MAWK(1) USER COMMANDS 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 exten-
sions.
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 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 compati-
ble 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.
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-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 sup-
port 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 recoginized 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 state-
ments 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
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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.
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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, expr918 rel-op expr928, 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"
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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 argu-
ment.
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.
[c918c928c938...] matches any character in the class
c918c928c938... . An interval of characters is
denoted c918-c928 inside a class [...].
[^c918c928c938...] matches any character not in the class
c918c928c938...
Regular expressions are built up from other regular expres-
sions as follows:
r918r928 matches r918 followed immediately by r92
8 (concatenation).
r918 | r928 matches r918 or r928 (alternation).
r* matches r repeated zero or more times.
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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, con-
catenation 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 regu-
lar 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 exam-
ple,
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.
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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 expres-
sion, expr in array evaluates to 1 if array[expr] exists,
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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[expr918,expr928] is
equivalent to array[expr918 SUBSEP expr928]. Testing for a mul-
tidimensional 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".
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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, ini-
tially = "\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 expres-
sion 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.
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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 -J and J.
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 gen-
erator 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
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writes $0 ORS to standard output.
print expr918, expr928, ..., expr9n
8 writes expr918 OFS expr928 OFS ... expr9n8 ORS to stan-
dard 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.
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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.
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 vari-
ables 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
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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 is
trimmed from the front and back of expr, and sep
becomes . mawk defines as the reg-
ular 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 indivi-
dual 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
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blank lines separate records. If FS = " " (the default),
then single newlines, by the rules for 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 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 argu-
ments 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 ter-
minates, else an input stream is opened and execution con-
tinues. 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
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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 con-
sidered 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.
EXAMPLES
1. emulate cat.
{ print }
2. emulate wc.
{ chars += length($0) + 1 # add one for the \n
words += NF
}
END{ print NR, words, chars }
3. count the number of unique "real words".
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BEGIN { FS = "[^A-Za-z]+" }
{ for(i = 1 ; i <= NF ; i++) word[$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 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().
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MAWK(1) USER COMMANDS MAWK(1)
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 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 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,
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MAWK(1) USER COMMANDS MAWK(1)
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 stan-
dard.
Posix explicitly leaves the behavior of FS = "" undefined,
and mentions splitting the record into characters as a pos-
sible interpretation, but currently this use is not portable
across implementations.
Finally, here is how mawk handles exceptional cases not dis-
cussed 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 intersec-
tion 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
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MAWK(1) USER COMMANDS MAWK(1)
the last record.
SEE ALSO
egrep(1)
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.
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 inher-
its 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).
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