Once I execute ping in traditional interactive shell by hand, It succeeds. Why does it fail from cron ?

The particular purpose for the failure is that the atmosphere in cron is a special atmosphere than “… in traditional interactive shell”. If you wish to be taught how the atmosphere is completely different in cron vs your interactive shell, attempt operating printenv (or env) from every & observe the variations; i.e.:

% crontab -e
...  crontab opens within the editor you have chosen as default (an atmosphere variable!!) 
...  add following line to your crontab: 

* * * * *  /usr/bin/printenv > $HOME/mycronenv.txt 2>&1

... save & exit the editor

Now, out of your interactive shell, enter the next command:

% /usr/bin/printenv > $HOME/myshellenv.txt 2>&1 

Subsequent, evaluate the environments listed within the two information:

cat $HOME/myshellenv.txt
...
cat $HOME/mycronenv.txt
...

See the distinction? Be aware particularly the distinction within the PATH atmosphere variable.

Subsequent, discover out the place your ping command is positioned:

whereis ping
ping: /sbin/ping /usr/share/man/man8/ping.8

Ask your self, “Is /sbin included within the PATH atmosphere variable for cron?”

The reply is: “No”

What ought to I do about that?

Most likely the best factor to do is change a line in your script:

FROM: 
ping localhost 

TO:
/sbin/ping -c 1 localhost

Please word the addition of -c 1 has nothing to do with atmosphere, that’s merely to maintain you from clogging your system with a bunch of ping processes!!

One different factor you could have to do beneath macOS:

1. Open System Settings, then choose Privateness and Safety, after which Full Disk Entry

2. Examine to confirm that cron is listed (and enabled) for “Full Disk Entry”

3. IF IT IS LISTED, BUT NOT ENABLED: Click on the toggle to allow it

4. IF IT IS NOT LISTED: Click on the + on the backside of the itemizing

5. Add cron (beneath /usr/sbin/cron to present cron “Full Disk Entry”)

And FWIW, see this recipe masking utilization of cron & crontab