gawk.hlp

早期freebsd实现

 Each action can contain more than one statement or expression to be
 executed, provided that they're separated by semicolons (;) and/or
 on separate lines.

 An omitted action is equivalent to
       { print $0 }
 which prints the current record.
3 operators
 Relational operators
    ==    compare for equality
    !=    compare for inequality
    <, <=, >, >=  numerical or lexical comparison (less than, less or
                    equal, greater than, greater or equal, respectively)
    ~     match against a regular expression
    !~    match against a regular expression, but accept failed matches
            instead of successful ones
 Arithmetic operators
    +     addition
    -     subtraction
    *     multiplication
    /     division
    %     remainder
    ^, ** exponentiation ('**' is a synonym for '^', unless POSIX
            compatibility is specified, in which case it's invalid)
 Boolean operators (aka Logical operators)
          a value is considered false if it's 0 or a null string,
            it is true otherwise; the result of a boolean operation
            (and also of a comparison operation) will be 0 when false
            or 1 when true
    ||    or [expression (a || b) is true if either a is true or b
            is true or both a and b are true; it is false otherwise;
            b is not evaluated unless a is false (ie, short-circuit)]
    &&    and [expression (a && b) is true if both a and b are true;
            it is false otherwise; b is only evaluated if a is true]
    !     not [expression (!a) is true if a is false, false otherwise]
    in    array membership; the keyword 'in' tests whether the value
            on the left represents a current subscript in the array
            named on the right
 Conditional operator
    ? :   the conditional operator takes three operands; the first is
            an expression to evaluate, the second is the expression to
            use if the first was true, the third is the expression to
            use if it was false [simple example (a < b ? b : a) gives
            the maximum of a and b]
 Assignment operators
    =     store the value on the right into the variable or array slot
            on the left [expression (a = b) stores the value of b in a]
    +=, -=, *=, /=, %=, ^=, **=  perform the indicated arithmetic
           operation using the current value of the variable or array
            element of the left side and the expression on the right
            side, then store the result in the left side
    ++    increment by 1 [expression (++a) gets the current value of
            a and adds 1 to it, stores that back in a, and returns the
            new value; expression (a++) gets the current value of a,
            adds 1 to it, stores that back in a, but returns the
            original value of a]
    --    decrement by 1 (analogous to increment)
 String operators
          there is no explicit operator for string concatenation;
            two values and/or variables side-by-side are implicitly
            concatenated into a string (numeric values are first
            converted into their string equivalents)
 Conversion between numeric and string values
          there is no explicit operator for conversion; adding 0
            to a string with force it to be converted to a number
            (the numeric value will be 0 if the string does not
            represent an integer or floating point number); the
            reverse, converting a number into a string, is done by
            concatenating a null string ("") to it [the expression
            (5.75 "") evaluates to "5.75"]
 Field 'operator'
    $     prefixing a number or variable with a dollar sign ($)
            causes the appropriate record field to be returned [($2)
            gives the second field of the record, ($NF) gives the
            last field (since the builtin variable NF is set to the
            number of fields in the current record)]
 Array subscript operator
    ,     multi-dimensional arrays are simulated by using comma (,)
            separated array indices; the actual index is generated
            by replacing commas with the value of builtin SUBSEP,
            then concatenating the expression into a string index
          [comma is also used to separate arguments in function
            calls and user-defined function definitions]
          [comma is *also* used to indicate a range pattern in an
            awk rule]
 Escape 'operator'
    \     In quoted character strings, the backslash (\) character
            causes the following character to be interpreted in a
            special manner [string "one\ntwo" has an embedded newline
            character (linefeed on VMS, but treated as if it were both
            carriage-return and linefeed); string "\033[" has an ASCII
            'escape' character (which has octal value 033) followed by
            a 'right-bracket' character]
          Backslash is also used in regular expressions
 Redirection operators
    <     Read-from -- valid with 'getline'
    >     Write-to (create new file) -- valid with 'print' and 'printf'
    >>    Append-to (create file if it doesn't already exist)
    |     Pipe-from/to -- valid with 'getline', 'print', and 'printf'
4 precedence
 Operator precedence, listed from highest to lowest.  Assignment,
 conditional, and exponentiation operators group from right to left;
 all others group from left to right.  Parentheses may be used to
 override the normal order.

     field ($)
     increment (++), decrement (--)
     exponentiation (^, **)
     unary plus (+), unary minus (-), boolean not (!)
     multiplication (*), division (/), remainder (%)
     addition (+), subtraction (-)
     concatenation (no special symbol; implied by context)
     relational (==, !=, <, >=, etc), and redirection (<, >, >>, |)
       Relational and redirection operators have the same precedence
       and use similar symbols; context distinguishes between them
     matching (~, !~)
     array membership ('in')
     boolean and (&&)
     boolean or (||)
     conditional (? :)
     assignment (=, +=, etc)
4 escaped_characters
 Inside of a quoted string or constant regular expression, the
 backslash (\) character gives special meaning to the character(s)
 after it.  Special character letters are case sensitive.
    \\    results in one backslash in the string
    \a    is an 'alert' (<ctrl/G>. the ASCII <bell> character)
    \b    is a backspace (BS, <ctrl/H>)
    \f    is a form feed (FF, <ctrl/L>)
    \n    'newline' (<ctrl/J> [line feed treated as CR+LF]
    \r    carriage return (CR, <ctrl/M> [re-positions at the
            beginning of the current line]
    \t    tab (HT, <ctrl/I>)
    \v    vertical tab (VT, <ctrl/K>)
    \###  is an arbitrary character, where '###' represents 1 to 3
            octal (ie, 0 thru 7) digits
    \x##  is an alternate arbitrary character, where '##' represents
            1 or more hexadecimal (ie, 0 thru 9 and/or A through E
            and/or a through e) digits; if more than two digits
            follow, the result is undefined; not recognized if POSIX
            compatibility mode is specified.
3 statements
 A statement refers to a unit of instruction found in the action
 part of an awk rule, and also found in the definition of a function.
 The distinction between action, statement, and expression usually
 won't matter to an awk programmer.

 Compound statements consist of multiple statements separated by
 semicolons or newlines and enclosed within braces ({}).  They are
 sometimes referred to as 'blocks'.
4 expressions
 An expression such as 'a = 10' or 'n += i++' is a valid statement.

 Function invocations such as 'reformat_field($3)' are also valid
 statements.
4 if-then-else
 A conditional statement in awk uses the same syntax as for the 'C'
 programming language:  the 'if' keyword, followed by an expression
 in parentheses, followed by a statement--or block of statements
 enclosed within braces ({})--which will be executed if the expression
 is true but skipped if it's false.  This can optionally be followed
 by the 'else' keyword and another statement--or block of statements--
 which will be executed if (and only if) the expression was false.
5 examples
 Simple example showing a statement used to control how many numbers
 are printed on a given line.
       if ( ++i <= 10 )     #check whether this would be the 11th
              printf(" %5d", k)     #print on current line if not
       else {
              printf("\n %5d", k)   #print on next line if so
              i = 1                 #and reset the counter
       }
 Another example ('next' is described under 'action-controls')
       if ($1 > $2) { print "rejected"; next } else diff = $2 - $1
4 loops
 Three types of loop statements are available in awk.  Each uses
 the same syntax as 'C'.  The simplest of the three is the 'while'
 statement.  It consists of the 'while' keyword, followed by an
 expression enclosed within parentheses, followed by a statement--or
 block of statements in braces ({})--which will be executed if the
 expression evaluates to true.  The expression is evaluated before
 attempting to execute the statement; if it's true, the statement is
 executed (the entire block of statements if there is a block) and
 then the expression is re-evaluated.

 The second type of loop is the do-while loop.  It consists of the
 'do' keyword, followed by a statement (usually a block of statements
 enclosed within braces), followed by the 'while' keyword, followed
 by a test expression enclosed within parentheses.  The statement--or
 block--is always executed at least once.  Then the test expression
 is evaluated, and the statement(s) re-executed if the result was
 true (followed by re-evaluation of the test, and so on).

 The most complex of the three loops is the 'for' statement, and it
 has a second variant that is not found in 'C'.  The ordinary for-loop
 consists of the 'for' keyword, followed by three semicolon-separated
 expressions enclosed within parentheses, followed by a statement or
 brace-enclosed block of statements.  The first of the three
 expressions is an initialization clause; it is done before starting
 the loop.  The second expression is used as a test, just like the
 expression in a while-loop.  It is checked before attempting to
 execute the statement block, and then re-checked after each execution
 (if any) of the block.  The third expression is an 'increment' clause;
 it is evaluated after an execution of the statement block and before
 re-evaluation of the test (2nd) expression.  Normally, the increment
 clause will change a variable used in the test clause, in such a
 fashion that the test clause will eventually evaluate to false and
 cause the loop to finish.

 Note to 'C' programmers:  the comma (,) operator commonly used in
 'C' for-loop expressions is not valid in awk.

 The awk-specific variant of the for-loop is used for processing
 arrays.  Its syntax is 'for' keyword, followed by variable_name 'in'
 array_name (where 'var in array' is enclosed in parentheses),
 followed by a statement (or block).  Each valid subscript value for
 the array in question is successively placed--in no particular
 order--into the specified 'index' variable.
5 while_example
 # strip fields from the input record until there's nothing left
 while (NF > 0) {
     $1 = ""    #this will affect the value of $0
     $0 = $0    #this causes $0 and NF to be re-evaluated
     print
 }
5 do_while_example
 # This is a variation of the while_example; it gives a slightly
 #   different display due to the order of operation.
 # echo input record until all fields have been stripped
 do {
     print      #output $0
     $1 = ""    #this will affect the value of $0
     $0 = $0    #this causes $0 and NF to be re-evaluated
 } while (NF > 0)
5 for_example
 # echo command line arguments (won't include option switches)
 for ( i = 0; i < ARGC; i++ )  print ARGV[i]

 # display contents of builtin environment array
 for (itm in ENVIRON)
     print itm, ENVIRON[itm]
4 loop-controls
 There are two special statements--both from 'C'--for changing the
 behavior of loop execution.  The 'continue' statement is useful in
 a compound (block) statement; when executed, it effectively skips
 the rest of the block so that the increment-expression (only for
 for-loops) and loop-termination expression can be re-evaluated.

 The 'break' statement, when executed, effectively skips the rest
 of the block and also treats the test expression as if it were
 false (instead of actually re-evaluating it).  In this case, the
 increment-expression of a for-loop is also skipped.

 'break' is only allowed within a loop ('for', 'while', or
 'do-while').  If 'continue' is used outside of a loop, it is
 treated like 'next' (see action-controls).  Inside nested loops,
 both 'break' and 'continue' only apply to the innermost loop.
4 action-controls
 There are two special statements for controlling statement execution.
 The 'next' statement, when executed, causes the rest of the current
 action and all further pattern-action rules to be skipped, so that
 the next input record will be immediately processed.  This is useful
 if any early action knows that the current record will fail all the
 remaining patterns; skipping those rules will reduce processing time.
 An extended form, 'next file', is also available.  It causes the
 remainder of the current file to be skipped, and then either the
 next input file will be processed, if any, or the END action will be
 performed.  'next file' is not available in traditional awk.

 The 'exit' statement causes GAWK execution to terminate.  All open
 files are closed, and no further processing is done.  The END rule,
 if any, is executed.  'exit' takes an optional numeric value as a
 argument which is used as an exit status value, so that some sort
 of indication of why execution has stopped can be passed on to the
 user's environment.
4 other_statements
 The delete statement is used to remove an element from an array.
 The syntax is 'delete' keyword followed by array name, followed
 by index value enclosed in square brackets ([]).

 The return statement is used in user-defined functions.  The syntax
 is the keyword 'return' optionally followed by a string or numeric
 expression.

 See also subtopic 'functions IO_functions' for a description of
 'print', 'printf', and 'getline'.
3 fields
 When an input record is read, it is automatically split into fields
 based on the current values of FS (builtin variable defining field
 separator expression) and RS (builtin variable defining record
 separator character).  The default value of FS is an expression
 which matches one or more spaces and tabs; the default for RS is
 newline.  If the FIELDWIDTHS variable is set to a space separated
 list of numbers (as in ``FIELDWIDTHS = "2 3 2"'') then the input
 is treated as if it had fixed-width fields of the indicated sizes
 and the FS value will be ignored.