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gawk: ARGC and ARGV

 
 7.5.3 Using 'ARGC' and 'ARGV'
 -----------------------------
 
 ⇒Auto-set presented the following program describing the
 information contained in 'ARGC' and 'ARGV':
 
      $ awk 'BEGIN {
      >        for (i = 0; i < ARGC; i++)
      >            print ARGV[i]
      >      }' inventory-shipped mail-list
      -| awk
      -| inventory-shipped
      -| mail-list
 
 In this example, 'ARGV[0]' contains 'awk', 'ARGV[1]' contains
 'inventory-shipped', and 'ARGV[2]' contains 'mail-list'.  Notice that
 the 'awk' program is not entered in 'ARGV'.  The other command-line
 options, with their arguments, are also not entered.  This includes
 variable assignments done with the '-v' option (⇒Options).
 Normal variable assignments on the command line _are_ treated as
 arguments and do show up in the 'ARGV' array.  Given the following
 program in a file named 'showargs.awk':
 
      BEGIN {
          printf "A=%d, B=%d\n", A, B
          for (i = 0; i < ARGC; i++)
              printf "\tARGV[%d] = %s\n", i, ARGV[i]
      }
      END   { printf "A=%d, B=%d\n", A, B }
 
 Running it produces the following:
 
      $ awk -v A=1 -f showargs.awk B=2 /dev/null
      -| A=1, B=0
      -|        ARGV[0] = awk
      -|        ARGV[1] = B=2
      -|        ARGV[2] = /dev/null
      -| A=1, B=2
 
    A program can alter 'ARGC' and the elements of 'ARGV'.  Each time
 'awk' reaches the end of an input file, it uses the next element of
 'ARGV' as the name of the next input file.  By storing a different
 string there, a program can change which files are read.  Use '"-"' to
 represent the standard input.  Storing additional elements and
 incrementing 'ARGC' causes additional files to be read.
 
    If the value of 'ARGC' is decreased, that eliminates input files from
 the end of the list.  By recording the old value of 'ARGC' elsewhere, a
 program can treat the eliminated arguments as something other than file
 names.
 
    To eliminate a file from the middle of the list, store the null
 string ('""') into 'ARGV' in place of the file's name.  As a special
 feature, 'awk' ignores file names that have been replaced with the null
 string.  Another option is to use the 'delete' statement to remove
 elements from 'ARGV' (⇒Delete).
 
    All of these actions are typically done in the 'BEGIN' rule, before
DONTPRINTYET  actual processing of the input begins.  ⇒Split Program and *noteDONTPRINTYET  actual processing of the input begins.  ⇒Split Program and ⇒
 Tee Program for examples of each way of removing elements from 'ARGV'.
 
    To actually get options into an 'awk' program, end the 'awk' options
 with '--' and then supply the 'awk' program's options, in the following
 manner:
 
      awk -f myprog.awk -- -v -q file1 file2 ...
 
    The following fragment processes 'ARGV' in order to examine, and then
 remove, the previously mentioned command-line options:
 
      BEGIN {
          for (i = 1; i < ARGC; i++) {
              if (ARGV[i] == "-v")
                  verbose = 1
              else if (ARGV[i] == "-q")
                  debug = 1
              else if (ARGV[i] ~ /^-./) {
                  e = sprintf("%s: unrecognized option -- %c",
                          ARGV[0], substr(ARGV[i], 2, 1))
                  print e > "/dev/stderr"
              } else
                  break
              delete ARGV[i]
          }
      }
 
    Ending the 'awk' options with '--' isn't necessary in 'gawk'.  Unless
 '--posix' has been specified, 'gawk' silently puts any unrecognized
 options into 'ARGV' for the 'awk' program to deal with.  As soon as it
 sees an unknown option, 'gawk' stops looking for other options that it
 might otherwise recognize.  The previous command line with 'gawk' would
 be:
 
      gawk -f myprog.awk -q -v file1 file2 ...
 
 Because '-q' is not a valid 'gawk' option, it and the following '-v' are
 passed on to the 'awk' program.  (⇒Getopt Function for an 'awk'
 library function that parses command-line options.)
 
    When designing your program, you should choose options that don't
 conflict with 'gawk''s, because it will process any options that it
 accepts before passing the rest of the command line on to your program.
 Using '#!' with the '-E' option may help (⇒Executable Scripts and
 ⇒Options).
 
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