" is the contents of an input file and RS = ":+", then there are two records "a" and "b".
RS = " " is not special.
If FS = "", then lmawk breaks the record into individual characters, and, similarly, split(s,A,"") places
the individual characters of s into A.
12.Multi-linerecords
Since lmawk interprets RS as a regular expression, multi-line records are easy. Setting RS = "\n\n+",
makes one or more blank lines separate records. If FS = " " (the default), then single newlines, by the
rules for <SPACE> above, become space and single newlines are field separators.
For example, if a file is "a b\nc\n\n", RS = "\n\n+" and FS = " ", then there is one record
"a b\nc" with three fields "a", "b" and "c". Changing FS = "\n", gives two fields "a b" and "c";
changing FS = "", gives one field identical to the record.
If you want lines with spaces or tabs to be considered blank, set RS = "\n([ \t]*\n)+". For
compatibility with other awks, setting RS = "" has the same effect as if blank lines are stripped from
the front and back of files and then records are determined as if RS = "\n\n+". Posix requires that "\n"
always separates records when RS = "" regardless of the value of FS. lmawk does not support this
convention, because defining "\n" as <SPACE> makes it unnecessary.
Most of the time when you change RS for multi-line records, you will also want to change ORS to "\n\n" so
the record spacing is preserved on output.
13.Programexecution
This section describes the order of program execution. First ARGC is set to the total number of command
line arguments passed to the execution phase of the program. ARGV[0] is set the name of the AWK
interpreter and ARGV[1] ... ARGV[ARGC-1] holds the remaining command line arguments exclusive of options
and program source. For example with
lmawk -f prog v=1 A t=hello B
ARGC = 5 with ARGV[0] = "lmawk", ARGV[1] = "v=1", ARGV[2] = "A", ARGV[3] = "t=hello" and ARGV[4] = "B".
Next, each BEGIN block is executed in order. If the program consists entirely of BEGIN blocks, then
execution terminates, else an input stream is opened and execution continues. If ARGC equals 1, the
input stream is set to stdin, else the command line arguments ARGV[1] ... ARGV[ARGC-1] are examined for
a file argument.
The command line arguments divide into three sets: file arguments, assignment arguments and empty strings
"". An assignment has the form var=string. When an ARGV[i] is examined as a possible file argument, if
it is empty it is skipped; if it is an assignment argument, the assignment to var takes place and i skips
to the next argument; else ARGV[i] is opened for input. If it fails to open, execution terminates with
exit code 2. If no command line argument is a file argument, then input comes from stdin. Getline in a
BEGIN action opens input. "-" as a file argument denotes stdin.
Once an input stream is open, each input record is tested against each pattern, and if it matches, the
associated action is executed. An expression pattern matches if it is boolean true (see the end of
section 2). A BEGIN pattern matches before any input has been read, and an END pattern matches after all
input has been read. A range pattern, expr1,expr2 , matches every record between the match of expr1 and
the match expr2 inclusively.
When end of file occurs on the input stream, the remaining command line arguments are examined for a file
argument, and if there is one it is opened, else the ENDpattern is considered matched and all ENDactions are executed.
In the example, the assignment v=1 takes place after the BEGINactions are executed, and the data placed
in v is typed number and string. Input is then read from file A. On end of file A, t is set to the
string "hello", and B is opened for input. On end of file B, the ENDactions are executed.
Program flow at the pattern{action} level can be changed with the
nextexitopt_expr
statements. A next statement causes the next input record to be read and pattern testing to restart with
the first pattern{action} pair in the program. An exit statement causes immediate execution of the END
actions or program termination if there are none or if the exit occurs in an END action. The opt_expr
sets the exit value of the program unless overridden by a later exit or subsequent error.
14.include
libmawk introduces source inclusion feature. Syntax is:
include "filename"
Include statements must be on top level (outside of blocks). If file name
starts with a plus sign ('+'), the script file is not loaded if it has
been already loaded (by another include or -f command line argument).
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