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gawk: Getopt Function

 10.4 Processing Command-Line Options
 Most utilities on POSIX-compatible systems take options on the command
 line that can be used to change the way a program behaves.  'awk' is an
 example of such a program (⇒Options).  Often, options take
 "arguments" (i.e., data that the program needs to correctly obey the
 command-line option).  For example, 'awk''s '-F' option requires a
 string to use as the field separator.  The first occurrence on the
 command line of either '--' or a string that does not begin with '-'
 ends the options.
    Modern Unix systems provide a C function named 'getopt()' for
 processing command-line arguments.  The programmer provides a string
 describing the one-letter options.  If an option requires an argument,
 it is followed in the string with a colon.  'getopt()' is also passed
 the count and values of the command-line arguments and is called in a
 loop.  'getopt()' processes the command-line arguments for option
 letters.  Each time around the loop, it returns a single character
 representing the next option letter that it finds, or '?' if it finds an
 invalid option.  When it returns -1, there are no options left on the
 command line.
    When using 'getopt()', options that do not take arguments can be
 grouped together.  Furthermore, options that take arguments require that
 the argument be present.  The argument can immediately follow the option
 letter, or it can be a separate command-line argument.
    Given a hypothetical program that takes three command-line options,
 '-a', '-b', and '-c', where '-b' requires an argument, all of the
 following are valid ways of invoking the program:
      prog -a -b foo -c data1 data2 data3
      prog -ac -bfoo -- data1 data2 data3
      prog -acbfoo data1 data2 data3
    Notice that when the argument is grouped with its option, the rest of
 the argument is considered to be the option's argument.  In this
 example, '-acbfoo' indicates that all of the '-a', '-b', and '-c'
 options were supplied, and that 'foo' is the argument to the '-b'
    'getopt()' provides four external variables that the programmer can
      The index in the argument value array ('argv') where the first
      nonoption command-line argument can be found.
      The string value of the argument to an option.
      Usually 'getopt()' prints an error message when it finds an invalid
      option.  Setting 'opterr' to zero disables this feature.  (An
      application might want to print its own error message.)
      The letter representing the command-line option.
    The following C fragment shows how 'getopt()' might process
 command-line arguments for 'awk':
      main(int argc, char *argv[])
          /* print our own message */
          opterr = 0;
          while ((c = getopt(argc, argv, "v:f:F:W:")) != -1) {
              switch (c) {
              case 'f':    /* file */
              case 'F':    /* field separator */
              case 'v':    /* variable assignment */
              case 'W':    /* extension */
              case '?':
    As a side point, 'gawk' actually uses the GNU 'getopt_long()'
 function to process both normal and GNU-style long options (⇒
    The abstraction provided by 'getopt()' is very useful and is quite
 handy in 'awk' programs as well.  Following is an 'awk' version of
 'getopt()'.  This function highlights one of the greatest weaknesses in
 'awk', which is that it is very poor at manipulating single characters.
 Repeated calls to 'substr()' are necessary for accessing individual
 characters (⇒String Functions).(1)
    The discussion that follows walks through the code a bit at a time:
      # getopt.awk --- Do C library getopt(3) function in awk
      # External variables:
      #    Optind -- index in ARGV of first nonoption argument
      #    Optarg -- string value of argument to current option
      #    Opterr -- if nonzero, print our own diagnostic
      #    Optopt -- current option letter
      # Returns:
      #    -1     at end of options
      #    "?"    for unrecognized option
      #    <c>    a character representing the current option
      # Private Data:
      #    _opti  -- index in multiflag option, e.g., -abc
    The function starts out with comments presenting a list of the global
 variables it uses, what the return values are, what they mean, and any
 global variables that are "private" to this library function.  Such
 documentation is essential for any program, and particularly for library
    The 'getopt()' function first checks that it was indeed called with a
 string of options (the 'options' parameter).  If 'options' has a zero
 length, 'getopt()' immediately returns -1:
      function getopt(argc, argv, options,    thisopt, i)
          if (length(options) == 0)    # no options given
              return -1
          if (argv[Optind] == "--") {  # all done
              _opti = 0
              return -1
          } else if (argv[Optind] !~ /^-[^:[:space:]]/) {
              _opti = 0
              return -1
    The next thing to check for is the end of the options.  A '--' ends
 the command-line options, as does any command-line argument that does
 not begin with a '-'.  'Optind' is used to step through the array of
 command-line arguments; it retains its value across calls to 'getopt()',
 because it is a global variable.
    The regular expression that is used, '/^-[^:[:space:]/', checks for a
 '-' followed by anything that is not whitespace and not a colon.  If the
 current command-line argument does not match this pattern, it is not an
 option, and it ends option processing.  Continuing on:
          if (_opti == 0)
              _opti = 2
          thisopt = substr(argv[Optind], _opti, 1)
          Optopt = thisopt
          i = index(options, thisopt)
          if (i == 0) {
              if (Opterr)
                  printf("%c -- invalid option\n", thisopt) > "/dev/stderr"
              if (_opti >= length(argv[Optind])) {
                  _opti = 0
              } else
              return "?"
    The '_opti' variable tracks the position in the current command-line
 argument ('argv[Optind]').  If multiple options are grouped together
 with one '-' (e.g., '-abx'), it is necessary to return them to the user
 one at a time.
    If '_opti' is equal to zero, it is set to two, which is the index in
 the string of the next character to look at (we skip the '-', which is
 at position one).  The variable 'thisopt' holds the character, obtained
 with 'substr()'.  It is saved in 'Optopt' for the main program to use.
    If 'thisopt' is not in the 'options' string, then it is an invalid
 option.  If 'Opterr' is nonzero, 'getopt()' prints an error message on
 the standard error that is similar to the message from the C version of
    Because the option is invalid, it is necessary to skip it and move on
 to the next option character.  If '_opti' is greater than or equal to
 the length of the current command-line argument, it is necessary to move
 on to the next argument, so 'Optind' is incremented and '_opti' is reset
 to zero.  Otherwise, 'Optind' is left alone and '_opti' is merely
    In any case, because the option is invalid, 'getopt()' returns '"?"'.
 The main program can examine 'Optopt' if it needs to know what the
 invalid option letter actually is.  Continuing on:
          if (substr(options, i + 1, 1) == ":") {
              # get option argument
              if (length(substr(argv[Optind], _opti + 1)) > 0)
                  Optarg = substr(argv[Optind], _opti + 1)
                  Optarg = argv[++Optind]
              _opti = 0
          } else
              Optarg = ""
    If the option requires an argument, the option letter is followed by
 a colon in the 'options' string.  If there are remaining characters in
 the current command-line argument ('argv[Optind]'), then the rest of
 that string is assigned to 'Optarg'.  Otherwise, the next command-line
 argument is used ('-xFOO' versus '-x FOO').  In either case, '_opti' is
 reset to zero, because there are no more characters left to examine in
 the current command-line argument.  Continuing:
          if (_opti == 0 || _opti >= length(argv[Optind])) {
              _opti = 0
          } else
          return thisopt
    Finally, if '_opti' is either zero or greater than the length of the
 current command-line argument, it means this element in 'argv' is
 through being processed, so 'Optind' is incremented to point to the next
 element in 'argv'.  If neither condition is true, then only '_opti' is
 incremented, so that the next option letter can be processed on the next
 call to 'getopt()'.
    The 'BEGIN' rule initializes both 'Opterr' and 'Optind' to one.
 'Opterr' is set to one, because the default behavior is for 'getopt()'
 to print a diagnostic message upon seeing an invalid option.  'Optind'
 is set to one, because there's no reason to look at the program name,
 which is in 'ARGV[0]':
      BEGIN {
          Opterr = 1    # default is to diagnose
          Optind = 1    # skip ARGV[0]
          # test program
          if (_getopt_test) {
              while ((_go_c = getopt(ARGC, ARGV, "ab:cd")) != -1)
                  printf("c = <%c>, Optarg = <%s>\n",
                                             _go_c, Optarg)
              printf("non-option arguments:\n")
              for (; Optind < ARGC; Optind++)
                  printf("\tARGV[%d] = <%s>\n",
                                          Optind, ARGV[Optind])
    The rest of the 'BEGIN' rule is a simple test program.  Here are the
 results of two sample runs of the test program:
      $ awk -f getopt.awk -v _getopt_test=1 -- -a -cbARG bax -x
      -| c = <a>, Optarg = <>
      -| c = <c>, Optarg = <>
      -| c = <b>, Optarg = <ARG>
      -| non-option arguments:
      -|         ARGV[3] = <bax>
      -|         ARGV[4] = <-x>
      $ awk -f getopt.awk -v _getopt_test=1 -- -a -x -- xyz abc
      -| c = <a>, Optarg = <>
      error-> x -- invalid option
      -| c = <?>, Optarg = <>
      -| non-option arguments:
      -|         ARGV[4] = <xyz>
      -|         ARGV[5] = <abc>
    In both runs, the first '--' terminates the arguments to 'awk', so
 that it does not try to interpret the '-a', etc., as its own options.
      NOTE: After 'getopt()' is through, user-level code must clear out
      all the elements of 'ARGV' from 1 to 'Optind', so that 'awk' does
      not try to process the command-line options as file names.
    Using '#!' with the '-E' option may help avoid conflicts between your
 program's options and 'gawk''s options, as '-E' causes 'gawk' to abandon
DONTPRINTYET  processing of further options (⇒Executable Scripts and *noteDONTPRINTYET  processing of further options (⇒Executable Scripts and ⇒
    Several of the sample programs presented in ⇒Sample Programs,
 use 'getopt()' to process their arguments.
    ---------- Footnotes ----------
    (1) This function was written before 'gawk' acquired the ability to
 split strings into single characters using '""' as the separator.  We
 have left it alone, as using 'substr()' is more portable.
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