This turns on compatibility
mode, with the following additional restrictions: \escape sequences are
not recognized. FS is set to a single space, newline does not. ? and :. func
for the keyword function is not recognized. ** and **= cannot be used in
place of ^ and ^=. fflush() function is not available.
- -W profile[=prof_file]
- --profile[=prof_file]
- Send profiling data to prof_file. The default is awkprof.out.
When run with gawk, the profile is just a ‘pretty printed’ version of the
program. When run with pgawk, the profile contains execution counts of each
statement in the program in the left margin and function call counts for
each user-defined function.
- -W re-interval
- --re-interval
- Enable the use of interval
expressions in regular expression matching (see Regular Expressions, below).
Interval expressions were not traditionally available in the language.
The standard added them, to make awk and egrep consistent with each other.
However, their use is likely to break old programs, so gawk only provides
them if they are requested with this option, or when --posix is specified.
- -W source program-text
- --source program-text
- Use program-text as program source
code. This option allows the easy intermixing of library functions (used
via the -f and --file options) with source code entered on the command line.
It is intended primarily for medium to large programs used in shell scripts.
- -W use-lc-numeric
- --use-lc-numeric
- This forces gawk to use the locale’s decimal
point character when parsing input data. Although the POSIX standard requires
this behavior, and gawk does so when --posix is in effect, the default is
to follow traditional behavior and use a period as the decimal point, even
in locales where the period is not the decimal point character. This option
overrides the default behavior, without the full draconian strictness of
the --posix option.
- -W version
- --version
- Print version information for this
particular copy of gawk on the standard output. This is useful mainly for
knowing if the current copy of gawk on your system is up to date with respect
to whatever the Free Software Foundation is distributing. This is also useful
when reporting bugs. (Per the GNU Coding Standards, these options cause
an immediate, successful exit.)
- --
- Signal the end of options. This is useful
to allow further arguments to the program itself to start with a ‘-’. This
provides consistency with the argument parsing convention used by most
other programs.
In compatibility mode, any other options are flagged as
invalid, but are otherwise ignored. In normal operation, as long as program
text has been supplied, unknown options are passed on to the program in
the ARGV array for processing. This is particularly useful for running
programs via the ‘#!’ executable interpreter mechanism.
An
program consists of a sequence of pattern-action statements and optional
function definitions.
pattern { action statements }
function name(parameter list) { statements }
Gawk first reads the program
source from the program-file(s) if specified, from arguments to --source,
or from the first non-option argument on the command line. The -f and --source
options may be used multiple times on the command line. Gawk reads the program
text as if all the program-files and command line source texts had been
concatenated together. This is useful for building libraries of functions,
without having to include them in each new program that uses them. It
also provides the ability to mix library functions with command line programs.
The environment variable AWKPATH specifies a search path to use when finding
source files named with the -f option. If this variable does not exist,
the default path is ".:/usr/local/share/awk". (The actual directory may vary,
depending upon how gawk was built and installed.) If a file name given to
the -f option contains a ‘/’ character, no path search is performed.
Gawk executes
programs in the following order. First, all variable assignments specified
via the -v option are performed. Next, gawk compiles the program into an
internal form. Then, gawk executes the code in the BEGIN block(s) (if any),
and then proceeds to read each file named in the ARGV array. If there are
no files named on the command line, gawk reads the standard input.
If a
filename on the command line has the form var=val it is treated as a variable
assignment. The variable var will be assigned the value val. (This happens
after any BEGIN block(s) have been run.) Command line variable assignment
is most useful for dynamically assigning values to the variables uses
to control how input is broken into fields and records. It is also useful
for controlling state if multiple passes are needed over a single data
file.
If the value of a particular element of ARGV is empty (""), gawk skips
over it.
For each record in the input, gawk tests to see if it matches any
pattern in the program. For each pattern that the record matches, the associated
action is executed. The patterns are tested in the order they occur in the
program.
Finally, after all the input is exhausted, gawk executes the code
in the END block(s) (if any).
variables are
dynamic; they come into existence when they are first used. Their values
are either floating-point numbers or strings, or both, depending upon how
they are used. also has one dimensional arrays; arrays with multiple dimensions
may be simulated. Several pre-defined variables are set as a program runs;
these are described as needed and summarized below.
Normally, records
are separated by newline characters. You can control how records are separated
by assigning values to the built-in variable RS. If RS is any single character,
that character separates records. Otherwise, RS is a regular expression.
Text in the input that matches this regular expression separates the record.
However, in compatibility mode, only the first character of its string
value is used for separating records. If RS is set to the null string, then
records are separated by blank lines. When RS is set to the null string,
the newline character always acts as a field separator, in addition to
whatever value FS may have.
As each input record is read, gawk splits
the record into fields, using the value of the FS variable as the field
separator. If FS is a single character, fields are separated by that character.
If FS is the null string, then each individual character becomes a separate
field. Otherwise, FS is expected to be a full regular expression. In the
special case that FS is a single space, fields are separated by runs of
spaces and/or tabs and/or newlines. (But see the section POSIX COMPATIBILITY,
below). NOTE: The value of IGNORECASE (see below) also affects how fields
are split when FS is a regular expression, and how records are separated
when RS is a regular expression.
If the FIELDWIDTHS variable is set to a
space separated list of numbers, each field is expected to have fixed width,
and gawk splits up the record using the specified widths. The value of
FS is ignored. Assigning a new value to FS overrides the use of FIELDWIDTHS,
and restores the default behavior.
Each field in the input record may be
referenced by its position, $1, $2, and so on. $0 is the whole record. Fields
need not be referenced by constants:
n = 5
print $n
prints the fifth field in the input record.
The variable NF is
set to the total number of fields in the input record.
References to non-existent
fields (i.e. fields after $NF) produce the null-string. However, assigning
to a non-existent field (e.g., $(NF+2) = 5) increases the value of NF, creates
any intervening fields with the null string as their value, and causes
the value of $0 to be recomputed, with the fields being separated by the
value of OFS. References to negative numbered fields cause a fatal error.
Decrementing NF causes the values of fields past the new value to be lost,
and the value of $0 to be recomputed, with the fields being separated by
the value of OFS.
Assigning a value to an existing field causes the whole
record to be rebuilt when $0 is referenced. Similarly, assigning a value
to $0 causes the record to be resplit, creating new values for the fields.
Gawk’s built-in variables are:
- ARGC
- The number of command
line arguments (does not include options to gawk, or the program source).
- ARGIND
- The index in ARGV of the current file being processed.
- ARGV
- Array
of command line arguments. The array is indexed from 0 to ARGC - 1. Dynamically
changing the contents of ARGV can control the files used for data.
- BINMODE
- On non-POSIX systems, specifies use of ‘binary’ mode for all file I/O. Numeric
values of 1, 2, or 3, specify that input files, output files, or all files,
respectively, should use binary I/O. String values of "r", or "w" specify
that input files, or output files, respectively, should use binary I/O.
String values of "rw" or "wr" specify that all files should use binary
I/O. Any other string value is treated as "rw", but generates a warning
message.
- CONVFMT
- The conversion format for numbers, "%.6g", by default.
- ENVIRON
- An array containing the values of the current environment. The array is
indexed by the environment variables, each element being the value of that
variable (e.g., ENVIRON["HOME"] might be /home/arnold). Changing this array
does not affect the environment seen by programs which gawk spawns via
redirection or the system() function.
- ERRNO
- If a system error occurs either
doing a redirection for getline, during a read for getline, or during a
close(), then ERRNO will contain a string describing the error. The value
is subject to translation in non-English locales.
- FIELDWIDTHS
- A white-space
separated list of fieldwidths. When set, gawk parses the input into fields
of fixed width, instead of using the value of the FS variable as the field
separator.
- FILENAME
- The name of the current input file. If no files are specified
on the command line, the value of FILENAME is ‘-’. However, FILENAME is undefined
inside the BEGIN block (unless set by getline).
- FNR
- The input record number
in the current input file.
- FS
- The input field separator, a space by default.
See Fields, above.
- IGNORECASE
- Controls the case-sensitivity of all regular
expression and string operations. If IGNORECASE has a non-zero value, then
string comparisons and pattern matching in rules, field splitting with
FS, record separating with RS, regular expression matching with ~ and !~,
and the gensub(), gsub(), index(), match(), split(), and sub() built-in
functions all ignore case when doing regular expression operations. NOTE:
Array subscripting is not affected. However, the asort() and asorti() functions
are affected.
Thus, if IGNORECASE is not equal to zero, /aB/ matches all
of the strings "ab", "aB", "Ab", and "AB". As with all variables, the initial
value of IGNORECASE is zero, so all regular expression and string operations
are normally case-sensitive. Under Unix, the full ISO 8859-1 Latin-1 character
set is used when ignoring case. As of gawk 3.1.4, the case equivalencies are
fully locale-aware, based on the C <ctype.h> facilities such as isalpha(),
and toupper().
- LINT
- Provides dynamic control of the --lint option from within
an program. When true, gawk prints lint warnings. When false, it does not.
When assigned the string value "fatal", lint warnings become fatal errors,
exactly like --lint=fatal. Any other true value just prints warnings.
- NF
- The
number of fields in the current input record.
- NR
- The total number of input
records seen so far.
- OFMT
- The output format for numbers, "%.6g", by default.
- OFS
- The output field separator, a space by default.
- ORS
- The output record
separator, by default a newline.
- PROCINFO
- The elements of this array provide
access to information about the running program. On some systems, there
may be elements in the array, "group1" through "groupn" for some n, which
is the number of supplementary groups that the process has. Use the in operator
to test for these elements. The following elements are guaranteed to be
available:
- PROCINFO["egid"]
- the value of the getegid(2)
system call.
- PROCINFO["euid"]
- the value of the geteuid(2)
system call.
- PROCINFO["FS"]
- "FS" if field splitting
with FS is in effect, or "FIELDWIDTHS" if field splitting with FIELDWIDTHS
is in effect.
- PROCINFO["gid"]
- the value of the getgid(2)
system call.
- PROCINFO["pgrpid"]
- the process group ID of the current process.
- PROCINFO["pid"]
- the process
ID of the current process.
- PROCINFO["ppid"]
- the parent process ID of the
current process.
- PROCINFO["uid"]
- the value of the getuid(2)
system call.
- PROCINFO["version"]
- The version of gawk. This is available from version
3.1.4 and later.
- RS
- The input record separator, by default a newline.
- RT
- The
record terminator. Gawk sets RT to the input text that matched the character
or regular expression specified by RS.
- RSTART
- The index of the first character
matched by match(); 0 if no match. (This implies that character indices
start at one.)
- RLENGTH
- The length of the string matched by match(); -1 if
no match.
- SUBSEP
- The character used to separate multiple subscripts in array
elements, by default "\034".
- TEXTDOMAIN
- The text domain of the program;
used to find the localized translations for the program’s strings.
Arrays
are subscripted with an expression between square brackets ([ and ]). If
the expression is an expression list (expr, expr ...) then the array subscript
is a string consisting of the concatenation of the (string) value of each
expression, separated by the value of the SUBSEP variable. This facility
is used to simulate multiply dimensioned arrays. For example:
i = "A";
j = "B"; k = "C"
x[i, j, k] = "hello, world\n"
assigns the string "hello, world\n" to the
element of the array x which is indexed by the string "A\034B\034C". All
arrays in are associative, i.e. indexed by string values.
The special operator
in may be used to test if an array has an index consisting of a particular
value.
if (val in array)
print array[val]
If the array has multiple subscripts, use (i, j) in array.
The in construct
may also be used in a for loop to iterate over all the elements of an array.
An element may be deleted from an array using the delete statement. The
delete statement may also be used to delete the entire contents of an array,
just by specifying the array name without a subscript.
Variables and fields may be (floating point) numbers, or strings,
or both. How the value of a variable is interpreted depends upon its context.
If used in a numeric expression, it will be treated as a number; if used
as a string it will be treated as a string.
To force a variable to be treated
as a number, add 0 to it; to force it to be treated as a string, concatenate
it with the null string.
When a string must be converted to a number, the
conversion is accomplished using strtod(3)
. A number is converted to a string
by using the value of CONVFMT as a format string for sprintf(3)
, with the
numeric value of the variable as the argument. However, even though all
numbers in are floating-point, integral values are always converted as
integers. Thus, given
CONVFMT = "%2.2f"
a = 12
b = a ""
the variable b has a string value of "12" and not "12.00".
When operating
in POSIX mode (such as with the --posix command line option), beware that
locale settings may interfere with the way decimal numbers are treated:
the decimal separator of the numbers you are feeding to gawk must conform
to what your locale would expect, be it a comma (,) or a period (.).
Gawk
performs comparisons as follows: If two variables are numeric, they are
compared numerically. If one value is numeric and the other has a string
value that is a ‘numeric string,’ then comparisons are also done numerically.
Otherwise, the numeric value is converted to a string and a string comparison
is performed. Two strings are compared, of course, as strings.
Note that
string constants, such as "57", are not numeric strings, they are string
constants. The idea of ‘numeric string’ only applies to fields, getline input,
FILENAME, ARGV elements, ENVIRON elements and the elements of an array
created by split() that are numeric strings. The basic idea is that user
input, and only user input, that looks numeric, should be treated that
way.
Uninitialized variables have the numeric value 0 and the string value
"" (the null, or empty, string).
Starting
with version 3.1 of gawk , you may use C-style octal and hexadecimal constants
in your AWK program source code. For example, the octal value 011 is equal
to decimal 9, and the hexadecimal value 0x11 is equal to decimal 17.
String constants in are sequences of characters enclosed between
double quotes ("). Within strings, certain escape sequences are recognized,
as in C. These are:
- \\
- A literal backslash.
- \a
- The ‘alert’ character; usually
the ASCII
BEL
character.
- \b
- backspace.
- \f
- form-feed.
- \n
- newline.
- \r
- carriage return.
- \t
- horizontal tab.
- \v
- vertical tab.
- \xhex digits
- The character represented by
the string of hexadecimal digits following the \x. As in C, all following
hexadecimal digits are considered part of the escape sequence. (This feature
should tell us something about language design by committee.) E.g., "\x1B"
is the ASCII
ESC
(escape) character.
- \ddd
- The character represented by the
1-, 2-, or 3-digit sequence of octal digits. E.g., "\033" is the ASCII
ESC
(escape)
character.
- \c
- The literal character c.
The escape sequences may also be used
inside constant regular expressions (e.g., /[ \t\f\n\r\v]/ matches whitespace characters).
In compatibility mode, the characters represented by octal and hexadecimal
escape sequences are treated literally when used in regular expression
constants. Thus, /a\52b/ is equivalent to /a\*b/.
is
a line-oriented language. The pattern comes first, and then the action.
Action statements are enclosed in { and }. Either the pattern may be missing,
or the action may be missing, but, of course, not both. If the pattern
is missing, the action is executed for every single record of input. A missing
action is equivalent to
{ print }
which prints the entire record.
Comments
begin with the ‘#’ character, and continue until the end of the line. Blank
lines may be used to separate statements. Normally, a statement ends with
a newline, however, this is not the case for lines ending in a ‘,’, {, ?,
:, &&, or ||. Lines ending in do or else also have their statements automatically
continued on the following line. In other cases, a line can be continued
by ending it with a ‘\’, in which case the newline will be ignored.
Multiple
statements may be put on one line by separating them with a ‘;’. This applies
to both the statements within the action part of a pattern-action pair (the
usual case), and to the pattern-action statements themselves.
patterns
may be one of the following:
BEGINEND/regular expression/relational expressionpattern && patternpattern
|| patternpattern ? pattern : pattern(pattern)! patternpattern1, pattern2
BEGIN
and END are two special kinds of patterns which are not tested against
the input. The action parts of all BEGIN patterns are merged as if all the
statements had been written in a single BEGIN block. They are executed
before any of the input is read. Similarly, all the END blocks are merged,
and executed when all the input is exhausted (or when an exit statement
is executed). BEGIN and END patterns cannot be combined with other patterns
in pattern expressions. BEGIN and END patterns cannot have missing action
parts.
For /regular expression/ patterns, the associated statement is executed
for each input record that matches the regular expression. Regular expressions
are the same as those in egrep(1)
, and are summarized below.
A relational
expression may use any of the operators defined below in the section on
actions. These generally test whether certain fields match certain regular
expressions.
The &&, ||, and ! operators are logical AND, logical OR, and logical
NOT, respectively, as in C. They do short-circuit evaluation, also as in
C, and are used for combining more primitive pattern expressions. As in
most languages, parentheses may be used to change the order of evaluation.
The ?: operator is like the same operator in C. If the first pattern is
true then the pattern used for testing is the second pattern, otherwise
it is the third. Only one of the second and third patterns is evaluated.
The pattern1, pattern2 form of an expression is called a range pattern.
It matches all input records starting with a record that matches pattern1,
and continuing until a record that matches pattern2, inclusive. It does
not combine with any other sort of pattern expression.
Regular
expressions are the extended kind found in egrep. They are composed of characters
as follows:
- c
- matches the non-metacharacter c.
- \c
- matches the literal character
c. .
- matches any character including newline.
- ^
- matches the beginning of a string.
- $
- matches the end of a string.
- [abc...]
- character list, matches any of the
characters abc....
- [^abc...]
- negated character list, matches any character except
abc....
- r1|r2
- alternation: matches either r1 or r2.
- r1r2
- concatenation: matches
r1, and then r2.
- r+
- matches one or more r’s.
- r*
- matches zero or more r’s.
- r?
- matches zero or one r’s.
- (r)
- grouping: matches r.
- r{n}
- r{n,}
- r{n,m}
- One
or two numbers inside braces denote an interval expression. If there is
one number in the braces, the preceding regular expression r is repeated
n times. If there are two numbers separated by a comma, r is repeated n
to m times. If there is one number followed by a comma, then r is repeated
at least n times.
Interval expressions are only available if either --posix
or --re-interval is specified on the command line.
- \y
- matches the empty string
at either the beginning or the end of a word.
- \B
- matches the empty string
within a word.
- \<
- matches the empty string at the beginning of a word.
- \>
- matches
the empty string at the end of a word.
- \w
- matches any word-constituent character
(letter, digit, or underscore).
- \W
- matches any character that is not word-constituent.
- \‘
- matches the empty string at the beginning of a buffer (string).
- \’
- matches
the empty string at the end of a buffer.
The escape sequences that are valid
in string constants (see below) are also valid in regular expressions.
Character
classes are a feature introduced in the standard. A character class is
a special notation for describing lists of characters that have a specific
attribute, but where the actual characters themselves can vary from country
to country and/or from character set to character set. For example, the
notion of what is an alphabetic character differs in the USA and in France.
A character class is only valid in a regular expression inside the brackets
of a character list. Character classes consist of [:, a keyword denoting
the class, and :]. The character classes defined by the standard are:
- [:alnum:]
- Alphanumeric characters.
- [:alpha:]
- Alphabetic characters.
- [:blank:]
- Space
or tab characters.
- [:cntrl:]
- Control characters.
- [:digit:]
- Numeric characters.
- [:graph:]
- Characters that are both printable and visible. (A space is printable,
but not visible, while an a is both.)
- [:lower:]
- Lower-case alphabetic characters.
- [:print:]
- Printable characters (characters that are not control characters.)
- [:punct:]
- Punctuation characters (characters that are not letter, digits,
control characters, or space characters).
- [:space:]
- Space characters (such
as space, tab, and formfeed, to name a few).
- [:upper:]
- Upper-case alphabetic
characters.
- [:xdigit:]
- Characters that are hexadecimal digits.
For example,
before the standard, to match alphanumeric characters, you would have
had to write /[A-Za-z0-9]/. If your character set had other alphabetic characters
in it, this would not match them, and if your character set collated differently
from ASCII
, this might not even match the ASCII
alphanumeric characters.
With the character classes, you can write /[[:alnum:]]/, and this matches
the alphabetic and numeric characters in your character set, no matter
what it is.
Two additional special sequences can appear in character lists.
These apply to non-ASCII
character sets, which can have single symbols (called
collating elements) that are represented with more than one character,
as well as several characters that are equivalent for collating, or sorting,
purposes. (E.g., in French, a plain ‘e’ and a grave-accented ‘e‘’ are equivalent.)
- Collating Symbols
- A collating symbol is a multi-character collating element
enclosed in [. and .]. For example, if ch is a collating element, then [[.ch.]]
is a regular expression that matches this collating element, while [ch]
is a regular expression that matches either c or h.
- Equivalence Classes
- An equivalence class is a locale-specific name for a list of characters
that are equivalent. The name is enclosed in [= and =]. For example, the
name e might be used to represent all of ‘e,’ ‘e’,’ and ‘e‘.’ In this case, [[=e=]]
is a regular expression that matches any of e, e’, or e‘.
These features are
very valuable in non-English speaking locales. The library functions that
gawk uses for regular expression matching currently only recognize character
classes; they do not recognize collating symbols or equivalence classes.
The \y, \B, \<, \>, \w, \W, \‘, and \’ operators are specific to gawk; they are extensions
based on facilities in the regular expression libraries.
The various command
line options control how gawk interprets characters in regular expressions.
- No options
- In the default case, gawk provide all the facilities of regular
expressions and the regular expression operators described above. However,
interval expressions are not supported.
- --posix
- Only regular expressions
are supported, the operators are not special. (E.g., \w matches a literal
w). Interval expressions are allowed.
- --traditional
- Traditional Unix awk regular
expressions are matched. The operators are not special, interval expressions
are not available, and neither are the character classes ([[:alnum:]]
and so on). Characters described by octal and hexadecimal escape sequences
are treated literally, even if they represent regular expression metacharacters.
- --re-interval
- Allow interval expressions in regular expressions, even if --traditional
has been provided.
Action statements are enclosed in braces, { and
}. Action statements consist of the usual assignment, conditional, and looping
statements found in most languages. The operators, control statements,
and input/output statements available are patterned after those in C.
The
operators in , in order of decreasing precedence, are
- (...)
- Grouping
- $
- Field
reference.
- ++ --
- Increment and decrement, both prefix and postfix.
- ^
- Exponentiation
(** may also be used, and **= for the assignment operator).
- + - !
- Unary plus,
unary minus, and logical negation.
- * / %
- Multiplication, division, and modulus.
- + -
- Addition and subtraction.
- space
- String concatenation.
- | |&
- Piped I/O for
getline, print, and printf.
- < >
- <= >=
- != ==
- The regular relational operators.
- ~ !~
- Regular expression match, negated match. NOTE: Do not use a constant
regular expression (/foo/) on the left-hand side of a ~ or !~. Only use one
on the right-hand side. The expression /foo/ ~ exp has the same meaning
as (($0 ~ /foo/) ~ exp). This is usually not what was intended.
- in
- Array
membership.
- &&
- Logical AND.
- ||
- Logical OR.
- ?:
- The C conditional expression. This
has the form expr1 ? expr2 : expr3. If expr1 is true, the value of the expression
is expr2, otherwise it is expr3. Only one of expr2 and expr3 is evaluated.
- = += -=
- *= /= %= ^=
- Assignment. Both absolute assignment (var = value)
and operator-assignment (the other forms) are supported.
The
control statements are as follows:
if (condition) statement [ else statement ]
while (condition) statement
do statement while (condition)
for (expr1; expr2; expr3) statement
for (var in array) statement
break
continue
delete array[index]
delete array
exit [ expression ]
{ statements }
The input/output statements are as follows:
- close(file [,
how])
- Close file, pipe or co-process. The optional how should only be used
when closing one end of a two-way pipe to a co-process. It must be a string
value, either "to" or "from".
- getline
- Set $0 from next input record; set
NF, NR, FNR.
- getline <file
- Set $0 from next record of file; set NF.
- getline
var
- Set var from next input record; set NR, FNR.
- getline var <file
- Set var
from next record of file.
- command | getline [var]
- Run command piping the
output either into $0 or var, as above.
- command |& getline [var]
- Run command
as a co-process piping the output either into $0 or var, as above. Co-processes
are a gawk extension. (command can also be a socket. See the subsection
Special File Names, below.)
- next
- Stop processing the current input record.
The next input record is read and processing starts over with the first
pattern in the program. If the end of the input data is reached, the END
block(s), if any, are executed.
- nextfile
- Stop processing the current input
file. The next input record read comes from the next input file. FILENAME
and ARGIND are updated, FNR is reset to 1, and processing starts over with
the first pattern in the program. If the end of the input data is reached,
the END block(s), if any, are executed.
- print
- Prints the current record.
The output record is terminated with the value of the ORS variable.
- print
expr-list
- Prints expressions. Each expression is separated by the value of
the OFS variable. The output record is terminated with the value of the
ORS variable.
- print expr-list >file
- Prints expressions on file. Each expression
is separated by the value of the OFS variable. The output record is terminated
with the value of the ORS variable.
- printf fmt, expr-list
- Format and print.
- printf fmt, expr-list >file
- Format and print on file.
- system(cmd-line)
- Execute
the command cmd-line, and return the exit status. (This may not be available
on non- systems.)
- fflush([file])
- Flush any buffers associated with the open
output file or pipe file. If file is missing, then standard output is flushed.
If file is the null string, then all open output files and pipes have their
buffers flushed.
Additional output redirections are allowed for print and
printf.
- print ... >> file
- Appends output to the file.
- print ... | command
- Writes on
a pipe.
- print ... |& command
- Sends data to a co-process or socket. (See also the
subsection Special File Names, below.)
The getline command returns 0 on
end of file and -1 on an error. Upon an error, ERRNO contains a string describing
the problem.
NOTE: If using a pipe, co-process, or socket to getline, or
from print or printf within a loop, you must use close() to create new
instances of the command or socket. does not automatically close pipes,
sockets, or co-processes when they return EOF.
The versions
of the printf statement and sprintf() function (see below) accept the following
conversion specification formats:
- %c
- An ASCII
character. If the argument
used for %c is numeric, it is treated as a character and printed. Otherwise,
the argument is assumed to be a string, and the only first character of
that string is printed.
- %d, %i
- A decimal number (the integer part).
- %e, %E
- A floating point number of the form [-]d.dddddde[+-]dd. The %E format uses
E instead of e.
- %f, %F
- A floating point number of the form [-]ddd.dddddd. If
the system library supports it, %F is available as well. This is like %f,
but uses capital letters for special ‘not a number’ and ‘infinity’ values. If
%F is not available, gawk uses %f.
- %g, %G
- Use %e or %f conversion, whichever
is shorter, with nonsignificant zeros suppressed. The %G format uses %E
instead of %e.
- %o
- An unsigned octal number (also an integer).
- %u
- An unsigned
decimal number (again, an integer).
- %s
- A character string.
- %x, %X
- An unsigned
hexadecimal number (an integer). The %X format uses ABCDEF instead of abcdef.
- %%
- A single % character; no argument is converted.
NOTE: When using the
integer format-control letters for values that are outside the range of
a C long integer, gawk switches to the %0f format specifier. If --lint is
provided on the command line gawk warns about this. Other versions of awk
may print invalid values or do something else entirely.
Optional, additional
parameters may lie between the % and the control letter:
- count$
- Use the
count’th argument at this point in the formatting. This is called a positional
specifier and is intended primarily for use in translated versions of format
strings, not in the original text of an AWK program. It is a gawk extension.
- -
- The expression should be left-justified within its field.
- space
- For numeric
conversions, prefix positive values with a space, and negative values with
a minus sign.
- +
- The plus sign, used before the width modifier (see below),
says to always supply a sign for numeric conversions, even if the data
to be formatted is positive. The + overrides the space modifier.
- #
- Use an
‘alternate form’ for certain control letters. For %o, supply a leading zero.
For %x, and %X, supply a leading 0x or 0X for a nonzero result. For %e,
%E, %f and %F, the result always contains a decimal point. For %g, and %G,
trailing zeros are not removed from the result.
- A leading 0 (zero) acts
as a flag, that indicates output should be padded with zeroes instead of
spaces. This applies even to non-numeric output formats. This flag only has
an effect when the field width is wider than the value to be printed.
- width
- The field should be padded to this width. The field is normally padded
with spaces. If the 0 flag has been used, it is padded with zeroes. .
- A number
that specifies the precision to use when printing. For the %e, %E, %f and
%F, formats, this specifies the number of digits you want printed to the
right of the decimal point. For the %g, and %G formats, it specifies the
maximum number of significant digits. For the %d, %o, %i, %u, %x, and %X
formats, it specifies the minimum number of digits to print. For %s, it
specifies the maximum number of characters from the string that should
be printed.
The dynamic width and prec capabilities of the C printf() routines
are supported. A * in place of either the width or prec specifications causes
their values to be taken from the argument list to printf or sprintf().
To use a positional specifier with a dynamic width or precision, supply
the count$ after the * in the format string. For example, "%3$*2$.*1$s".
When doing I/O redirection from either print or printf into a
file, or via getline from a file, gawk recognizes certain special filenames
internally. These filenames allow access to open file descriptors inherited
from gawk’s parent process (usually the shell). These file names may also
be used on the command line to name data files. The filenames are:
- /dev/stdin
- The standard input.
- /dev/stdout
- The standard output.
- /dev/stderr
- The standard
error output.
- /dev/fd/n
- The file associated with the open file descriptor
n.
These are particularly useful for error messages. For example:
print
"You blew it!" > "/dev/stderr"
whereas you would otherwise have to use
print
"You blew it!" | "cat 1>&2"
The following special filenames may be used with
the |& co-process operator for creating TCP/IP network connections.
- /inet/tcp/lport/rhost/rport
- File for TCP/IP connection on local port lport to remote host rhost on
remote port rport. Use a port of 0 to have the system pick a port.
- /inet/udp/lport/rhost/rport
- Similar, but use UDP/IP instead of TCP/IP.
- /inet/raw/lport/rhost/rport
-
Reserved for future use.
Other special filenames provide access to information
about the running gawk process. These filenames are now obsolete. Use the
PROCINFO array to obtain the information they provide. The filenames are:
- /dev/pid
- Reading this file returns the process ID of the current process,
in decimal, terminated with a newline.
- /dev/ppid
- Reading this file returns
the parent process ID of the current process, in decimal, terminated with
a newline.
- /dev/pgrpid
- Reading this file returns the process group ID of
the current process, in decimal, terminated with a newline.
- /dev/user
- Reading
this file returns a single record terminated with a newline. The fields
are separated with spaces. $1 is the value of the getuid(2)
system call,
$2 is the value of the geteuid(2)
system call, $3 is the value of the getgid(2)
system call, and $4 is the value of the getegid(2)
system call. If there
are any additional fields, they are the group IDs returned by getgroups(2)
.
Multiple groups may not be supported on all systems.
has
the following built-in arithmetic functions:
- atan2(y, x)
- Returns the arctangent
of y/x in radians.
- cos(expr)
- Returns the cosine of expr, which is in radians.
- exp(expr)
- The exponential function.
- int(expr)
- Truncates to integer.
- log(expr)
- The natural logarithm function.
- rand()
- Returns a random number N, between
0 and 1, such that 0 <= N < 1.
- sin(expr)
- Returns the sine of expr, which is
in radians.
- sqrt(expr)
- The square root function.
- srand([expr])
- Uses expr
as a new seed for the random number generator. If no expr is provided,
the time of day is used. The return value is the previous seed for the random
number generator.
Gawk has the following built-in string
functions:
- asort(s [, d])
- Returns the number of elements in the source
array s. The contents of s are sorted using gawk’s normal rules for comparing
values, and the indices of the sorted values of s are replaced with sequential
integers starting with 1. If the optional destination array d is specified,
then s is first duplicated into d, and then d is sorted, leaving the indices
of the source array s unchanged.
- asorti(s [, d])
- Returns the number of elements
in the source array s. The behavior is the same as that of asort(), except
that the array indices are used for sorting, not the array values. When
done, the array is indexed numerically, and the values are those of the
original indices. The original values are lost; thus provide a second array
if you wish to preserve the original.
- gensub(r, s, h [, t])
- Search the target
string t for matches of the regular expression r. If h is a string beginning
with g or G, then replace all matches of r with s. Otherwise, h is a number
indicating which match of r to replace. If t is not supplied, $0 is used
instead. Within the replacement text s, the sequence \n, where n is a digit
from 1 to 9, may be used to indicate just the text that matched the n’th
parenthesized subexpression. The sequence \0 represents the entire matched
text, as does the character &. Unlike sub() and gsub(), the modified string
is returned as the result of the function, and the original target string
is not changed.
- gsub(r, s [, t])
- For each substring matching the regular
expression r in the string t, substitute the string s, and return the number
of substitutions. If t is not supplied, use $0. An & in the replacement text
is replaced with the text that was actually matched. Use \& to get a literal
&. (This must be typed as "\\&"; see P for a fuller discussion of the rules
for &’s and backslashes in the replacement text of sub(), gsub(), and gensub().)
- index(s, t)
- Returns the index of the string t in the string s, or 0 if
t is not present. (This implies that character indices start at one.)
- length([s])
- Returns the length of the string s, or the length of $0 if s is not supplied.
Starting with version 3.1.5, as a non-standard extension, with an array argument,
length() returns the number of elements in the array.
- match(s, r [, a])
- Returns the position in s where the regular expression r occurs, or 0 if
r is not present, and sets the values of RSTART and RLENGTH. Note that the
argument order is the same as for the ~ operator: str ~ re. If array a is
provided, a is cleared and then elements 1 through n are filled with the
portions of s that match the corresponding parenthesized subexpression
in r. The 0’th element of a contains the portion of s matched by the entire
regular expression r. Subscripts a[n, "start"], and a[n, "length"] provide
the starting index in the string and length respectively, of each matching
substring.
- split(s, a [, r])
- Splits the string s into the array a on the
regular expression r, and returns the number of fields. If r is omitted,
FS is used instead. The array a is cleared first. Splitting behaves identically
to field splitting, described above.
- sprintf(fmt, expr-list)
- Prints expr-list
according to fmt, and returns the resulting string.
- strtonum(str)
- Examines
str, and returns its numeric value. If str begins with a leading 0, strtonum()
assumes that str is an octal number. If str begins with a leading 0x or
0X, strtonum() assumes that str is a hexadecimal number.
- sub(r, s [, t])
- Just like gsub(), but only the first matching substring is replaced.
- substr(s,
i [, n])
- Returns the at most n-character substring of s starting at i. If
n is omitted, the rest of s is used.
- tolower(str)
- Returns a copy of the
string str, with all the upper-case characters in str translated to their
corresponding lower-case counterparts. Non-alphabetic characters are left
unchanged.
- toupper(str)
- Returns a copy of the string str, with all the lower-case
characters in str translated to their corresponding upper-case counterparts.
Non-alphabetic characters are left unchanged.
As of version 3.1.5, gawk is
multibyte aware. This means that index(), length(), substr() and match()
all work in terms of characters, not bytes.
Since one of the
primary uses of programs is processing log files that contain time stamp
information, gawk provides the following functions for obtaining time stamps
and formatting them.
- mktime(datespec)
- Turns datespec into a time stamp of
the same form as returned by systime(). The datespec is a string of the
form YYYY MM DD HH MM SS[ DST]. The contents of the string are six or seven
numbers representing respectively the full year including century, the
month from 1 to 12, the day of the month from 1 to 31, the hour of the
day from 0 to 23, the minute from 0 to 59, and the second from 0 to 60,
and an optional daylight saving flag. The values of these numbers need not
be within the ranges specified; for example, an hour of -1 means 1 hour
before midnight. The origin-zero Gregorian calendar is assumed, with year
0 preceding year 1 and year -1 preceding year 0. The time is assumed to be
in the local timezone. If the daylight saving flag is positive, the time
is assumed to be daylight saving time; if zero, the time is assumed to
be standard time; and if negative (the default), mktime() attempts to determine
whether daylight saving time is in effect for the specified time. If datespec
does not contain enough elements or if the resulting time is out of range,
mktime() returns -1.
- strftime([format [, timestamp[, utc-flag]]])
- Formats
timestamp according to the specification in format. If utc-flag is present
and is non-zero or non-null, the result is in UTC, otherwise the result is
in local time. The timestamp should be of the same form as returned by systime().
If timestamp is missing, the current time of day is used. If format is missing,
a default format equivalent to the output of date(1)
is used. See the specification
for the strftime() function in C for the format conversions that are guaranteed
to be available.
- systime()
- Returns the current time of day as the number
of seconds since the Epoch (1970-01-01 00:00:00 UTC on systems).
Starting with version 3.1 of gawk, the following bit manipulation
functions are available. They work by converting double-precision floating
point values to uintmax_t integers, doing the operation, and then converting
the result back to floating point. The functions are:
- and(v1, v2)
- Return
the bitwise AND of the values provided by v1 and v2.
- compl(val)
- Return the
bitwise complement of val.
- lshift(val, count)
- Return the value of val, shifted
left by count bits.
- or(v1, v2)
- Return the bitwise OR of the values provided
by v1 and v2.
- rshift(val, count)
- Return the value of val, shifted right
by count bits.
- xor(v1, v2)
- Return the bitwise XOR of the values provided
by v1 and v2.
Starting with version 3.1 of
gawk, the following functions may be used from within your AWK program
for translating strings at run-time. For full details, see P.
- bindtextdomain(directory
[, domain])
- Specifies the directory where gawk looks for the .mo files,
in case they will not or cannot be placed in the ‘‘standard’’ locations (e.g.,
during testing). It returns the directory where domain is ‘‘bound.’’
The default
domain is the value of TEXTDOMAIN. If directory is the null string (""),
then bindtextdomain() returns the current binding for the given domain.
- dcgettext(string [, domain [, category]])
- Returns the translation of string
in text domain domain for locale category category. The default value for
domain is the current value of TEXTDOMAIN. The default value for category
is "LC_MESSAGES".
If you supply a value for category, it must be a string
equal to one of the known locale categories described in P. You must also
supply a text domain. Use TEXTDOMAIN if you want to use the current domain.
- dcngettext(string1 , string2 , number [, domain [, category]])
- Returns
the plural form used for number of the translation of string1 and string2
in text domain domain for locale category category. The default value for
domain is the current value of TEXTDOMAIN. The default value for category
is "LC_MESSAGES".
If you supply a value for category, it must be a string
equal to one of the known locale categories described in P. You must also
supply a text domain. Use TEXTDOMAIN if you want to use the current domain.
Functions in are defined as follows:
function name(parameter
list) { statements }
Functions are executed when they are called from within
expressions in either patterns or actions. Actual parameters supplied in
the function call are used to instantiate the formal parameters declared
in the function. Arrays are passed by reference, other variables are passed
by value.
Since functions were not originally part of the language, the
provision for local variables is rather clumsy: They are declared as extra
parameters in the parameter list. The convention is to separate local variables
from real parameters by extra spaces in the parameter list. For example:
function f(p, q, a, b) # a and b are local
{
...
}
/abc/ { ... ; f(1, 2) ; ... }
The left parenthesis in a function call is required to immediately follow
the function name, without any intervening white space. This avoids a syntactic
ambiguity with the concatenation operator. This restriction does not apply
to the built-in functions listed above.
Functions may call each other and
may be recursive. Function parameters used as local variables are initialized
to the null string and the number zero upon function invocation.
Use return
expr to return a value from a function. The return value is undefined if
no value is provided, or if the function returns by ‘falling off’ the end.
If --lint has been provided, gawk warns about calls to undefined functions
at parse time, instead of at run time. Calling an undefined function at
run time is a fatal error.
The word func may be used in place of function.
Beginning with version 3.1 of gawk, you
can dynamically add new built-in functions to the running gawk interpreter.
The full details are beyond the scope of this manual page; see P for the
details.
- extension(object, function)
- Dynamically link the shared object
file named by object, and invoke function in that object, to perform initialization.
These should both be provided as strings. Returns the value returned by
function.
This function is provided and documented in P, but everything
about this feature is likely to change eventually. We STRONGLY recommend
that you do not use this feature for anything that you aren’t willing to
redo.
Signals
pgawk accepts two signals. SIGUSR1 causes it to dump a profile
and function call stack to the profile file, which is either awkprof.out,
or whatever file was named with the --profile option. It then continues to
run. SIGHUP causes pgawk to dump the profile and function call stack and
then exit.
Print and sort the login names of all users:
BEGIN { FS = ":" }
{ print $1 | "sort" }
Count lines in a file:
{ nlines++ }
END { print nlines }
Precede each line by its number in the file:
{ print FNR, $0 }
Concatenate and line number (a variation on a theme):
{ print NR, $0 }
Run an external command for particular lines of data:
tail -f access_log |
awk ’/myhome.html/ { system("nmap " $1 ">> logdir/myhome.html") }’
String constants are sequences of characters enclosed
in double quotes. In non-English speaking environments, it is possible to
mark strings in the program as requiring translation to the native natural
language. Such strings are marked in the program with a leading underscore
(‘_’). For example,
gawk ’BEGIN { print "hello, world" }’
always prints hello,
world. But,
gawk ’BEGIN { print _"hello, world" }’
might print bonjour, monde
in France.
There are several steps involved in producing and running a localizable
program.
.- Add a BEGIN action to assign a value to the TEXTDOMAIN variable
to set the text domain to a name associated with your program.
BEGIN { TEXTDOMAIN = "myprog" }
This allows gawk to find the .mo file associated with your program. Without
this step, gawk uses the messages text domain, which likely does not contain
translations for your program.
.- Mark all strings that should be translated
with leading underscores.
.- If necessary, use the dcgettext() and/or bindtextdomain()
functions in your program, as appropriate.
.- Run gawk --gen-po -f myprog.awk > myprog.po
to generate a .po file for your program.
.- Provide appropriate translations,
and build and install the corresponding .mo files.
The internationalization
features are described in full detail in P.
Posix Compatibility
A primary
goal for gawk is compatibility with the standard, as well as with the
latest version of awk. To this end, gawk incorporates the following user
visible features which are not described in the book, but are part of
the Bell Laboratories version of awk, and are in the standard.
The book
indicates that command line variable assignment happens when awk would
otherwise open the argument as a file, which is after the BEGIN block is
executed. However, in earlier implementations, when such an assignment
appeared before any file names, the assignment would happen before the
BEGIN block was run. Applications came to depend on this ‘feature.’ When awk
was changed to match its documentation, the -v option for assigning variables
before program execution was added to accommodate applications that depended
upon the old behavior. (This feature was agreed upon by both the Bell Laboratories
and the developers.)
The -W option for implementation specific features
is from the standard.
When processing arguments, gawk uses the special
option ‘--’ to signal the end of arguments. In compatibility mode, it warns
about but otherwise ignores undefined options. In normal operation, such
arguments are passed on to the program for it to process.
The book does
not define the return value of srand(). The standard has it return the
seed it was using, to allow keeping track of random number sequences. Therefore
srand() in gawk also returns its current seed.
Other new features are: The
use of multiple -f options (from MKS awk); the ENVIRON array; the \a, and
\v escape sequences (done originally in gawk and fed back into the Bell
Laboratories version); the tolower() and toupper() built-in functions (from
the Bell Laboratories version); and the C conversion specifications in
printf (done first in the Bell Laboratories version).
There
are two features of historical implementations that gawk supports. First,
it is possible to call the length() built-in function not only with no argument,
but even without parentheses! Thus,
a = length # Holy Algol 60, Batman!
is the same as either of
a = length()
a = length($0)
This feature is marked as ‘deprecated’ in the standard, and
gawk issues a warning about its use if --lint is specified on the command
line.
The other feature is the use of either the continue or the break statements
outside the body of a while, for, or do loop. Traditional implementations
have treated such usage as equivalent to the next statement. Gawk supports
this usage if --traditional has been specified.
Gawk has a number
of extensions to awk. They are described in this section. All the extensions
described here can be disabled by invoking gawk with the --traditional or
--posix options.
The following features of gawk are not available in awk.
- -f option. Therefore the AWKPATH environment variable is not special.
- \x escape sequence. (Disabled with --posix.)
- fflush() function. (Disabled with
--posix.)
- ? and :. (Disabled with --posix.)
.-
- ARGIND, BINMODE, ERRNO, LINT, RT
and TEXTDOMAIN variables are not special.
- IGNORECASE variable and its side-effects
are not available.
- FIELDWIDTHS variable and fixed-width field splitting.
- PROCINFO array is not available.
- RS as a regular expression.
.- |& operator for
creating co-processes.
- as the value of FS, and as the third argument to
split().
- close() function.
- match() function.
- printf and sprintf().
- length().
- delete array to delete the entire contents of an array.
- nextfile to abandon
processing of the current input file.
- and(), asort(), asorti(), bindtextdomain(),
compl(), dcgettext(), dcngettext(), gensub(), lshift(), mktime(), or(),
rshift(), strftime(), strtonum(), systime() and xor() functions.
.-
-- extension()
function.
The book does not define the return value of the close() function.
Gawk’s close() returns the value from fclose(3)
, or pclose(3)
, when closing
an output file or pipe, respectively. It returns the process’s exit status
when closing an input pipe. The return value is -1 if the named file, pipe
or co-process was not opened with a redirection.
When gawk is invoked with
the --traditional option, if the fs argument to the -F option is ‘t’, then FS
is set to the tab character. Note that typing gawk -F\t ... simply causes the
shell to quote the ‘t,’ and does not pass ‘\t’ to the -F option. Since this is
a rather ugly special case, it is not the default behavior. This behavior
also does not occur if --posix has been specified. To really get a tab character
as the field separator, it is best to use single quotes: gawk -F’\t’ ....
If gawk
is configured with the --enable-switch option to the configure command, then
it accepts an additional control-flow statement:
switch (expression) {
case value|regex : statement
...
[ default: statement ]
}
If gawk is configured with the --disable-directories-fatal option, then it
will silently skip directories named on the command line. Otherwise, it
will do so only if invoked with the --traditional option.
The
AWKPATH environment variable can be used to provide a list of directories
that gawk searches when looking for files named via the -f and --file options.
If POSIXLY_CORRECT exists in the environment, then gawk behaves exactly
as if --posix had been specified on the command line. If --lint has been specified,
gawk issues a warning message to this effect.
egrep(1)
, getpid(2)
,
getppid(2)
, getpgrp(2)
, getuid(2)
, geteuid(2)
, getgid(2)
, getegid(2)
, getgroups(2)
The AWK Programming Language, Alfred V. Aho, Brian W. Kernighan, Peter J.
Weinberger, Addison-Wesley, 1988. ISBN 0-201-07981-X.
P, Edition 3.0, published
by the Free Software Foundation, 2001. The current version of this document
is available online at http://www.gnu.org/software/gawk/manual.
Bugs
The -F
option is not necessary given the command line variable assignment feature;
it remains only for backwards compatibility.
Syntactically invalid single
character programs tend to overflow the parse stack, generating a rather
unhelpful message. Such programs are surprisingly difficult to diagnose
in the completely general case, and the effort to do so really is not worth
it.
The original version of awk was designed and implemented by
Alfred Aho, Peter Weinberger, and Brian Kernighan of Bell Laboratories.
Brian Kernighan continues to maintain and enhance it.
Paul Rubin and Jay
Fenlason, of the Free Software Foundation, wrote gawk, to be compatible
with the original version of awk distributed in Seventh Edition . John Woods
contributed a number of bug fixes. David Trueman, with contributions from
Arnold Robbins, made gawk compatible with the new version of awk. Arnold
Robbins is the current maintainer.
The initial DOS port was done by Conrad
Kwok and Scott Garfinkle. Scott Deifik is the current DOS maintainer. Pat
Rankin did the port to VMS, and Michal Jaegermann did the port to the Atari
ST. The port to OS/2 was done by Kai Uwe Rommel, with contributions and
help from Darrel Hankerson. Juan M. Guerrero now maintains the OS/2 port.
Fred Fish supplied support for the Amiga, and Martin Brown provided the
BeOS port. Stephen Davies provided the original Tandem port, and Matthew
Woehlke provided changes for Tandem’s POSIX-compliant systems.
This
man page documents gawk, version 3.1.6.
If you find a bug in gawk,
please send electronic mail to bug-gawk@gnu.org. Please include your operating
system and its revision, the version of gawk (from gawk --version), what
C compiler you used to compile it, and a test program and data that are
as small as possible for reproducing the problem.
Before sending a bug report,
please do the following things. First, verify that you have the latest
version of gawk. Many bugs (usually subtle ones) are fixed at each release,
and if yours is out of date, the problem may already have been solved. Second,
please see if setting the environment variable LC_ALL to LC_ALL=C causes
things to behave as you expect. If so, it’s a locale issue, and may or may
not really be a bug. Finally, please read this man page and the reference
manual carefully to be sure that what you think is a bug really is, instead
of just a quirk in the language.
Whatever you do, do NOT post a bug report
in comp.lang.awk. While the gawk developers occasionally read this newsgroup,
posting bug reports there is an unreliable way to report bugs. Instead,
please use the electronic mail addresses given above.
If you’re using a GNU/Linux
system or BSD-based system, you may wish to submit a bug report to the vendor
of your distribution. That’s fine, but please send a copy to the official
email address as well, since there’s no guarantee that the bug will be forwarded
to the gawk maintainer.
Brian Kernighan of Bell Laboratories
provided valuable assistance during testing and debugging. We thank him.
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Inc.
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Permission is granted to copy and distribute modified
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Permission is granted
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under the above conditions for modified versions, except that this permission
notice may be stated in a translation approved by the Foundation.
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