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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.

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:


A string constant consists of a sequence of characters enclosed in double-quote marks. For example:


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, `\'.

Represents the "alert" character, control-g, ASCII code 7.

Represents a backspace, control-h, ASCII code 8.

Represents a formfeed, control-l, ASCII code 12.

Represents a newline, control-j, ASCII code 10.

Represents a carriage return, control-m, ASCII code 13.

Represents a horizontal tab, control-i, ASCII code 9.

Represents a vertical tab, control-k, ASCII code 11.

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'.

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 (see section How to Use Regular Expressions).

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/)' (see section Expressions as Patterns). This means that the two code segments,

if ($0 ~ /barfly/ || $0 ~ /camelot/)
    print "found"


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 (see section Built-in Functions for String Manipulation).

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.


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. See section Assignment Expressions.

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. See section 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.

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 (see section Invoking awk). Such an assignment has this form:


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 (see section Built-in Variables).

awk processes the values of command line assignments for escape sequences (see section Constant 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

x - y

- x

+ x
Unary plus. No real effect on the expression.

x * y

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.)

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 `=').

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 (see section Numeric and String 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 (see section Conversion of Strings and Numbers). 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 (see section How to Use Regular Expressions).

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 `!~'! See section Constant Expressions, where this is discussed in more detail.

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 (see section Boolean Operators and 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.

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

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 (see section Variables); it can also be a field (see section Changing the Contents of a Field) or an array element (see section Arrays in awk). 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.)

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.

This expression increments lvalue and the new value becomes the value of this expression.

This expression causes the contents of lvalue to be incremented. The value of the expression is the old value of lvalue.

Like ++lvalue, but instead of adding, it subtracts. It decrements lvalue and delivers the value that results.

Like lvalue++, but instead of adding, it subtracts. It decrements lvalue. The value of the expression is the old value of lvalue.

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 (see section Built-in Variables). Numbers are converted using a special version of the sprintf function (see section Built-in Functions) 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!

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 (see section Conversion of Strings and Numbers) changes in the meantime, the old string value will still be used. For example:

    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'.

See section Conversion of Strings and Numbers, for the rules that specify how string values are made from numeric values.

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.

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. See section Built-in Functions, 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. See section User-defined Functions, 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:


Some of the built-in functions allow you to omit the final argument. If you do so, they use a reasonable default. See section Built-in Functions, 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 (see section User-defined Functions).

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) }'

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:

`=', `+=', `-=', `*=', `/=', `%=', `^=', `**='. These operators group right-to-left. (The `**=' operator is not specified by POSIX.)

`?:'. This operator groups right-to-left.

logical "or".

logical "and".

array membership

`~', `!~'.

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.

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"
`+', `-', `!'.

`^', `**'. These operators group right-to-left. (The `**' operator is not specified by POSIX.)

increment, decrement
`++', `--'.


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