MAWK(1)
NAME
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 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 inter-
pretation. 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 deter-
mined 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 compat-
ible 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 compila-
tion).
-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 recom-
piled.
-W posix_space forces mawk not to consider '\n' to be
space.
The short forms -W[vdiesp] are recognized and on some sys-
tems -We is mandatory to avoid command line length limita-
tions.
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 parenthe-
sis 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 computa-
tions are done in floating point arithmetic. So for exam-
ple, the expression 0.2e2 == 20 is true and true is repre-
sented 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 num-
ber 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(CON-
VFMT, 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 num-
ber 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 expres-
sions 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 evalu-
ates 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 argu-
ment to a built-in function that expects a regular expres-
sion 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 fol-
lows:
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 new-
line).
^ 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 recog-
nized 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 exam-
ple,
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 incre-
mented by 1. Fields above $NF are set to "".
Assignment to $0 causes the fields and NF to be recom-
puted. 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 expres-
sions 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 refer-
enced 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 expres-
sion, 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.
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, ini-
tially = " ".
ORS terminates each record on output, ini-
tially = "\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".
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 - and .
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 repeat-
ing 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 conver-
sion 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 associ-
ated with expr. Close returns 0 if expr is an open file,
the exit status if expr is a piped command, and -1 other-
wise. 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.
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 con-
catenation.
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 expres-
sion. 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-charac-
ters 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 indi-
vidual 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 separa-
tors.
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 argu-
ments exclusive of options and program source. For exam-
ple 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 pro-
gram 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, execu-
tion 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 pat-
tern 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 pro-
gram unless overridden by a later exit or subsequent
error.
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
}
COMPATIBILITY ISSUES
The Posix 1003.2(draft 11.3) definition of the AWK lan-
guage 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 char-
acter, 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 expres-
sion. 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 pro-
grams 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 advo-
cates \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 sub-
str("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 con-
dition, index(s,t) != 0 if and only t is a sub-
string 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.
SEE ALSO
egrep(1)
Aho, Kernighan and Weinberger, The AWK Programming Lan-
guage, Addison-Wesley Publishing, 1988, (the AWK book),
defines the language, opening with a tutorial and advanc-
ing 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 excel-
lent. It also discusses Posix requirements for AWK.
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.
AUTHOR
Mike Brennan brennan@boeing.com.