If I were in your situation, I would just run a nightly job that appended their history file to a central file ('/tmp/user_logs/user.history').

@users = qw(fred wilma barney); #watch these users
foreach $user (@users) {
    open(LOG,">>$user.history");
    open(HSTRY,"/$user/.bash_history");
    while(<HSTRY>) {
        print LOG $_;
    }
}
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If that wouldn't work for you and you really don't want to write a simple shell (which would amount to a bunch or fork's and exec's, really); you could try to write a daemon that would run and when a specified user logged in, it would dup() his STDIN to a file. (Note: the following code does not do this, but is an example of dup() ).

open(LOG, ">>/tmp/$user.logfile");
open(STDIN, ">&LOG");
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But, again, you would have to make that a daemon that would watch people logging in, grab their username, expect the username, and then permanently dup() their STDIN.

Good Luck,
Jeremy