4.4BSD/usr/src/contrib/gawk-2.15.2/gawk.info-4
This is Info file gawk.info, produced by Makeinfo-1.54 from the input
file gawk.texi.
This file documents `awk', a program that you can use to select
particular records in a file and perform operations upon them.
This is Edition 0.15 of `The GAWK Manual',
for the 2.15 version of the GNU implementation
of AWK.
Copyright (C) 1989, 1991, 1992, 1993 Free Software Foundation, Inc.
Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.
Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided that
the entire resulting derived work is distributed under the terms of a
permission notice identical to this one.
Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that this permission notice may be stated in a
translation approved by the Foundation.
File: gawk.info, Node: Actions, Next: Expressions, Prev: Patterns, Up: Top
Overview of Actions
*******************
An `awk' program or script consists of a series of rules and
function definitions, interspersed. (Functions are described later.
*Note User-defined Functions: User-defined.)
A rule contains a pattern and an action, either of which may be
omitted. The purpose of the "action" is to tell `awk' what to do once
a match for the pattern is found. Thus, the entire program looks
somewhat like this:
[PATTERN] [{ ACTION }]
[PATTERN] [{ ACTION }]
...
function NAME (ARGS) { ... }
...
An action consists of one or more `awk' "statements", enclosed in
curly braces (`{' and `}'). Each statement specifies one thing to be
done. The statements are separated by newlines or semicolons.
The curly braces around an action must be used even if the action
contains only one statement, or even if it contains no statements at
all. However, if you omit the action entirely, omit the curly braces as
well. (An omitted action is equivalent to `{ print $0 }'.)
Here are the kinds of statements supported in `awk':
* Expressions, which can call functions or assign values to variables
(*note Expressions as Action Statements: Expressions.). Executing
this kind of statement simply computes the value of the expression
and then ignores it. This is useful when the expression has side
effects (*note Assignment Expressions: Assignment Ops.).
* Control statements, which specify the control flow of `awk'
programs. The `awk' language gives you C-like constructs (`if',
`for', `while', and so on) as well as a few special ones (*note
Control Statements in Actions: Statements.).
* Compound statements, which consist of one or more statements
enclosed in curly braces. A compound statement is used in order
to put several statements together in the body of an `if',
`while', `do' or `for' statement.
* Input control, using the `getline' command (*note Explicit Input
with `getline': Getline.), and the `next' statement (*note The
`next' Statement: Next Statement.).
* Output statements, `print' and `printf'. *Note Printing Output:
Printing.
* Deletion statements, for deleting array elements. *Note The
`delete' Statement: Delete.
File: gawk.info, Node: Expressions, Next: Statements, Prev: Actions, Up: Top
Expressions as Action Statements
********************************
Expressions are the basic building block of `awk' actions. An
expression evaluates to a value, which you can print, test, store in a
variable or pass to a function. But beyond that, an expression can
assign a new value to a variable or a field, with an assignment
operator.
An expression can serve as a statement on its own. Most other kinds
of statements contain one or more expressions which specify data to be
operated on. As in other languages, expressions in `awk' include
variables, array references, constants, and function calls, as well as
combinations of these with various operators.
* Menu:
* Constants:: String, numeric, and regexp constants.
* Variables:: Variables give names to values for later use.
* Arithmetic Ops:: Arithmetic operations (`+', `-', etc.)
* Concatenation:: Concatenating strings.
* Comparison Ops:: Comparison of numbers and strings
with `<', etc.
* Boolean Ops:: Combining comparison expressions
using boolean operators
`||' ("or"), `&&' ("and") and `!' ("not").
* Assignment Ops:: Changing the value of a variable or a field.
* Increment Ops:: Incrementing the numeric value of a variable.
* Conversion:: The conversion of strings to numbers
and vice versa.
* Values:: The whole truth about numbers and strings.
* Conditional Exp:: Conditional expressions select
between two subexpressions under control
of a third subexpression.
* Function Calls:: A function call is an expression.
* Precedence:: How various operators nest.
File: gawk.info, Node: Constants, Next: Variables, Prev: Expressions, Up: Expressions
Constant Expressions
====================
The simplest type of expression is the "constant", which always has
the same value. There are three types of constants: numeric constants,
string constants, and regular expression constants.
A "numeric constant" stands for a number. This number can be an
integer, a decimal fraction, or a number in scientific (exponential)
notation. Note that all numeric values are represented within `awk' in
double-precision floating point. Here are some examples of numeric
constants, which all have the same value:
105
1.05e+2
1050e-1
A string constant consists of a sequence of characters enclosed in
double-quote marks. For example:
"parrot"
represents the string whose contents are `parrot'. Strings in `gawk'
can be of any length and they can contain all the possible 8-bit ASCII
characters including ASCII NUL. Other `awk' implementations may have
difficulty with some character codes.
Some characters cannot be included literally in a string constant.
You represent them instead with "escape sequences", which are character
sequences beginning with a backslash (`\').
One use of an escape sequence is to include a double-quote character
in a string constant. Since a plain double-quote would end the string,
you must use `\"' to represent a single double-quote character as a
part of the string. The backslash character itself is another
character that cannot be included normally; you write `\\' to put one
backslash in the string. Thus, the string whose contents are the two
characters `"\' must be written `"\"\\"'.
Another use of backslash is to represent unprintable characters such
as newline. While there is nothing to stop you from writing most of
these characters directly in a string constant, they may look ugly.
Here is a table of all the escape sequences used in `awk':
`\\'
Represents a literal backslash, `\'.
`\a'
Represents the "alert" character, control-g, ASCII code 7.
`\b'
Represents a backspace, control-h, ASCII code 8.
`\f'
Represents a formfeed, control-l, ASCII code 12.
`\n'
Represents a newline, control-j, ASCII code 10.
`\r'
Represents a carriage return, control-m, ASCII code 13.
`\t'
Represents a horizontal tab, control-i, ASCII code 9.
`\v'
Represents a vertical tab, control-k, ASCII code 11.
`\NNN'
Represents the octal value NNN, where NNN are one to three digits
between 0 and 7. For example, the code for the ASCII ESC (escape)
character is `\033'.
`\xHH...'
Represents the hexadecimal value HH, where HH are hexadecimal
digits (`0' through `9' and either `A' through `F' or `a' through
`f'). Like the same construct in ANSI C, the escape sequence
continues until the first non-hexadecimal digit is seen. However,
using more than two hexadecimal digits produces undefined results.
(The `\x' escape sequence is not allowed in POSIX `awk'.)
A "constant regexp" is a regular expression description enclosed in
slashes, such as `/^beginning and end$/'. Most regexps used in `awk'
programs are constant, but the `~' and `!~' operators can also match
computed or "dynamic" regexps (*note How to Use Regular Expressions:
Regexp Usage.).
Constant regexps may be used like simple expressions. When a
constant regexp is not on the right hand side of the `~' or `!~'
operators, it has the same meaning as if it appeared in a pattern, i.e.
`($0 ~ /foo/)' (*note Expressions as Patterns: Expression Patterns.).
This means that the two code segments,
if ($0 ~ /barfly/ || $0 ~ /camelot/)
print "found"
and
if (/barfly/ || /camelot/)
print "found"
are exactly equivalent. One rather bizarre consequence of this rule is
that the following boolean expression is legal, but does not do what
the user intended:
if (/foo/ ~ $1) print "found foo"
This code is "obviously" testing `$1' for a match against the regexp
`/foo/'. But in fact, the expression `(/foo/ ~ $1)' actually means
`(($0 ~ /foo/) ~ $1)'. In other words, first match the input record
against the regexp `/foo/'. The result will be either a 0 or a 1,
depending upon the success or failure of the match. Then match that
result against the first field in the record.
Since it is unlikely that you would ever really wish to make this
kind of test, `gawk' will issue a warning when it sees this construct in
a program.
Another consequence of this rule is that the assignment statement
matches = /foo/
will assign either 0 or 1 to the variable `matches', depending upon the
contents of the current input record.
Constant regular expressions are also used as the first argument for
the `sub' and `gsub' functions (*note Built-in Functions for String
Manipulation: String Functions.).
This feature of the language was never well documented until the
POSIX specification.
You may be wondering, when is
$1 ~ /foo/ { ... }
preferable to
$1 ~ "foo" { ... }
Since the right-hand sides of both `~' operators are constants, it
is more efficient to use the `/foo/' form: `awk' can note that you have
supplied a regexp and store it internally in a form that makes pattern
matching more efficient. In the second form, `awk' must first convert
the string into this internal form, and then perform the pattern
matching. The first form is also better style; it shows clearly that
you intend a regexp match.
File: gawk.info, Node: Variables, Next: Arithmetic Ops, Prev: Constants, Up: Expressions
Variables
=========
Variables let you give names to values and refer to them later. You
have already seen variables in many of the examples. The name of a
variable must be a sequence of letters, digits and underscores, but it
may not begin with a digit. Case is significant in variable names; `a'
and `A' are distinct variables.
A variable name is a valid expression by itself; it represents the
variable's current value. Variables are given new values with
"assignment operators" and "increment operators". *Note Assignment
Expressions: Assignment Ops.
A few variables have special built-in meanings, such as `FS', the
field separator, and `NF', the number of fields in the current input
record. *Note Built-in Variables::, for a list of them. These
built-in variables can be used and assigned just like all other
variables, but their values are also used or changed automatically by
`awk'. Each built-in variable's name is made entirely of upper case
letters.
Variables in `awk' can be assigned either numeric or string values.
By default, variables are initialized to the null string, which is
effectively zero if converted to a number. There is no need to
"initialize" each variable explicitly in `awk', the way you would in C
or most other traditional languages.
* Menu:
* Assignment Options:: Setting variables on the command line
and a summary of command line syntax.
This is an advanced method of input.
File: gawk.info, Node: Assignment Options, Prev: Variables, Up: Variables
Assigning Variables on the Command Line
---------------------------------------
You can set any `awk' variable by including a "variable assignment"
among the arguments on the command line when you invoke `awk' (*note
Invoking `awk': Command Line.). Such an assignment has this form:
VARIABLE=TEXT
With it, you can set a variable either at the beginning of the `awk'
run or in between input files.
If you precede the assignment with the `-v' option, like this:
-v VARIABLE=TEXT
then the variable is set at the very beginning, before even the `BEGIN'
rules are run. The `-v' option and its assignment must precede all the
file name arguments, as well as the program text.
Otherwise, the variable assignment is performed at a time determined
by its position among the input file arguments: after the processing of
the preceding input file argument. For example:
awk '{ print $n }' n=4 inventory-shipped n=2 BBS-list
prints the value of field number `n' for all input records. Before the
first file is read, the command line sets the variable `n' equal to 4.
This causes the fourth field to be printed in lines from the file
`inventory-shipped'. After the first file has finished, but before the
second file is started, `n' is set to 2, so that the second field is
printed in lines from `BBS-list'.
Command line arguments are made available for explicit examination by
the `awk' program in an array named `ARGV' (*note Built-in
Variables::.).
`awk' processes the values of command line assignments for escape
sequences (*note Constant Expressions: Constants.).
File: gawk.info, Node: Arithmetic Ops, Next: Concatenation, Prev: Variables, Up: Expressions
Arithmetic Operators
====================
The `awk' language uses the common arithmetic operators when
evaluating expressions. All of these arithmetic operators follow normal
precedence rules, and work as you would expect them to. This example
divides field three by field four, adds field two, stores the result
into field one, and prints the resulting altered input record:
awk '{ $1 = $2 + $3 / $4; print }' inventory-shipped
The arithmetic operators in `awk' are:
`X + Y'
Addition.
`X - Y'
Subtraction.
`- X'
Negation.
`+ X'
Unary plus. No real effect on the expression.
`X * Y'
Multiplication.
`X / Y'
Division. Since all numbers in `awk' are double-precision
floating point, the result is not rounded to an integer: `3 / 4'
has the value 0.75.
`X % Y'
Remainder. The quotient is rounded toward zero to an integer,
multiplied by Y and this result is subtracted from X. This
operation is sometimes known as "trunc-mod." The following
relation always holds:
b * int(a / b) + (a % b) == a
One possibly undesirable effect of this definition of remainder is
that `X % Y' is negative if X is negative. Thus,
-17 % 8 = -1
In other `awk' implementations, the signedness of the remainder
may be machine dependent.
`X ^ Y'
`X ** Y'
Exponentiation: X raised to the Y power. `2 ^ 3' has the value 8.
The character sequence `**' is equivalent to `^'. (The POSIX
standard only specifies the use of `^' for exponentiation.)
File: gawk.info, Node: Concatenation, Next: Comparison Ops, Prev: Arithmetic Ops, Up: Expressions
String Concatenation
====================
There is only one string operation: concatenation. It does not have
a specific operator to represent it. Instead, concatenation is
performed by writing expressions next to one another, with no operator.
For example:
awk '{ print "Field number one: " $1 }' BBS-list
produces, for the first record in `BBS-list':
Field number one: aardvark
Without the space in the string constant after the `:', the line
would run together. For example:
awk '{ print "Field number one:" $1 }' BBS-list
produces, for the first record in `BBS-list':
Field number one:aardvark
Since string concatenation does not have an explicit operator, it is
often necessary to insure that it happens where you want it to by
enclosing the items to be concatenated in parentheses. For example, the
following code fragment does not concatenate `file' and `name' as you
might expect:
file = "file"
name = "name"
print "something meaningful" > file name
It is necessary to use the following:
print "something meaningful" > (file name)
We recommend you use parentheses around concatenation in all but the
most common contexts (such as in the right-hand operand of `=').
File: gawk.info, Node: Comparison Ops, Next: Boolean Ops, Prev: Concatenation, Up: Expressions
Comparison Expressions
======================
"Comparison expressions" compare strings or numbers for
relationships such as equality. They are written using "relational
operators", which are a superset of those in C. Here is a table of
them:
`X < Y'
True if X is less than Y.
`X <= Y'
True if X is less than or equal to Y.
`X > Y'
True if X is greater than Y.
`X >= Y'
True if X is greater than or equal to Y.
`X == Y'
True if X is equal to Y.
`X != Y'
True if X is not equal to Y.
`X ~ Y'
True if the string X matches the regexp denoted by Y.
`X !~ Y'
True if the string X does not match the regexp denoted by Y.
`SUBSCRIPT in ARRAY'
True if array ARRAY has an element with the subscript SUBSCRIPT.
Comparison expressions have the value 1 if true and 0 if false.
The rules `gawk' uses for performing comparisons are based on those
in draft 11.2 of the POSIX standard. The POSIX standard introduced the
concept of a "numeric string", which is simply a string that looks like
a number, for example, `" +2"'.
When performing a relational operation, `gawk' considers the type of
an operand to be the type it received on its last *assignment*, rather
than the type of its last *use* (*note Numeric and String Values:
Values.). This type is *unknown* when the operand is from an
"external" source: field variables, command line arguments, array
elements resulting from a `split' operation, and the value of an
`ENVIRON' element. In this case only, if the operand is a numeric
string, then it is considered to be of both string type and numeric
type. If at least one operand of a comparison is of string type only,
then a string comparison is performed. Any numeric operand will be
converted to a string using the value of `CONVFMT' (*note Conversion of
Strings and Numbers: Conversion.). If one operand of a comparison is
numeric, and the other operand is either numeric or both numeric and
string, then `gawk' does a numeric comparison. If both operands have
both types, then the comparison is numeric. Strings are compared by
comparing the first character of each, then the second character of
each, and so on. Thus `"10"' is less than `"9"'. If there are two
strings where one is a prefix of the other, the shorter string is less
than the longer one. Thus `"abc"' is less than `"abcd"'.
Here are some sample expressions, how `gawk' compares them, and what
the result of the comparison is.
`1.5 <= 2.0'
numeric comparison (true)
`"abc" >= "xyz"'
string comparison (false)
`1.5 != " +2"'
string comparison (true)
`"1e2" < "3"'
string comparison (true)
`a = 2; b = "2"'
`a == b'
string comparison (true)
echo 1e2 3 | awk '{ print ($1 < $2) ? "true" : "false" }'
prints `false' since both `$1' and `$2' are numeric strings and thus
have both string and numeric types, thus dictating a numeric comparison.
The purpose of the comparison rules and the use of numeric strings is
to attempt to produce the behavior that is "least surprising," while
still "doing the right thing."
String comparisons and regular expression comparisons are very
different. For example,
$1 == "foo"
has the value of 1, or is true, if the first field of the current input
record is precisely `foo'. By contrast,
$1 ~ /foo/
has the value 1 if the first field contains `foo', such as `foobar'.
The right hand operand of the `~' and `!~' operators may be either a
constant regexp (`/.../'), or it may be an ordinary expression, in
which case the value of the expression as a string is a dynamic regexp
(*note How to Use Regular Expressions: Regexp Usage.).
In very recent implementations of `awk', a constant regular
expression in slashes by itself is also an expression. The regexp
`/REGEXP/' is an abbreviation for this comparison expression:
$0 ~ /REGEXP/
In some contexts it may be necessary to write parentheses around the
regexp to avoid confusing the `gawk' parser. For example, `(/x/ - /y/)
> threshold' is not allowed, but `((/x/) - (/y/)) > threshold' parses
properly.
One special place where `/foo/' is *not* an abbreviation for `$0 ~
/foo/' is when it is the right-hand operand of `~' or `!~'! *Note
Constant Expressions: Constants, where this is discussed in more detail.
File: gawk.info, Node: Boolean Ops, Next: Assignment Ops, Prev: Comparison Ops, Up: Expressions
Boolean Expressions
===================
A "boolean expression" is a combination of comparison expressions or
matching expressions, using the boolean operators "or" (`||'), "and"
(`&&'), and "not" (`!'), along with parentheses to control nesting.
The truth of the boolean expression is computed by combining the truth
values of the component expressions.
Boolean expressions can be used wherever comparison and matching
expressions can be used. They can be used in `if', `while' `do' and
`for' statements. They have numeric values (1 if true, 0 if false),
which come into play if the result of the boolean expression is stored
in a variable, or used in arithmetic.
In addition, every boolean expression is also a valid boolean
pattern, so you can use it as a pattern to control the execution of
rules.
Here are descriptions of the three boolean operators, with an
example of each. It may be instructive to compare these examples with
the analogous examples of boolean patterns (*note Boolean Operators and
Patterns: Boolean Patterns.), which use the same boolean operators in
patterns instead of expressions.
`BOOLEAN1 && BOOLEAN2'
True if both BOOLEAN1 and BOOLEAN2 are true. For example, the
following statement prints the current input record if it contains
both `2400' and `foo'.
if ($0 ~ /2400/ && $0 ~ /foo/) print
The subexpression BOOLEAN2 is evaluated only if BOOLEAN1 is true.
This can make a difference when BOOLEAN2 contains expressions that
have side effects: in the case of `$0 ~ /foo/ && ($2 == bar++)',
the variable `bar' is not incremented if there is no `foo' in the
record.
`BOOLEAN1 || BOOLEAN2'
True if at least one of BOOLEAN1 or BOOLEAN2 is true. For
example, the following command prints all records in the input
file `BBS-list' that contain *either* `2400' or `foo', or both.
awk '{ if ($0 ~ /2400/ || $0 ~ /foo/) print }' BBS-list
The subexpression BOOLEAN2 is evaluated only if BOOLEAN1 is false.
This can make a difference when BOOLEAN2 contains expressions
that have side effects.
`!BOOLEAN'
True if BOOLEAN is false. For example, the following program
prints all records in the input file `BBS-list' that do *not*
contain the string `foo'.
awk '{ if (! ($0 ~ /foo/)) print }' BBS-list
File: gawk.info, Node: Assignment Ops, Next: Increment Ops, Prev: Boolean Ops, Up: Expressions
Assignment Expressions
======================
An "assignment" is an expression that stores a new value into a
variable. For example, let's assign the value 1 to the variable `z':
z = 1
After this expression is executed, the variable `z' has the value 1.
Whatever old value `z' had before the assignment is forgotten.
Assignments can store string values also. For example, this would
store the value `"this food is good"' in the variable `message':
thing = "food"
predicate = "good"
message = "this " thing " is " predicate
(This also illustrates concatenation of strings.)
The `=' sign is called an "assignment operator". It is the simplest
assignment operator because the value of the right-hand operand is
stored unchanged.
Most operators (addition, concatenation, and so on) have no effect
except to compute a value. If you ignore the value, you might as well
not use the operator. An assignment operator is different; it does
produce a value, but even if you ignore the value, the assignment still
makes itself felt through the alteration of the variable. We call this
a "side effect".
The left-hand operand of an assignment need not be a variable (*note
Variables::.); it can also be a field (*note Changing the Contents of a
Field: Changing Fields.) or an array element (*note Arrays in `awk':
Arrays.). These are all called "lvalues", which means they can appear
on the left-hand side of an assignment operator. The right-hand
operand may be any expression; it produces the new value which the
assignment stores in the specified variable, field or array element.
It is important to note that variables do *not* have permanent types.
The type of a variable is simply the type of whatever value it happens
to hold at the moment. In the following program fragment, the variable
`foo' has a numeric value at first, and a string value later on:
foo = 1
print foo
foo = "bar"
print foo
When the second assignment gives `foo' a string value, the fact that it
previously had a numeric value is forgotten.
An assignment is an expression, so it has a value: the same value
that is assigned. Thus, `z = 1' as an expression has the value 1. One
consequence of this is that you can write multiple assignments together:
x = y = z = 0
stores the value 0 in all three variables. It does this because the
value of `z = 0', which is 0, is stored into `y', and then the value of
`y = z = 0', which is 0, is stored into `x'.
You can use an assignment anywhere an expression is called for. For
example, it is valid to write `x != (y = 1)' to set `y' to 1 and then
test whether `x' equals 1. But this style tends to make programs hard
to read; except in a one-shot program, you should rewrite it to get rid
of such nesting of assignments. This is never very hard.
Aside from `=', there are several other assignment operators that do
arithmetic with the old value of the variable. For example, the
operator `+=' computes a new value by adding the right-hand value to
the old value of the variable. Thus, the following assignment adds 5
to the value of `foo':
foo += 5
This is precisely equivalent to the following:
foo = foo + 5
Use whichever one makes the meaning of your program clearer.
Here is a table of the arithmetic assignment operators. In each
case, the right-hand operand is an expression whose value is converted
to a number.
`LVALUE += INCREMENT'
Adds INCREMENT to the value of LVALUE to make the new value of
LVALUE.
`LVALUE -= DECREMENT'
Subtracts DECREMENT from the value of LVALUE.
`LVALUE *= COEFFICIENT'
Multiplies the value of LVALUE by COEFFICIENT.
`LVALUE /= QUOTIENT'
Divides the value of LVALUE by QUOTIENT.
`LVALUE %= MODULUS'
Sets LVALUE to its remainder by MODULUS.
`LVALUE ^= POWER'
`LVALUE **= POWER'
Raises LVALUE to the power POWER. (Only the `^=' operator is
specified by POSIX.)
File: gawk.info, Node: Increment Ops, Next: Conversion, Prev: Assignment Ops, Up: Expressions
Increment Operators
===================
"Increment operators" increase or decrease the value of a variable
by 1. You could do the same thing with an assignment operator, so the
increment operators add no power to the `awk' language; but they are
convenient abbreviations for something very common.
The operator to add 1 is written `++'. It can be used to increment
a variable either before or after taking its value.
To pre-increment a variable V, write `++V'. This adds 1 to the
value of V and that new value is also the value of this expression.
The assignment expression `V += 1' is completely equivalent.
Writing the `++' after the variable specifies post-increment. This
increments the variable value just the same; the difference is that the
value of the increment expression itself is the variable's *old* value.
Thus, if `foo' has the value 4, then the expression `foo++' has the
value 4, but it changes the value of `foo' to 5.
The post-increment `foo++' is nearly equivalent to writing `(foo +=
1) - 1'. It is not perfectly equivalent because all numbers in `awk'
are floating point: in floating point, `foo + 1 - 1' does not
necessarily equal `foo'. But the difference is minute as long as you
stick to numbers that are fairly small (less than a trillion).
Any lvalue can be incremented. Fields and array elements are
incremented just like variables. (Use `$(i++)' when you wish to do a
field reference and a variable increment at the same time. The
parentheses are necessary because of the precedence of the field
reference operator, `$'.)
The decrement operator `--' works just like `++' except that it
subtracts 1 instead of adding. Like `++', it can be used before the
lvalue to pre-decrement or after it to post-decrement.
Here is a summary of increment and decrement expressions.
`++LVALUE'
This expression increments LVALUE and the new value becomes the
value of this expression.
`LVALUE++'
This expression causes the contents of LVALUE to be incremented.
The value of the expression is the *old* value of LVALUE.
`--LVALUE'
Like `++LVALUE', but instead of adding, it subtracts. It
decrements LVALUE and delivers the value that results.
`LVALUE--'
Like `LVALUE++', but instead of adding, it subtracts. It
decrements LVALUE. The value of the expression is the *old* value
of LVALUE.
File: gawk.info, Node: Conversion, Next: Values, Prev: Increment Ops, Up: Expressions
Conversion of Strings and Numbers
=================================
Strings are converted to numbers, and numbers to strings, if the
context of the `awk' program demands it. For example, if the value of
either `foo' or `bar' in the expression `foo + bar' happens to be a
string, it is converted to a number before the addition is performed.
If numeric values appear in string concatenation, they are converted to
strings. Consider this:
two = 2; three = 3
print (two three) + 4
This eventually prints the (numeric) value 27. The numeric values of
the variables `two' and `three' are converted to strings and
concatenated together, and the resulting string is converted back to the
number 23, to which 4 is then added.
If, for some reason, you need to force a number to be converted to a
string, concatenate the null string with that number. To force a string
to be converted to a number, add zero to that string.
A string is converted to a number by interpreting a numeric prefix
of the string as numerals: `"2.5"' converts to 2.5, `"1e3"' converts to
1000, and `"25fix"' has a numeric value of 25. Strings that can't be
interpreted as valid numbers are converted to zero.
The exact manner in which numbers are converted into strings is
controlled by the `awk' built-in variable `CONVFMT' (*note Built-in
Variables::.). Numbers are converted using a special version of the
`sprintf' function (*note Built-in Functions: Built-in.) with `CONVFMT'
as the format specifier.
`CONVFMT''s default value is `"%.6g"', which prints a value with at
least six significant digits. For some applications you will want to
change it to specify more precision. Double precision on most modern
machines gives you 16 or 17 decimal digits of precision.
Strange results can happen if you set `CONVFMT' to a string that
doesn't tell `sprintf' how to format floating point numbers in a useful
way. For example, if you forget the `%' in the format, all numbers
will be converted to the same constant string.
As a special case, if a number is an integer, then the result of
converting it to a string is *always* an integer, no matter what the
value of `CONVFMT' may be. Given the following code fragment:
CONVFMT = "%2.2f"
a = 12
b = a ""
`b' has the value `"12"', not `"12.00"'.
Prior to the POSIX standard, `awk' specified that the value of
`OFMT' was used for converting numbers to strings. `OFMT' specifies
the output format to use when printing numbers with `print'. `CONVFMT'
was introduced in order to separate the semantics of conversions from
the semantics of printing. Both `CONVFMT' and `OFMT' have the same
default value: `"%.6g"'. In the vast majority of cases, old `awk'
programs will not change their behavior. However, this use of `OFMT'
is something to keep in mind if you must port your program to other
implementations of `awk'; we recommend that instead of changing your
programs, you just port `gawk' itself!
File: gawk.info, Node: Values, Next: Conditional Exp, Prev: Conversion, Up: Expressions
Numeric and String Values
=========================
Through most of this manual, we present `awk' values (such as
constants, fields, or variables) as *either* numbers *or* strings.
This is a convenient way to think about them, since typically they are
used in only one way, or the other.
In truth though, `awk' values can be *both* string and numeric, at
the same time. Internally, `awk' represents values with a string, a
(floating point) number, and an indication that one, the other, or both
representations of the value are valid.
Keeping track of both kinds of values is important for execution
efficiency: a variable can acquire a string value the first time it is
used as a string, and then that string value can be used until the
variable is assigned a new value. Thus, if a variable with only a
numeric value is used in several concatenations in a row, it only has
to be given a string representation once. The numeric value remains
valid, so that no conversion back to a number is necessary if the
variable is later used in an arithmetic expression.
Tracking both kinds of values is also important for precise numerical
calculations. Consider the following:
a = 123.321
CONVFMT = "%3.1f"
b = a " is a number"
c = a + 1.654
The variable `a' receives a string value in the concatenation and
assignment to `b'. The string value of `a' is `"123.3"'. If the
numeric value was lost when it was converted to a string, then the
numeric use of `a' in the last statement would lose information. `c'
would be assigned the value 124.954 instead of 124.975. Such errors
accumulate rapidly, and very adversely affect numeric computations.
Once a numeric value acquires a corresponding string value, it stays
valid until a new assignment is made. If `CONVFMT' (*note Conversion
of Strings and Numbers: Conversion.) changes in the meantime, the old
string value will still be used. For example:
BEGIN {
CONVFMT = "%2.2f"
a = 123.456
b = a "" # force `a' to have string value too
printf "a = %s\n", a
CONVFMT = "%.6g"
printf "a = %s\n", a
a += 0 # make `a' numeric only again
printf "a = %s\n", a # use `a' as string
}
This program prints `a = 123.46' twice, and then prints `a = 123.456'.
*Note Conversion of Strings and Numbers: Conversion, for the rules
that specify how string values are made from numeric values.
File: gawk.info, Node: Conditional Exp, Next: Function Calls, Prev: Values, Up: Expressions
Conditional Expressions
=======================
A "conditional expression" is a special kind of expression with
three operands. It allows you to use one expression's value to select
one of two other expressions.
The conditional expression looks the same as in the C language:
SELECTOR ? IF-TRUE-EXP : IF-FALSE-EXP
There are three subexpressions. The first, SELECTOR, is always
computed first. If it is "true" (not zero and not null) then
IF-TRUE-EXP is computed next and its value becomes the value of the
whole expression. Otherwise, IF-FALSE-EXP is computed next and its
value becomes the value of the whole expression.
For example, this expression produces the absolute value of `x':
x > 0 ? x : -x
Each time the conditional expression is computed, exactly one of
IF-TRUE-EXP and IF-FALSE-EXP is computed; the other is ignored. This
is important when the expressions contain side effects. For example,
this conditional expression examines element `i' of either array `a' or
array `b', and increments `i'.
x == y ? a[i++] : b[i++]
This is guaranteed to increment `i' exactly once, because each time one
or the other of the two increment expressions is executed, and the
other is not.
File: gawk.info, Node: Function Calls, Next: Precedence, Prev: Conditional Exp, Up: Expressions
Function Calls
==============
A "function" is a name for a particular calculation. Because it has
a name, you can ask for it by name at any point in the program. For
example, the function `sqrt' computes the square root of a number.
A fixed set of functions are "built-in", which means they are
available in every `awk' program. The `sqrt' function is one of these.
*Note Built-in Functions: Built-in, for a list of built-in functions
and their descriptions. In addition, you can define your own functions
in the program for use elsewhere in the same program. *Note
User-defined Functions: User-defined, for how to do this.
The way to use a function is with a "function call" expression,
which consists of the function name followed by a list of "arguments"
in parentheses. The arguments are expressions which give the raw
materials for the calculation that the function will do. When there is
more than one argument, they are separated by commas. If there are no
arguments, write just `()' after the function name. Here are some
examples:
sqrt(x^2 + y^2) # One argument
atan2(y, x) # Two arguments
rand() # No arguments
*Do not put any space between the function name and the
open-parenthesis!* A user-defined function name looks just like the
name of a variable, and space would make the expression look like
concatenation of a variable with an expression inside parentheses.
Space before the parenthesis is harmless with built-in functions, but
it is best not to get into the habit of using space to avoid mistakes
with user-defined functions.
Each function expects a particular number of arguments. For
example, the `sqrt' function must be called with a single argument, the
number to take the square root of:
sqrt(ARGUMENT)
Some of the built-in functions allow you to omit the final argument.
If you do so, they use a reasonable default. *Note Built-in Functions:
Built-in, for full details. If arguments are omitted in calls to
user-defined functions, then those arguments are treated as local
variables, initialized to the null string (*note User-defined
Functions: User-defined.).
Like every other expression, the function call has a value, which is
computed by the function based on the arguments you give it. In this
example, the value of `sqrt(ARGUMENT)' is the square root of the
argument. A function can also have side effects, such as assigning the
values of certain variables or doing I/O.
Here is a command to read numbers, one number per line, and print the
square root of each one:
awk '{ print "The square root of", $1, "is", sqrt($1) }'
File: gawk.info, Node: Precedence, Prev: Function Calls, Up: Expressions
Operator Precedence (How Operators Nest)
========================================
"Operator precedence" determines how operators are grouped, when
different operators appear close by in one expression. For example,
`*' has higher precedence than `+'; thus, `a + b * c' means to multiply
`b' and `c', and then add `a' to the product (i.e., `a + (b * c)').
You can overrule the precedence of the operators by using
parentheses. You can think of the precedence rules as saying where the
parentheses are assumed if you do not write parentheses yourself. In
fact, it is wise to always use parentheses whenever you have an unusual
combination of operators, because other people who read the program may
not remember what the precedence is in this case. You might forget,
too; then you could make a mistake. Explicit parentheses will help
prevent any such mistake.
When operators of equal precedence are used together, the leftmost
operator groups first, except for the assignment, conditional and
exponentiation operators, which group in the opposite order. Thus, `a
- b + c' groups as `(a - b) + c'; `a = b = c' groups as `a = (b = c)'.
The precedence of prefix unary operators does not matter as long as
only unary operators are involved, because there is only one way to
parse them--innermost first. Thus, `$++i' means `$(++i)' and `++$x'
means `++($x)'. However, when another operator follows the operand,
then the precedence of the unary operators can matter. Thus, `$x^2'
means `($x)^2', but `-x^2' means `-(x^2)', because `-' has lower
precedence than `^' while `$' has higher precedence.
Here is a table of the operators of `awk', in order of increasing
precedence:
assignment
`=', `+=', `-=', `*=', `/=', `%=', `^=', `**='. These operators
group right-to-left. (The `**=' operator is not specified by
POSIX.)
conditional
`?:'. This operator groups right-to-left.
logical "or".
`||'.
logical "and".
`&&'.
array membership
`in'.
matching
`~', `!~'.
relational, and redirection
The relational operators and the redirections have the same
precedence level. Characters such as `>' serve both as
relationals and as redirections; the context distinguishes between
the two meanings.
The relational operators are `<', `<=', `==', `!=', `>=' and `>'.
The I/O redirection operators are `<', `>', `>>' and `|'.
Note that I/O redirection operators in `print' and `printf'
statements belong to the statement level, not to expressions. The
redirection does not produce an expression which could be the
operand of another operator. As a result, it does not make sense
to use a redirection operator near another operator of lower
precedence, without parentheses. Such combinations, for example
`print foo > a ? b : c', result in syntax errors.
concatenation
No special token is used to indicate concatenation. The operands
are simply written side by side.
add, subtract
`+', `-'.
multiply, divide, mod
`*', `/', `%'.
unary plus, minus, "not"
`+', `-', `!'.
exponentiation
`^', `**'. These operators group right-to-left. (The `**'
operator is not specified by POSIX.)
increment, decrement
`++', `--'.
field
`$'.
File: gawk.info, Node: Statements, Next: Arrays, Prev: Expressions, Up: Top
Control Statements in Actions
*****************************
"Control statements" such as `if', `while', and so on control the
flow of execution in `awk' programs. Most of the control statements in
`awk' are patterned on similar statements in C.
All the control statements start with special keywords such as `if'
and `while', to distinguish them from simple expressions.
Many control statements contain other statements; for example, the
`if' statement contains another statement which may or may not be
executed. The contained statement is called the "body". If you want
to include more than one statement in the body, group them into a
single compound statement with curly braces, separating them with
newlines or semicolons.
* Menu:
* If Statement:: Conditionally execute
some `awk' statements.
* While Statement:: Loop until some condition is satisfied.
* Do Statement:: Do specified action while looping until some
condition is satisfied.
* For Statement:: Another looping statement, that provides
initialization and increment clauses.
* Break Statement:: Immediately exit the innermost enclosing loop.
* Continue Statement:: Skip to the end of the innermost
enclosing loop.
* Next Statement:: Stop processing the current input record.
* Next File Statement:: Stop processing the current file.
* Exit Statement:: Stop execution of `awk'.
File: gawk.info, Node: If Statement, Next: While Statement, Prev: Statements, Up: Statements
The `if' Statement
==================
The `if'-`else' statement is `awk''s decision-making statement. It
looks like this:
if (CONDITION) THEN-BODY [else ELSE-BODY]
CONDITION is an expression that controls what the rest of the statement
will do. If CONDITION is true, THEN-BODY is executed; otherwise,
ELSE-BODY is executed (assuming that the `else' clause is present).
The `else' part of the statement is optional. The condition is
considered false if its value is zero or the null string, and true
otherwise.
Here is an example:
if (x % 2 == 0)
print "x is even"
else
print "x is odd"
In this example, if the expression `x % 2 == 0' is true (that is,
the value of `x' is divisible by 2), then the first `print' statement
is executed, otherwise the second `print' statement is performed.
If the `else' appears on the same line as THEN-BODY, and THEN-BODY
is not a compound statement (i.e., not surrounded by curly braces),
then a semicolon must separate THEN-BODY from `else'. To illustrate
this, let's rewrite the previous example:
awk '{ if (x % 2 == 0) print "x is even"; else
print "x is odd" }'
If you forget the `;', `awk' won't be able to parse the statement, and
you will get a syntax error.
We would not actually write this example this way, because a human
reader might fail to see the `else' if it were not the first thing on
its line.
File: gawk.info, Node: While Statement, Next: Do Statement, Prev: If Statement, Up: Statements
The `while' Statement
=====================
In programming, a "loop" means a part of a program that is (or at
least can be) executed two or more times in succession.
The `while' statement is the simplest looping statement in `awk'.
It repeatedly executes a statement as long as a condition is true. It
looks like this:
while (CONDITION)
BODY
Here BODY is a statement that we call the "body" of the loop, and
CONDITION is an expression that controls how long the loop keeps
running.
The first thing the `while' statement does is test CONDITION. If
CONDITION is true, it executes the statement BODY. (CONDITION is true
when the value is not zero and not a null string.) After BODY has been
executed, CONDITION is tested again, and if it is still true, BODY is
executed again. This process repeats until CONDITION is no longer
true. If CONDITION is initially false, the body of the loop is never
executed.
This example prints the first three fields of each record, one per
line.
awk '{ i = 1
while (i <= 3) {
print $i
i++
}
}'
Here the body of the loop is a compound statement enclosed in braces,
containing two statements.
The loop works like this: first, the value of `i' is set to 1.
Then, the `while' tests whether `i' is less than or equal to three.
This is the case when `i' equals one, so the `i'-th field is printed.
Then the `i++' increments the value of `i' and the loop repeats. The
loop terminates when `i' reaches 4.
As you can see, a newline is not required between the condition and
the body; but using one makes the program clearer unless the body is a
compound statement or is very simple. The newline after the open-brace
that begins the compound statement is not required either, but the
program would be hard to read without it.
File: gawk.info, Node: Do Statement, Next: For Statement, Prev: While Statement, Up: Statements
The `do'-`while' Statement
==========================
The `do' loop is a variation of the `while' looping statement. The
`do' loop executes the BODY once, then repeats BODY as long as
CONDITION is true. It looks like this:
do
BODY
while (CONDITION)
Even if CONDITION is false at the start, BODY is executed at least
once (and only once, unless executing BODY makes CONDITION true).
Contrast this with the corresponding `while' statement:
while (CONDITION)
BODY
This statement does not execute BODY even once if CONDITION is false to
begin with.
Here is an example of a `do' statement:
awk '{ i = 1
do {
print $0
i++
} while (i <= 10)
}'
prints each input record ten times. It isn't a very realistic example,
since in this case an ordinary `while' would do just as well. But this
reflects actual experience; there is only occasionally a real use for a
`do' statement.