Q:

Find out ID of 'at' job from within it

When I schedule a job with 'at' it is assigned an id, viz:
job 44 at 2014-01-28 17:30
When that job runs I would like to get at that id from within it.  This is on Centos, FWIW.  I have established that no environment variable contains the ID.  When the Perl code in that job runs I would like it to be able to print the job ID (44 in this example).
Yes, I know that atq shows an = next to jobs that are executing, but there might be more than one of those at a time.
I could do something like pass a unique argument to the job when scheduling it, capture the ID, save that and the argument to a file somewhere, read that from the job.  That's a lot of work I'd rather not go to if I don't have to, and it seems like this should be simple but I'm drawing a blank.

A:

What follows is figured out by reading sources of at-3.14. The way at puts job id and the time when it is run into the file name should be similar for any version, but I haven't checked this.
To begin whith at encodes the job id and the time when a particular job should be run into the file name describing a job. The file name has format aJJJJJTTTTTTTT, where JJJJJ is 5 character hexadecimal string, the job id, and TTTTTTTT is an 8 character hexadecimal string, the time when the job should be run. The time is stored as seconds from the epoch.
At jobs are run by feeding a job description file as the standard input to sh -c. Fortunately the Linux kernel provides a symbolic link, /proc/self/fd/0, which will point to the standard input of the process currently being executed (play with ls -l /proc/self/fd/0 in case you need to assure yourself that this indeed is so).
A file describing a job has been deleted by the time a job is run. However, the file is still available for the kernel because it has been duplicated with dup(2) before being used as the standard input for a job. So, actually we are resolving a symbolic link to a file name which is not visible any more. In the perl script at the end we need to take this into account as readlink will return something like /foo/bar/baz (deleted) instead of /foo/bar/baz. And we're interested in just the file name which has all the information we need.
The reason why the symbolic link points to a deleted file is because at daemon unlinks the original before executing the job. Unlinking gets done only after creating a copy, a hard link, which begins with = instead of a. With this the at daemon tries to ensure there will be only one copy of a job running: the daemon will not execle(2), ie. it will bail out, should the link(2) fail. Because the original file has been subject to open(2) and dup(2) the inode is still there for the kernel to use because it still has hard links pointing to it.
After a fairly long and possibly confusing introduction, here is how to put it all together:
#!/usr/bin/perl

use strict;
use warnings;

my $job_file = readlink("/proc/self/fd/0");
if (index($job_file, " ") > 0) {
        $job_file = substr($job_file, 0, index($job_file, " ") - 1);
}
my $tmp = substr($job_file, rindex($job_file, "/") + 1);
$tmp =~ s/^a([0-9a-f]{5})[0-9a-f]+/$1/;
my $job_id = hex($tmp);

if ($job_id > 0) {
        printf("My AT job id is %d.\n", $job_id);
}

# end of file.

