Cron is a command that executes commands periodically as specified in /etc/crontab and user-defined crontabs. In Debian 1.3 there is a standard mechanism to execute commands in /etc/cron.daily, /etc/cron.weekly and /etc/cron.monthly.
Cron is started from boot scripts but it seems to change it's PATH to a pretty strange one:
THIS IS LIKELY A BUG IN CRON. This is the init path where there is /usr/bin:/bin written over the beginning without terminating 0! This bug does not exist in all the systems.
In crontab there can be PATH definition. In Debian 1.3 there is the following default line in the beginning of /etc/crontab:
Because of this, the PATH of crond program is never used in user programs. All the scripts in /etc/cron.* directories get this path by default. This path is used even if a program is executed as non-root.
at is a command that can be used to run a one-time program at specific time.
atd is run using init path. However, the user programs are always run in the user environment using sh command. Therefore the usual shell overwrites apply. Look the chapter on bash.