gawk: Fixed width data
4.6.1 Processing Fixed-Width Data
---------------------------------
An example of fixed-width data would be the input for old Fortran
programs where numbers are run together, or 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' does not
work well in this case. Although a portable 'awk' program can use a
series of 'substr()' calls on '$0' (
String Functions), 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. It is a fatal error to supply a field width that
has a negative value.
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 this input, converts the idle time to
number of seconds, and prints out the first two fields and the
calculated idle time:
BEGIN { FIELDWIDTHS = "9 6 10 6 7 7 35" }
NR > 2 {
idle = $4
sub(/^ +/, "", idle) # strip leading spaces
if (idle == "")
idle = 0
if (idle ~ /:/) { # hh:mm
split(idle, t, ":")
idle = t[1] * 60 + t[2]
}
if (idle ~ /days/)
idle *= 24 * 60 * 60
print $1, $2, idle
}
NOTE: The preceding program uses a number of 'awk' features that
haven't been introduced yet.
Running the program on the data produces the following results:
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
is 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. Because 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!)
Info Catalog
gawk: Constant Size
gawk: Skipping intervening