How does cron set the environment variables in /etc/cron.d/* and /etc/cron.d/?

Question

In Debian's manpage of cron,

The files in /etc/cron.d/ are independent of /etc/crontab: they do not, for example, inherit environment variable settings from it.

and from the LinuxQuestions.org Forum: /etc/crontab vs /etc/cron.d vs /var/spool/cron/crontabs/,

scripts that are in /etc/cron.d/ don't load environment variables.

I'm assuming you added your command as root in the /etc/crontab file. If that's the fact then executing the crontab line will load the user's environment variables which don't get loaded when you put the script in /etc/cron.d.

I was wondering what the sentences highlighted by me mean? What is the "inherit"ance from?

1. For /etc/cron.d/*, cron reset the environment variables, so don't load environment variables of the given users in the job definition lines.

   After creating /etc/cron.d/myjob

   35 * * * * t   echo $PATH  > /tmp/cron.log 2>&1
   

   /tmp/cron.log shows the default value of PATH is:

   /usr/bin:/bin
   

   which isn't the root's PATH:

   $ sudo su
   # echo $PATH
   /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games
   

2. In /etc/crontab, I added

   * * * * * root   echo $PATH    > /tmp/cron.log 2>&1
   * * * * * t    echo $PATH  > /tmp/cron.log.1 2>&1
   

   Then The PATH value for a root's cron job isn't the root's

   $ cat /tmp/cron.log
   /usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin
   

   The PATH value of my cron job isn't mine (modified in~/.profile) either

   $ cat /tmp/cron.log.1
   /usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin
   
   $ echo $PATH
   /home/t/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/opt/mssql-tools/bin
   
   $ less ~/.profile | grep PATH
   PATH="$HOME/bin:$PATH"
   PATH="$PATH:/opt/mssql-tools/bin"
   

Thanks.

Answer 1

From man 5 crontab:

An active line in a crontab will be either an environment setting or a cron command.

That is: a not commented line (#) could be:

PATH = /bin:/sbin

Which will set the value of PATH for the whole crontab file.

If that value is not set, the built-in value (in-code) as shown on this answer is used.

Example of a crontab file setting the PATH:

SHELL=/bin/bash 
MAILTO=root
PATH=~/bin:/usr/bin/:/bin

# Edit this file to introduce tasks to be run by cron.
#.---------------- minute (m) (0 - 59)
#|      .------------- hour (h) (0 - 23)
#|      |       .---------- day of month (dom) (1 - 31)
#|      |       |       .------- month (mon) (1 - 12) OR jan,feb,mar,apr ...
#|      |       |       |       .---- day of week (dow) (0 - 6) (Sunday=0 or 7)  OR sun,mon,tue,wed,thu,fri,sat
#                                       .---- user
#|      |       |       |       |       |
#*      *       *       *       *       root   echo "the command to be executed"
#
#m      h       dom     mon     dow     user   command
*       *       *       *       *       root   echo "A crontab file test"