mawk - pattern scanning and text processing language
SYNOPSIS
mawk [-W option] [-F value] [-v var=value] [--] 'program text' [file ...]
mawk [-W option] [-F value] [-v var=value] [-f program-file] [--] [file ...]
DESCRIPTION
mawk is an interpreter for the AWK Programming Language. The AWK lan-
guage is useful for manipulation of data files, text retrieval and pro-
cessing, 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 con-
forms 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 synony-
mous 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 com-
mand 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 implemen-
tation of AWK, and implementation specific options are prefaced with
-W. mawk provides six:
-W version mawk writes its version and copyright to stdout and com-
piled limits to stderr and exits 0.
-W dump writes an assembler like listing of the internal repre-
sentation of the program to stdout and exits 0 (on suc-
cessful 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.
THE AWK LANGUAGE
1. Program structure
An AWK program is a sequence of pattern {action} pairs and user func-
tion 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 state-
ment 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 state-
ments
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 neces-
sary 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 fol-
lows:
r1r2 matches r1 followed immediately by r2 (concatena-
tion).
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 !~ opera-
tors 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 iinn
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 paren-
thesized.
...
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 out-
put. Numeric expressions are converted to string with
OFMT.
printf format, expr-list
duplicates the printf C library function writing to stan-
dard 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 quali-
fiers 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 inte-
gers. "%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, subse-
quent 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 con-
serve 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 argu-
ment ("") 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.
...
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 execu-
tion phase of the program. ARGV[0] is set the name of the AWK inter-
preter 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 ii 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 sec-
tion 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.
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