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11.2.1 Cutting out Fields and Columns

The cut utility selects, or “cuts,” characters or fields from its standard input and sends them to its standard output. Fields are separated by TABs by default, but you may supply a command-line option to change the field delimiter (i.e., the field-separator character). cut's definition of fields is less general than awk's.

A common use of cut might be to pull out just the login name of logged-on users from the output of who. For example, the following pipeline generates a sorted, unique list of the logged-on users:

     who | cut -c1-8 | sort | uniq

The options for cut are:

-c list
Use list as the list of characters to cut out. Items within the list may be separated by commas, and ranges of characters can be separated with dashes. The list ‘1-8,15,22-35’ specifies characters 1 through 8, 15, and 22 through 35.
-f list
Use list as the list of fields to cut out.
-d delim
Use delim as the field-separator character instead of the TAB character.
-s
Suppress printing of lines that do not contain the field delimiter.

The awk implementation of cut uses the getopt() library function (see Getopt Function) and the join() library function (see Join Function).

The program begins with a comment describing the options, the library functions needed, and a usage() function that prints out a usage message and exits. usage() is called if invalid arguments are supplied:

     
     # cut.awk --- implement cut in awk
     
     
     
     # Options:
     #    -f list     Cut fields
     #    -d c        Field delimiter character
     #    -c list     Cut characters
     #
     #    -s          Suppress lines without the delimiter
     #
     # Requires getopt() and join() library functions
     
     function usage(    e1, e2)
     {
         e1 = "usage: cut [-f list] [-d c] [-s] [files...]"
         e2 = "usage: cut [-c list] [files...]"
         print e1 > "/dev/stderr"
         print e2 > "/dev/stderr"
         exit 1
     }
     

The variables e1 and e2 are used so that the function fits nicely on the page. screen.

Next comes a BEGIN rule that parses the command-line options. It sets FS to a single TAB character, because that is cut's default field separator. The rule then sets the output field separator to be the same as the input field separator. A loop using getopt() steps through the command-line options. Exactly one of the variables by_fields or by_chars is set to true, to indicate that processing should be done by fields or by characters, respectively. When cutting by characters, the output field separator is set to the null string:

     
     BEGIN    \
     {
         FS = "\t"    # default
         OFS = FS
         while ((c = getopt(ARGC, ARGV, "sf:c:d:")) != -1) {
             if (c == "f") {
                 by_fields = 1
                 fieldlist = Optarg
             } else if (c == "c") {
                 by_chars = 1
                 fieldlist = Optarg
                 OFS = ""
             } else if (c == "d") {
                 if (length(Optarg) > 1) {
                     printf("Using first character of %s" \
                            " for delimiter\n", Optarg) > "/dev/stderr"
                     Optarg = substr(Optarg, 1, 1)
                 }
                 FS = Optarg
                 OFS = FS
                 if (FS == " ")    # defeat awk semantics
                     FS = "[ ]"
             } else if (c == "s")
                 suppress++
             else
                 usage()
         }
     
         # Clear out options
         for (i = 1; i < Optind; i++)
             ARGV[i] = ""
     

The code must take special care when the field delimiter is a space. Using a single space (" ") for the value of FS is incorrect—awk would separate fields with runs of spaces, TABs, and/or newlines, and we want them to be separated with individual spaces. Also remember that after getopt() is through (as described in Getopt Function), we have to 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.

After dealing with the command-line options, the program verifies that the options make sense. Only one or the other of -c and -f should be used, and both require a field list. Then the program calls either set_fieldlist() or set_charlist() to pull apart the list of fields or characters:

     
         if (by_fields && by_chars)
             usage()
     
         if (by_fields == 0 && by_chars == 0)
             by_fields = 1    # default
     
         if (fieldlist == "") {
             print "cut: needs list for -c or -f" > "/dev/stderr"
             exit 1
         }
     
         if (by_fields)
             set_fieldlist()
         else
             set_charlist()
     }
     

set_fieldlist() splits the field list apart at the commas into an array. Then, for each element of the array, it looks to see if the element is actually a range, and if so, splits it apart. The function checks the range to make sure that the first number is smaller than the second. Each number in the list is added to the flist array, which simply lists the fields that will be printed. Normal field splitting is used. The program lets awk handle the job of doing the field splitting:

     
     function set_fieldlist(        n, m, i, j, k, f, g)
     {
         n = split(fieldlist, f, ",")
         j = 1    # index in flist
         for (i = 1; i <= n; i++) {
             if (index(f[i], "-") != 0) { # a range
                 m = split(f[i], g, "-")
                 if (m != 2 || g[1] >= g[2]) {
                     printf("bad field list: %s\n",
                                       f[i]) > "/dev/stderr"
                     exit 1
                 }
                 for (k = g[1]; k <= g[2]; k++)
                     flist[j++] = k
             } else
                 flist[j++] = f[i]
         }
         nfields = j - 1
     }
     

The set_charlist() function is more complicated than set_fieldlist(). The idea here is to use gawk's FIELDWIDTHS variable (see Constant Size), which describes constant-width input. When using a character list, that is exactly what we have.

Setting up FIELDWIDTHS is more complicated than simply listing the fields that need to be printed. We have to keep track of the fields to print and also the intervening characters that have to be skipped. For example, suppose you wanted characters 1 through 8, 15, and 22 through 35. You would use ‘-c 1-8,15,22-35’. The necessary value for FIELDWIDTHS is "8 6 1 6 14". This yields five fields, and the fields to print are $1, $3, and $5. The intermediate fields are filler, which is stuff in between the desired data. flist lists the fields to print, and t tracks the complete field list, including filler fields:

     
     function set_charlist(    field, i, j, f, g, t,
                               filler, last, len)
     {
         field = 1   # count total fields
         n = split(fieldlist, f, ",")
         j = 1       # index in flist
         for (i = 1; i <= n; i++) {
             if (index(f[i], "-") != 0) { # range
                 m = split(f[i], g, "-")
                 if (m != 2 || g[1] >= g[2]) {
                     printf("bad character list: %s\n",
                                    f[i]) > "/dev/stderr"
                     exit 1
                 }
                 len = g[2] - g[1] + 1
                 if (g[1] > 1)  # compute length of filler
                     filler = g[1] - last - 1
                 else
                     filler = 0
                 if (filler)
                     t[field++] = filler
                 t[field++] = len  # length of field
                 last = g[2]
                 flist[j++] = field - 1
             } else {
                 if (f[i] > 1)
                     filler = f[i] - last - 1
                 else
                     filler = 0
                 if (filler)
                     t[field++] = filler
                 t[field++] = 1
                 last = f[i]
                 flist[j++] = field - 1
             }
         }
         FIELDWIDTHS = join(t, 1, field - 1)
         nfields = j - 1
     }
     

Next is the rule that actually processes the data. If the -s option is given, then suppress is true. The first if statement makes sure that the input record does have the field separator. If cut is processing fields, suppress is true, and the field separator character is not in the record, then the record is skipped.

If the record is valid, then gawk has split the data into fields, either using the character in FS or using fixed-length fields and FIELDWIDTHS. The loop goes through the list of fields that should be printed. The corresponding field is printed if it contains data. If the next field also has data, then the separator character is written out between the fields:

     
     {
         if (by_fields && suppress && index($0, FS) != 0)
             next
     
         for (i = 1; i <= nfields; i++) {
             if ($flist[i] != "") {
                 printf "%s", $flist[i]
                 if (i < nfields && $flist[i+1] != "")
                     printf "%s", OFS
             }
         }
         print ""
     }
     

This version of cut relies on gawk's FIELDWIDTHS variable to do the character-based cutting. While it is possible in other awk implementations to use substr() (see String Functions), it is also extremely painful. The FIELDWIDTHS variable supplies an elegant solution to the problem of picking the input line apart by characters.