gawk: Cut Program
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 names 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 (
Getopt Function) and the 'join()' library function
(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()
{
print("usage: cut [-f list] [-d c] [-s] [files...]") > "/dev/stderr"
print("usage: cut [-c list] [files...]") > "/dev/stderr"
exit 1
}
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("cut: using first character of %s" \
" for delimiter\n", Optarg) > "/dev/stderr"
Optarg = substr(Optarg, 1, 1)
}
fs = FS = Optarg
OFS = FS
if (FS == " ") # defeat awk semantics
FS = "[ ]"
} else if (c == "s")
suppress = 1
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. To this end, we save
the original space character in the variable 'fs' for later use; after
setting 'FS' to '"[ ]"' we can't use it directly to see if the field
delimiter character is in the string.
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("cut: 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 (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, n, m, 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("cut: 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 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. It is possible in other 'awk'
implementations to use 'substr()' (String Functions), but it is
also extremely painful. The 'FIELDWIDTHS' variable supplies an elegant
solution to the problem of picking the input line apart by characters.
Info Catalog
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