(This section discusses an advanced, experimental feature. If you are
a novice awk user, you may wish to skip it on the first reading.)
gawk version 2.13 introduced a new facility for dealing with
fixed-width fields with no distinctive field separator. Data of this
nature arises, for example, in the input for old FORTRAN programs where
numbers are run together; or in the output of programs that did not
anticipate the use of their output as input for other programs.
An example of the latter is a table where all the columns are lined up by
the use of a variable number of spaces and empty fields are just
spaces. Clearly, awk's normal field splitting based on FS
will not work well in this case. Although a portable awk program
can use a series of substr calls on $0
(see section Built-in Functions for String Manipulation),
this is awkward and inefficient for a large number of fields.
The splitting of an input record into fixed-width fields is specified by
assigning a string containing space-separated numbers to the built-in
variable FIELDWIDTHS. Each number specifies the width of the field
including columns between fields. If you want to ignore the columns
between fields, you can specify the width as a separate field that is
subsequently ignored.
The following data is the output of the Unix w utility. It is useful
to illustrate the use of FIELDWIDTHS.
10:06pm up 21 days, 14:04, 23 users User tty login idle JCPU PCPU what hzuo ttyV0 8:58pm 9 5 vi p24.tex hzang ttyV3 6:37pm 50 -csh eklye ttyV5 9:53pm 7 1 em thes.tex dportein ttyV6 8:17pm 1:47 -csh gierd ttyD3 10:00pm 1 elm dave ttyD4 9:47pm 4 4 w brent ttyp0 26Jun91 4:46 26:46 4:41 bash dave ttyq4 26Jun9115days 46 46 wnewmail
The following program takes the above input, converts the idle time to
number of seconds and prints out the first two fields and the calculated
idle time. (This program uses a number of awk features that
haven't been introduced yet.)
BEGIN { FIELDWIDTHS = "9 6 10 6 7 7 35" }
NR > 2 {
idle = $4
sub(/^ */, "", idle) # strip leading spaces
if (idle == "")
idle = 0
if (idle ~ /:/) {
split(idle, t, ":")
idle = t[1] * 60 + t[2]
}
if (idle ~ /days/)
idle *= 24 * 60 * 60
print $1, $2, idle
}
Here is the result of running the program on the data:
hzuo ttyV0 0 hzang ttyV3 50 eklye ttyV5 0 dportein ttyV6 107 gierd ttyD3 1 dave ttyD4 0 brent ttyp0 286 dave ttyq4 1296000
Another (possibly more practical) example of fixed-width input data
would be the input from a deck of balloting cards. In some parts of
the United States, voters mark their choices by punching holes in computer
cards. These cards are then processed to count the votes for any particular
candidate or on any particular issue. Since a voter may choose not to
vote on some issue, any column on the card may be empty. An awk
program for processing such data could use the FIELDWIDTHS feature
to simplify reading the data. (Of course, getting gawk to run on
a system with card readers is another story!)
Assigning a value to FS causes gawk to return to using
FS for field splitting. Use `FS = FS' to make this happen,
without having to know the current value of FS.
This feature is still experimental, and may evolve over time.
Note that in particular, gawk does not attempt to verify
the sanity of the values used in the value of FIELDWIDTHS.
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