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 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 de-
finitions. Short programs are entered on the command line usually en-
closed 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. Implementation specific options are prefaced with -W.
mawk provides these:
-W dump
writes an assembler like listing of the internal representation
of the program to stdout and exits 0 (on successful compila-
tion).
-W exec file
Program text is read from file and this is the last option.
This is a useful alternative to -f on systems that support the
#! “magic number” convention for executable scripts. Those im-
plicitly pass the pathname of the script itself as the final pa-
rameter, and expect no more than one “-” option on the #! line.
Because mawk can combine multiple -W options separated by com-
mas, you can use this option when an additional -W option is
needed.
-W help
prints a usage message to stderr and exits (same as “-W usage”).
-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 posix
modifies mawk's behavior to be more POSIX-compliant:
• forces mawk not to consider '\n' to be space.
The original “posix_space” is recognized, but deprecated.
-W random=num
calls srand with the given parameter (and overrides the auto-
seeding behavior).
-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 re-
compiled.
-W traditional
Omit features such as interval expressions which were not sup-
ported by traditional awk.
-W usage
prints a usage message to stderr and exits (same as “-W help”).
-W version
mawk writes its version and copyright to stdout and compiled
limits to stderr and exits 0.
mawk accepts abbreviations for any of these options, e.g., “-W v” and
“-Wv” both tell mawk to show its version.
mawk allows multiple -W options to be combined by separating the op-
tions with commas, e.g., -Wsprint=2000,posix. This is useful for exe-
cutable #! “magic number” invocations in which only one argument is
supported, e.g., -Winteractive,exec.
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 nu-
meric, 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 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 nu-
meric, 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 exe-
cuted. 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 ex-
pression argument.
AWK uses extended regular expressions as with the -E option of grep(1).
The regular expression metacharacters, i.e., those with special meaning
in regular expressions are
\ ^ $ . [ ] | ( ) * + ? { }
If the command line option -W traditional is used, these are omitted:
{ }
are also regular expression metacharacters, and in this mode, require
escaping to be a literal character.
Regular expressions are built up from characters as follows:
c matches any non-metacharacter c.
\c matches a character defined by the same escape se-
quences used in string constants or the literal char-
acter 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. (repetition).
(r) matches r (grouping).
r{n} matches r exactly n times.
r{n,} matches r repeated n or more times.
r{n,m} matches r repeated n to m (inclusive) times.
r{,m} matches r repeated 0 to m times (a non-standard op-
tion).
The increasing precedence of operators is:
alternation concatenation repetition grouping
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 nu-
meric 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 se-
quence 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 paren-
thesized.
6. Arrays
Awk provides one-dimensional arrays. Array elements are expressed as
array[expr]. Expr is internally converted to string type, so, for ex-
ample, A[1] and A["1"] are the same element and the actual index is
"1". Arrays indexed by strings are called associative arrays. Ini-
tially an array is empty; elements exist when first accessed. An ex-
pression, 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 the delete array feature, which deletes all elements of
array.
Multidimensional arrays are synthesized with concatenation using the
built-in variable SUBSEP. array[expr1,expr2] is equivalent to ar-
ray[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 ex-
ecution.
ARGC number of command line arguments.
ARGV array of command line arguments, 0..ARGC-1.
CONVFMT
format for internal conversion of numbers to string, ini-
tially = "%.6g".
ENVIRON
array indexed by environment variables. An environment
string, var=value is stored as ENVIRON[var] = value.
FILENAME
name of the current input file.
FNR current record number in FILENAME.
FS splits records into fields as a regular expression.
NF number of fields in the current record.
NR current record number in the total input stream.
OFMT format for printing numbers; initially = "%.6g".
OFS inserted between fields on output, initially = " ".
ORS terminates each record on output, initially = "\n".
RLENGTH
length set by the last call to the built-in function,
match().
RS input record separator, initially = "\n".
RSTART index set by the last call to match().
SUBSEP used to build multiple array subscripts, initially =
"\034".
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 replace-
ments is returned. If t is omitted, $0 is used. An & in
the replacement string s is replaced by the matched sub-
string 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 or array s.
match(s,r)
Returns the index of the first longest match of regular ex-
pression 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 sub-
stitution.
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 con-
verted to lower case.
toupper(s)
Returns a copy of s with all lower case characters con-
verted to upper case.
Time functions
These are available on systems which support the corresponding C mktime
and strftime functions:
mktime(specification)
converts a date specification to a timestamp with the same
units as systime. The date specification is a string con-
taining the components of the date as decimal integers:
YYYY
the year, e.g., 2012
MM the month of the year starting at 1
DD the day of the month starting at 1
HH hour (0-23)
MM minute (0-59)
SS seconds (0-59)
DST
tells how to treat timezone versus daylight savings
time:
positive
DST is in effect
zero (default)
DST is not in effect
negative
mktime() should (use timezone information and sys-
tem databases to) attempt to determine whether DST
is in effect at the specified time.
strftime([format [, timestamp [, utc ]]])
formats the given timestamp using the format (passed to the
C strftime function):
• If the format parameter is missing, "%c" is used.
• If the timestamp parameter is missing, the current
value from systime is used.
• If the utc parameter is present and nonzero, the result
is in UTC. Otherwise local time is used.
systime()
returns the current time of day as the number of seconds
since the Epoch (1970-01-01 00:00:00 UTC on POSIX systems).
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.
Srand(expr) is useful for repeating pseudo random se-
quences.
Note: mawk is normally configured to seed the random number
generator from the clock at startup, making it unnecessary
to call srand(). This feature can be suppressed via condi-
tional compile, or overridden using the -Wrandom option.
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 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 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,
• "/dev/stdout" with stdout,
• "-" and "/dev/stdin" with stdin.
The association with stderr is especially useful because it allows
print and printf to be redirected to stderr. These names can also 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 the C runtime system call to execute
expr and returns the corresponding wait status of the command as fol-
lows:
• if the system call failed, setting the status to -1, mawk returns
that value.
• if the command exited normally, mawk returns its exit-status.
• if the command exited due to a signal such as SIGHUP, mawk returns
the signal number plus 256.
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 initial-
ized 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 con-
fusion with concatenation.
A function parameter is normally a scalar value (number or string). If
there is a forward reference to a function using an array as a parame-
ter, the function's corresponding parameter will be treated as an ar-
ray.
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 expres-
sion 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() re-
turns 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 separa-
tor. (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 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”:
• using FS = “\n”, gives two fields “a b” and “c”;
• using 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 = "" regard-
less of the value of FS. mawk does not support this convention, be-
cause 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 execu-
tion phase of the program.
• ARGV[0] is set to 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 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, as-
signment 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 as-
signment 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 ex-
pression 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. In-
put 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
nextfile
exit opt_expr
statements:
• A next statement causes the next input record to be read and pat-
tern testing to restart with the first pattern {action} pair in the
program.
• A nextfile statement tells mawk to stop processing the current in-
put file. It then updates FILENAME to the next file listed on the
command line, and resets FNR to 1.
• 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.
ENVIRONMENT
Mawk recognizes these variables:
MAWKBINMODE
(see COMPATIBILITY)
MAWK_LONG_OPTIONS
If this is set, mawk uses its value to decide what to do with
GNU-style long options:
allow Mawk allows the option to be checked against the (small)
set of long options it recognizes.
The long names from the -W option are recognized, e.g.,
--version is derived from -Wversion.
error Mawk prints an error message and exits. This is the de-
fault.
ignore Mawk ignores the option, unless it happens to be one of
the one it recognizes.
warn Print an warning message and otherwise ignore the op-
tion.
If the variable is unset, mawk prints an error message and exits.
WHINY_USERS
This is a gawk 3.1.0 feature, removed in the 4.0.0 release. It
tells mawk to sort array indices before it starts to iterate over
the elements of an array.
COMPATIBILITY
MAWK 1.3.3 versus POSIX 1003.2 Draft 11.3
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 Sys-
temVR4 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 im-
plementation options as arguments to -W.
• For systems (MS-DOS or Windows) which provide a setmode func-
tion, an environment variable MAWKBINMODE and a built-in vari-
able BINMODE. The bits of the BINMODE value tell mawk how to
modify the RS and ORS variables:
0 set standard input to binary mode, and if BIT-2 is unset, set
RS to "\r\n" (CR/LF) rather than "\n" (LF).
1 set standard output to binary mode, and if BIT-2 is unset,
set ORS to "\r\n" (CR/LF) rather than "\n" (LF).
2 suppress the assignment to RS and ORS of CR/LF, making it
possible to run scripts and generate output compatible with
Unix line-endings.
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 conven-
tion, RS = "", makes one or more blank lines separate records, allowing
multi-line records. When RS = "", "\n" is always a field separator re-
gardless 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 deter-
mines 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 sim-
plified 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 ex-
pression. On the string scan, mawk ignores the escape on non-escape
characters while the AWK book advocates \c be recognized as c which ne-
cessitates the double escaping of meta-characters in strings. POSIX
explicitly declines to define the behavior which passively forces pro-
grams 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{in,out,err}". Some systems pro-
vide an actual device for this, allowing AWKs which do not implement
the feature directly to support it.
POSIX AWK does not recognize \x hex escape sequences in strings. Un-
like ANSI C, mawk limits the number of digits that follows \x to two as
the current implementation only supports 8 bit characters.
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.
Some features were not part of the POSIX standard until long after
their introduction in mawk and other implementations. These have been
approved, though still (as of July 2020), are not part of a published
standard:
• The built-in fflush first appeared in a 1993 AT&T awk released to
netlib. It was approved for the POSIX standard in 2012.
• Aggregate deletion with delete array was approved in 2018.
Random numbers
POSIX does not prescribe a method for initializing random numbers at
startup.
In practice, most implementations do nothing special, which makes srand
and rand follow the C runtime library, making the initial seed value 1.
Some implementations (Solaris XPG4 and Tru64) return 0 from the first
call to srand, although the results from rand behave as if the initial
seed is 1. Other implementations return 1.
While mawk can call srand at startup with no parameter (initializing
random numbers from the clock), this feature may be suppressed using
conditional compilation.
Extensions added for compatibility for GAWK and BWK
Nextfile is a gawk extension (also implemented by BWK awk). It was ap-
proved for the POSIX standard in September 2012, and is expected to be
part of the next revision of the standard.
Mktime, strftime and systime are gawk extensions.
The "/dev/stdin" feature was added to mawk after 1.3.4, for compatibil-
ity with gawk and BWK awk. The corresponding "-" (alias for
/dev/stdin) was present in mawk 1.3.3.
Interval expressions, e.g., a range {m,n} in Extended Regular Expres-
sions (EREs), were not supported in awk (or even the original “nawk”):
• Gawk provided this feature in 1991 (and later, in 1998, options for
turning it off, for compatibility with “traditional awk”).
• Interval expressions, were introduced into awk regular expressions
in IEEE 1003.1-2001 (also known as Unix 03), along with some inter-
nationalization features.
• Apple modified its copy of the original awk in April 2006, making
this version of awk support interval expressions.
The updated source provides for compatibility with older “legacy”
versions using an environment variable, making this “Unix 2003”
feature (perhaps meant as Unix 03) the default.
• NetBSD developers copied this change in January 2018, omitting the
compatibility option, and then applied it to BWK awk.
• The interval expression implementation in mawk is based on changes
proposed by James Parkinson in April 2016.
Mawk also recognizes a few gawk-specific command line options for
script compatibility:
--help, --posix, -r, --re-interval, --traditional, --version
Subtle Differences not in POSIX or the AWK Book
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 un-
changed. Similarly, on entry to the END actions, $0, the fields
and NF have their value unaltered from the last record.
BUGS
mawk implements printf() and sprintf() using the C library functions,
printf and sprintf, so full ANSI compatibility requires an ANSI C li-
brary. 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 imagi-
nation when naming their programs.
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
}
AUTHORS
Mike Brennan (brennan@whidbey.com).
Thomas E. Dickey <dickey@invisible-island.net>.
SEE ALSO
grep(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 ar-
rays is excellent. It also discusses POSIX requirements for AWK.
mawk-arrays(7) discusses mawk's implementation of arrays.
mawk-code(7) gives more information on the -W dump option.
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