Modification of date format within a text file - awk

Question

I have some text files containing lines as follow :
07JAN01, -0.247297942769082E+07, -0.467133797284279E+07, 0.355810777473149E+07
07JAN02, -0.247297942405032E+07, -0.467133797586388E+07, 0.355810777517715E+07
07JAN03, -0.247297942584851E+07, -0.467133797727224E+07, 0.355810777627353E+07
. . . .
. . . .
I need to produce a script which will amend the format of the date to :
01/01/07, -0.247297942769082E+07, -0.467133797284279E+07, 0.355810777473149E+07
02/01/07, -0.247297942405032E+07, -0.467133797586388E+07, 0.355810777517715E+07
03/01/07, -0.247297942584851E+07, -0.467133797727224E+07, 0.355810777627353E+07
. . . .
. . . .
I was looking for an appropriate sed or grep command to extract only some characters of each line, to define it as a variable in my script. As I would like to "reorganize" the date, I was thinking about defining three variable, where, for the for the first line it would be :
a=07
b=JAN (need to implement a "case" in the script to deal with this I think?)
c=03
I looked at some grep examples, and tons of docs, but nothing really clear appeared ...
found something about the -cut command, but ... I'm not too sure it's appropriate here.
The other question I have is about the output, as sed doesn't modify the input data, how can I modify directly the files ? Is there a way ?
Any help would really be appreciated :)

Answer 1

I don't think grep is the right tool for the job myself. You need something a little more expressive like Perl or awk:
echo '07JAN01, -0.24729E+07, -0.46713E+07, 0.35581E+07
07JAN02, -0.24729E+07, -0.46713E+07, 0.35581E+07
07AUG03, -0.24729E+07, -0.46713E+07, 0.35581E+07' | awk -F, '
{
yy=substr($1,1,2);
mm=substr($1,3,3);
mm=(index(":JAN:FEB:MAR:APR:MAY:JUN:JUL:AUG:SEP:OCT:NOV:DEC",mm)+2)/4;
dd=substr($1,6,2);
printf "%02d/%02d/%02d,%s,%s,%s\n",dd,mm,yy,$2,$3,$4
}'
which generates:
01/01/07, -0.24729E+07, -0.46713E+07, 0.35581E+07
02/01/07, -0.24729E+07, -0.46713E+07, 0.35581E+07
03/08/07, -0.24729E+07, -0.46713E+07, 0.35581E+07
Obviously, that's just pumping some test data through a command line awk script. You'd be better off putting that into an actual awk script file and running your input through it.
If datchg.awk contains:
{
yy=substr($1,1,2);
mm=substr($1,3,3);
mm=(index(":JAN:FEB:MAR:APR:MAY:JUN:JUL:AUG:SEP:OCT:NOV:DEC",mm)+2)/4;
dd=substr($1,6,2);
printf "%02d/%02d/%02d,%s,%s,%s\n",dd,mm,yy,$2,$3,$4
}
then:
echo '07JAN01, -0.24729E+07, -0.46713E+07, 0.35581E+07
07JAN02, -0.24729E+07, -0.46713E+07, 0.35581E+07
07AUG03, -0.24729E+07, -0.46713E+07, 0.35581E+07' | awk -F, -fdatechg.awk
also produces:
01/01/07, -0.24729E+07, -0.46713E+07, 0.35581E+07
02/01/07, -0.24729E+07, -0.46713E+07, 0.35581E+07
03/08/07, -0.24729E+07, -0.46713E+07, 0.35581E+07
The way this works is as follows. Each line is split into fields (-F, sets the field separator to a comma) and we extract and process the relevant parts of field 1 (the date). By this I mean the year and day are reversed and the textual month is turned into a numeric month by searching a string for it and manipulating the index where it was found, so that it falls in the range 1 through 12.
This is the only (relatively) tricky bit and is done with some basic mathematics: the index function simply finds the position within the string of your month (where the first char is 1). So JAN is at position 2, FEB at 6, MAR at 10, ..., DEC at 46 (the set {2, 6, 10, ..., 46}). They're 4 apart so we're going to need to divide by 4 eventually to get consecutive month numbers but first we add 2 so the division will work well. Adding that 2 gives you the set {4, 8, 12, ..., 48}. Then you divide by 4 to get {1, 2, 3, ... 12} and there's your month number:
Text Pos +2 /4
---- --- -- --
JAN 2 4 1
FEB 6 8 2
MAR 10 12 3
APR 14 16 4
MAY 18 20 5
JUN 22 24 6
JUL 26 28 7
AUG 30 32 8
SEP 34 36 9
OCT 38 40 10
NOV 42 44 11
DEC 46 48 12
Then we just output the new information. Obviously, this is likely to barf if you provide bad data but I'm assuming either:
the data is good; or
you'll add your own error checks.
Regarding modifying the files directly, the time-honored UNIX tradition is to use a shell script to save the current file elsewhere, process it to create a new file, then overwrite the old file with the new file (but not touching the saved file, in case something goes horribly wrong).
I won't make my answer any longer by detailing that, you've probably fallen asleep already :-)

Answer 2

A bit clunky, but you could do:
sed -e 's/^\(..\)JAN\(..\)/\2\/01\/\1/'
sed -e 's/^\(..\)FEB\(..\)/\2\/02\/\1/'
...
In order to run sed in-place, see the -i commandline option:
sed -i -e ...
Edit
Just to point out that this answers a previous version of the question where AWK was not specified.

Answer 3

awk 'BEGIN{
OFS=FS=","
# create table of mapping of months to numbers
s=split("JAN:FEB:MAR:APR:MAY:JUN:JUL:AUG:SEP:OCT:NOV:DEC",d,":")
for(o=1;o<=s;o++){
m=sprintf("%02s",o) # add 0 is single digit
date[d[o]]=m
}
}
{
yr=substr($1,1,2)
mth=substr($1,3,3)
day=substr($1,6,2)
$1=day"/"date[mth]"/"yr
}1' file