UNIX Command Line

$ cat idfile
id='1' dsadsad adsad
id='31' dsadsad adsad 32432
id='231' dsadsad adsad 3234
id='123' 2332 dsadsad adsad
id='124' 2332


Output required:
Subtract 1 from the id value from each line of the above file. i.e. final output should like this:

id='0' dsadsad adsad
id='30' dsadsad adsad 32432
id='230' dsadsad adsad 3234
id='122' 2332 dsadsad adsad
id='123' 2332

The script:

$ cat idfile | while read line
> do
> R=`echo $line | sed "s/id='\([0-9].*\)'.*/\1/"`
> ((R-=1))
> echo $line | sed "s/\(id='\)\([0-9].*\)\('.*\)/\1$R\3/"
> done

Note:
If
((R-=1))
complians, then use
R=`expr $R - 1`


Some references:

$ echo "id='1' dsadsad adsad" | sed "s/\(id='\)\([0-9].*\)\('.*\)/\1/"
id='

$ echo "id='1' dsadsad adsad" | sed "s/\(id='\)\([0-9].*\)\('.*\)/\2/"
1

$ echo "id='1' dsadsad adsad" | sed "s/\(id='\)\([0-9].*\)\('.*\)/\3/"
' dsadsad adsad