The cut utility selects, or "cuts," either characters or fields
from its standard
input and sends them to its standard output. cut can cut out either
a list of characters, or a list of fields. By default, fields are separated
by tabs, 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
-f list
-d delim
-s
The awk implementation of cut uses the getopt library
function (see section Processing Command Line Options),
and the join library function
(see section Merging an Array Into a String).
The program begins with a comment describing the options and a usage
function which prints out a usage message and exits. usage is called
if invalid arguments are supplied.
# cut.awk --- implement cut in awk
# Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain
# May 1993
# Options:
# -f list Cut fields
# -d c Field delimiter character
# -c list Cut characters
#
# -s Suppress lines without the delimiter character
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.
Next comes a BEGIN rule that parses the command line options.
It sets FS to a single tab character, since that is cut's
default field separator. The output field separator is also set to be the
same as the input field separator. Then getopt is used to step
through the command line options. One or the other 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()
}
for (i = 1; i < Optind; i++)
ARGV[i] = ""
Special care is taken 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, note that after getopt is
through, we have to clear out all the elements of ARGV from one to
Optind, so that awk will 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 either set_fieldlist or
set_charlist is called 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()
}
Here is set_fieldlist. It first splits the field list apart
at the commas, into an array. Then, for each element of the array, it
looks to see if it is actually a range, and if so splits it apart. The range
is verified to make sure 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 section Reading Fixed-width Data),
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 be
printed, and also the intervening characters that have to be skipped.
For example, suppose you wanted characters one through eight, 15, and
22 through 35. You would use `-c 1-8,15,22-35'. The necessary value
for FIELDWIDTHS would be "8 6 1 6 14". This gives us five
fields, and what should be printed are $1, $3, and $5.
The intermediate fields are "filler," stuff in between the desired data.
flist lists the fields to be printed, 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
}
Here is the rule that actually processes the data. If the `-s' option
was given, then suppress will be 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 at this point, 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. If the corresponding field has data in it, it is
printed. If the next field also has data, then the separator character is
written out in between the fields.
{
if (by_fields && suppress && $0 !~ FS)
next
for (i = 1; i <= nfields; i++) {
if ($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 would be possible in
other awk implementations to use substr
(see section Built-in Functions for String Manipulation),
it would also be extremely painful to do so.
The FIELDWIDTHS variable supplies an elegant solution to the problem
of picking the input line apart by characters.
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