mawk
pattern scanning and text processing language
see also :
egrep
Synopsis
mawk
[-W option] [-F
value] [-v var=value]
[--] ’program text’ [file ...]
mawk [-W option] [-F
value] [-v var=value]
[-f program-file] [--] [file
...]
add an example, a script, a trick and tips
examples
1. emulate cat.
{ print }
2. emulate wc.
{ chars += length($0) + 1 # add one for the \n
words += NF
}
END{ print NR, words, chars }
3. count the number of unique "real words".
BEGIN { FS = "[^A-Za-z]+" }
{ for(i = 1 ; i <= NF ; i++) word[$i] = "" }
END { delete word[""]
for ( i in word ) cnt++
print cnt
}
4. sum the second field of every record based on the first field.
$1 ~ /credit|gain/ { sum += $2 }
$1 ~ /debit|loss/ { sum -= $2 }
END { print sum }
5. sort a file, comparing as string
{ line[NR] = $0 "" } # make sure of comparison type
# in case some lines look numeric
END { isort(line, NR)
for(i = 1 ; i <= NR ; i++) print line[i]
}
#insertion sort of A[1..n]
function isort( A, n,
i, j, hold)
{
for( i = 2 ; i <= n ; i++)
{
hold = A[j = i]
while ( A[j-1] > hold )
{ j-- ; A[j+1] = A[j] }
A[j] = hold
}
# sentinel A[0] = "" will be created if needed
}
description
mawk is
an interpreter for the AWK Programming Language. The AWK
language is useful for manipulation of data files, text
retrieval and processing, and for prototyping and
experimenting with algorithms. mawk is a new
awk meaning it implements the AWK language as defined in
Aho, Kernighan and Weinberger, The AWK Programming
Language, Addison-Wesley Publishing, 1988. (Hereafter
referred to as the AWK book.) mawk conforms to the
Posix 1003.2 (draft 11.3) definition of the AWK language
which contains a few features not described in the AWK book,
and mawk provides a small number of extensions.
An AWK program
is a sequence of pattern {action} pairs and function
definitions. Short programs are entered on the command line
usually enclosed in ’ ’ to avoid shell
interpretation. Longer programs can be read in from a file
with the -f option. Data input is read from the list
of files on the command line or from standard input when the
list is empty. The input is broken into records as
determined by the record separator variable, RS.
Initially, RS = "\n" and records are
synonymous with lines. Each record is compared against each
pattern and if it matches, the program text for
{action} is executed.
options
-F value
sets the field separator,
FS, to value.
-f file
Program text is read from file instead of from
the command line. Multiple -f options are
allowed.
-v var=value
assigns value to program variable var.
--
indicates the unambiguous end of options.
The above
options will be available with any Posix compatible
implementation of AWK, and implementation specific options
are prefaced with -W. mawk provides
six:
-W version
mawk writes its version
and copyright to stdout and compiled limits to stderr and
exits 0.
-W dump
writes an assembler like listing of the internal
representation of the program to stdout and exits 0 (on
successful compilation).
-W interactive
sets unbuffered writes to stdout and line buffered reads
from stdin. Records from stdin are lines regardless of the
value of RS.
-W exec file
Program text is read from file and this is the
last option. Useful on systems that support the #!
"magic number" convention for executable
scripts.
-W sprintf=num
adjusts the size of mawk’s internal sprintf
buffer to num bytes. More than rare use of this
option indicates mawk should be recompiled.
-W posix_space
forces mawk not to consider ’\n’ to
be space.
The short forms
-W[vdiesp] are recognized and on some systems
-We is mandatory to avoid command line length
limitations.
argc
number of command line arguments.
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8. Built-in functions
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.
mawk 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.
9. Input and output
There are two output statements, print and printf.
print
writes $0 ORS to standard output.
print expr1, expr2, ..., exprn
writes expr1 OFS expr2 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, mawk associates the
filename "/dev/stderr" with stderr which allows print and printf
to be redirected to stderr. mawk also associates "-" and
"/dev/stdout" with stdin and stdout which allows these streams to
be passed to functions.
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. User defined functions
The syntax for a user defined function is
function name( args ) { statements }
The function body can contain a return statement
return opt_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. Splitting strings, records and files
Awk programs use the same algorithm to split strings into arrays
with split(), and records into fields on FS. mawk
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>. mawk 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
"".
mawk 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 mawk breaks the record into
individual characters, and, similarly, split(s,A,"")
places the individual characters of s into A.
12. Multi-line records
Since mawk 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. mawk 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. Program execution
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
mawk -f prog v=1 A t=hello B
ARGC = 5 with ARGV[0] = "mawk", 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 END pattern is
considered matched and all END actions are
executed.
In the example, the assignment v=1 takes place after the
BEGIN actions 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 END actions are
executed.
Program flow at the pattern {action} level can be changed
with the
next
exit opt_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.
compatibility issues
The Posix 1003.2(draft 11.3) definition of the AWK language is
AWK as described in the AWK book with a few extensions that
appeared in SystemVR4 nawk. The extensions are:
New functions: toupper() and tolower().
New variables: ENVIRON[] and CONVFMT.
ANSI C conversion specifications for printf() and sprintf().
New command options: -v var=value, multiple -f options and
implementation options as arguments to -W.
Posix AWK is oriented to operate on files a line at a time.
RS can be changed from "\n" to another single character,
but it is hard to find any use for this — there are no examples
in the AWK book. By convention, RS = "", makes one or more
blank lines separate records, allowing multi-line records. When
RS = "", "\n" is always a field separator regardless of
the value in FS.
mawk, on the other hand, allows RS to be a regular
expression. When "\n" appears in records, it is treated as space,
and FS always determines fields.
Removing the line at a time paradigm can make some programs
simpler and can often improve performance. For example, redoing
example 3 from above,
BEGIN { RS = "[^A-Za-z]+" }
{ word[ $0 ] = "" }
END { delete word[ "" ]
for( i in word ) cnt++
print cnt
}
counts the number of unique words by making each word a record.
On moderate size files, mawk executes twice as fast,
because of the simplified inner loop.
The following program replaces each comment by a single space in
a C program file,
BEGIN {
RS = "/\*([^*]|\*+[^/*])*\*+/"
# comment is record separator
ORS = " "
getline hold
}
{ print hold ; hold = $0 }
END { printf "%s" , hold }
Buffering one record is needed to avoid terminating the last
record with a space.
With mawk, the following are all equivalent,
x ~ /a\+b/ x ~ "a\+b" x ~ "a\\+b"
The strings get scanned twice, once as string and once as regular
expression. On the string scan, mawk ignores the escape on
non-escape characters while the AWK book advocates \c be
recognized as c which necessitates the double escaping of
meta-characters in strings. Posix explicitly declines to define
the behavior which passively forces programs that must run under
a variety of awks to use the more portable but less readable,
double escape.
Posix AWK does not recognize "/dev/std{out,err}" or \x hex escape
sequences in strings. Unlike ANSI C, mawk limits the
number of digits that follows \x to two as the current
implementation only supports 8 bit characters. The built-in
fflush first appeared in a recent (1993) AT&T awk
released to netlib, and is not part of the posix standard.
Aggregate deletion with delete array is not part of
the posix standard.
Posix explicitly leaves the behavior of FS = "" undefined,
and mentions splitting the record into characters as a possible
interpretation, but currently this use is not portable across
implementations.
Finally, here is how mawk handles exceptional cases not
discussed in the AWK book or the Posix draft. It is unsafe to
assume consistency across awks and safe to skip to the next
section.
substr(s, i, n) returns the characters of s in the intersection
of the closed interval [1, length(s)] and the half-open interval
[i, i+n). When this intersection is empty, the empty string is
returned; so substr("ABC", 1, 0) = "" and substr("ABC", -4, 6) =
"A".
Every string, including the empty string, matches the empty
string at the front so, s ~ // and s ~ "", are always 1 as is
match(s, //) and match(s, ""). The last two set RLENGTH to
0.
index(s, t) is always the same as match(s, t1) where t1 is the
same as t with metacharacters escaped. Hence consistency with
match requires that index(s, "") always returns 1. Also the
condition, index(s,t) != 0 if and only t is a substring of s,
requires index("","") = 1.
If getline encounters end of file, getline var, leaves var
unchanged. Similarly, on entry to the END actions,
$0, the fields and NF have their value unaltered
from the last record.
the awk language
1. Program structure
An AWK program is a sequence of pattern {action} pairs and
user function definitions.
A pattern can be:
BEGIN
END
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 ) statement
if ( expr ) statement else
statement
while ( expr ) statement
do statement while ( expr )
for ( opt_expr ; opt_expr ; opt_expr
) statement
for ( var in array ) statement
continue
break
2. Data types, conversion and comparison
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.,
mawk ignores the escape.
There are really three basic data types; the third is number
and string 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-op expr2, 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. Regular expressions
In the AWK language, records, fields and strings are often tested
for matching a regular expression. 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.
mawk 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 | mawk { gsub(//, "X") ; print }
XaXbXcX
4. Records and fields
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 |
mawk ’{ 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. Expressions and operators
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
? :
logical or
||
logical and
&&
array membership
in
matching
~ !~
relational
< > <= >= == !=
concatenation
(no explicit operator)
add ops
+ -
mul ops
* / %
unary
+ -
logical not
!
exponentiation
^
inc and dec
++ -- (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, expr
in array 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 ( var in array ) 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, delete array[expr], causes
array[expr] not to exist. mawk supports an
extension, delete array, 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 SUBSEP expr2]. 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.
bugs
mawk
cannot handle ascii NUL \0 in the source or data files. You
can output NUL using printf with %c, and any other 8 bit
character is acceptable input.
mawk
implements printf() and sprintf() using the C library
functions, printf and sprintf, so full ANSI compatibility
requires an ANSI C library. In practice this means the h
conversion qualifier may not be available. Also mawk
inherits any bugs or limitations of the library
functions.
Implementors of
the AWK language have shown a consistent lack of imagination
when naming their programs.
see also
egrep
Aho, Kernighan
and Weinberger, The AWK Programming Language,
Addison-Wesley Publishing, 1988, (the AWK book), defines the
language, opening with a tutorial and advancing to many
interesting programs that delve into issues of software
design and analysis relevant to programming in any
language.
The GAWK
Manual, The Free Software Foundation, 1991, is a
tutorial and language reference that does not attempt the
depth of the AWK book and assumes the reader may be a novice
programmer. The section on AWK arrays is excellent. It also
discusses Posix requirements for AWK.
author
Mike Brennan
(brennan[:at:]whidbey[:dot:]com).