Date arithmetic in Unix shell scripts

Question

I need to do date arithmetic in Unix shell scripts that I use to control the execution of third party programs.

I\'m using a function to increment a day and another

Answer 1

I have bumped into this a couple of times. My thoughts are:

1. Date arithmetic is always a pain
2. It is a bit easier when using EPOCH date format
3. date on Linux converts to EPOCH, but not on Solaris
4. For a portable solution, you need to do one of the following:
   5. Install gnu date on solaris (already mentioned, needs human interaction to complete)
   6. Use perl for the date part (most unix installs include perl, so I would generally assume that this action does not require additional work).

A sample script (checks for the age of certain user files to see if the account can be deleted):

#!/usr/local/bin/perl

$today = time();

$user = $ARGV[0];

$command="awk -F: '/$user/ {print \$6}' /etc/passwd";

chomp ($user_dir = `$command`);

if ( -f "$user_dir/.sh_history" ) {
    @file_dates   = stat("$user_dir/.sh_history");
    $sh_file_date = $file_dates[8];
} else {
    $sh_file_date = 0;
}
if ( -f "$user_dir/.bash_history" ) {
    @file_dates     = stat("$user_dir/.bash_history");
    $bash_file_date = $file_dates[8];
} else {
    $bash_file_date = 0;
}
if ( $sh_file_date > $bash_file_date ) {
    $file_date = $sh_file_date;
} else {
    $file_date = $bash_file_date;
}
$difference = $today - $file_date;

if ( $difference >= 3888000 ) {
    print "User needs to be disabled, 45 days old or older!\n";
    exit (1);
} else {
    print "OK\n";
    exit (0);
}

Answer 2

Assuming you have GNU date, like so:

date --date='1 days ago' '+%a'

And similar phrases.