It should be clear from looking at the outputs we saw for PATH, both at and cron by default use /bin/sh. You might get lucky and have a distribution where /bin/sh defaults to Bash, but this is not necessarily the case, especially not if you followed along with our Ubuntu 18.04 installation! In this case, if we check out /bin/sh, we see something different entirely:

reader@ubuntu:~$ ls -l /bin/sh
lrwxrwxrwx 1 root root 4 Apr 26  2018 /bin/sh -> dash

Dash is the Debian Almquist shell, which is the default system shell on recent Debian systems (Ubuntu, as you might remember, belongs to the Debian distribution family). While Dash is a wonderful shell with its own set of advantages and disadvantages, this book is written for Bash. So, for our use case, it is not practical to have cron default to using a Dash shell, since that would not allow us to use cool Bash 4.x functions such as advanced redirections, certain expansions, and so on. Fortunately, we can easily set the shell that cron should use when we run our commands: we use the SHELL environment variable. Setting this is really simple:

reader@ubuntu:~$ crontab -e
crontab: installing new crontab
reader@ubuntu:~$ crontab -l
SHELL=/bin/bash
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/snap/bin
# m h  dom mon dow   command
* * * * * echo $0
reader@ubuntu:~$
You have mail in /var/mail/reader
reader@ubuntu:~$ tail -3 /var/mail/reader
/bin/bash
reader@ubuntu:~/scripts/chapter_14$ crontab -r

With just the simple addition of the SHELL environment variable, we made sure we do not have mind-boggling issues about why certain Bash functionality is not working. It is always a good idea to prevent these issues rather than hope you catch them quickly, especially if you're still mastering shell scripting.