Shell Programming and Scripting

$ 
$ cat file1
20090516 100034837SHDHSHE
second line              
third line               
$                        
$ head -1 file1 | perl -MDate::Calc -ne '{
>   ($y,$m,$d)=unpack"a4 a2 a2";
>   if (Date::Calc::check_date($y,$m,$d)) {print "Valid date\n"}
>   else {print "Invalid date\n"}}'
Valid date
$
$
$ cat file2
20090229 100034837SHDHSHE
second line
third line
$
$ head -1 file2 | perl -MDate::Calc -ne '{
>   ($y,$m,$d)=unpack"a4 a2 a2";
>   if (Date::Calc::check_date($y,$m,$d)) {print "Valid date\n"}
>   else {print "Invalid date\n"}}'
Invalid date
$
$

awk 'NR==1{
    year=substr($1,1,4)
    mth=substr($1,5,2)
    day=substr($1,7,2)
    cmd = "cal "mth" "year
    while ( cmd | getline result){
        m=split(result,a," ")
        for (i=1;i<=m;i++){      
            if (a[i] == day){
                print "Date ok"
                break
            }
        }
    }
    value=close(cmd)
    if (value!=0){
        print "Date not ok"
    }
}' file

# more file
20090432 100034837SHDHSHE
some lines
....
# ./test.sh
Date not ok

# more file
20090416 100034837SHDHSHE
some lines
....
# ./test.sh
Date ok