GAWK: Effective AWK Programming

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150 GAWK: Effective AWK Programming$ date ’+Today is %A, %B %d, %Y.’⊣ Today is Thursday, September 14, 2000.Here is the gawk version of the date utility. It has a shell “wrapper” to handle the ‘-u’option, which requires that date run as if the time zone is set to UTC:#! /bin/sh## date --- approximate the P1003.2 ’date’ commandcase $1 in-u) TZ=UTC0 # use UTCexport TZshift ;;esacgawk ’BEGIN {format = "%a %b %d %H:%M:%S %Z %Y"exitval = 0if (ARGC > 2)exitval = 1else if (ARGC == 2) {format = ARGV[1]if (format ~ /^\+/)format = substr(format, 2) # remove leading +}print strftime(format)exit exitval}’ "$@"8.1.6 Bit-Manipulation Functions of gawkI can explain it for you, but I can’t understand it for you.AnonymousMany languages provide the ability to perform bitwise operations on two integer numbers.In other words, the operation is performed on each successive pair of bits in theoperands. Three common operations are bitwise AND, OR, and XOR. The operations aredescribed in Table 8.6.Table 8.6: Bitwise OperationsBit operatorAND OR XOROperands 0 1 0 1 0 10 0 0 0 1 0 11 0 1 1 1 1 0
Chapter 8: Functions 151As you can see, the result of an AND operation is 1 only when both bits are 1. Theresult of an OR operation is 1 if either bit is 1. The result of an XOR operation is 1 ifeither bit is 1, but not both. The next operation is the complement; the complement of 1is 0 and the complement of 0 is 1. Thus, this operation “flips” all the bits of a given value.Finally, two other common operations are to shift the bits left or right. For example,if you have a bit string ‘10111001’ and you shift it right by three bits, you end up with‘00010111’. 14 If you start over again with ‘10111001’ and shift it left by three bits, you endup with ‘11001000’. gawk provides built-in functions that implement the bitwise operationsjust described. They are:and(v1, v2)or(v1, v2)xor(v1, v2)compl(val)lshift(val, count)Returns the bitwise AND of the values provided byv1 and v2.Returns the bitwise OR of the values provided by v1and v2.Returns the bitwise XOR of the values provided byv1 and v2.Returns the bitwise complement of val.Returns the value of val, shifted left by count bits.rshift(val, count) Returns the value of val, shifted right by count bits.For all of these functions, first the double-precision floating-point value is converted tothe widest C unsigned integer type, then the bitwise operation is performed. If the resultcannot be represented exactly as a C double, leading nonzero bits are removed one by oneuntil it can be represented exactly. The result is then converted back into a C double. (Ifyou don’t understand this paragraph, don’t worry about it.)Here is a user-defined function (see Section 8.2 [User-Defined Functions], page 153) thatillustrates the use of these functions:# bits2str --- turn a byte into readable 1’s and 0’sfunction bits2str(bits,{if (bits == 0)return "0"data, mask)mask = 1for (; bits != 0; bits = rshift(bits, 1))data = (and(bits, mask) ? "1" : "0") datawhile ((length(data) % 8) != 0)data = "0" data}return data14 This example shows that 0’s come in on the left side. For gawk, this is always true, but in some languages,it’s possible to have the left side fill with 1’s. Caveat emptor.
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GAWK: Effective AWK Programming

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