How do I convert an epoch timestamp to a human readable format on the cli?

On *BSD:

date -r 1234567890

On Linux (specifically, with GNU coreutils ≥5.3):

date -d @1234567890

With older versions of GNU date, you can calculate the relative difference to the UTC epoch:

date -d '1970-01-01 UTC + 1234567890 seconds'

If you need portability, you're out of luck. The only time you can format with a POSIX shell command (without doing the calculation yourself) line is the current time. In practice, Perl is often available:

perl -le 'print scalar localtime $ARGV[0]' 1234567890

date -d @1190000000 Replace 1190000000 with your epoch

If your epoch time is in milliseconds instead of seconds, remove the last three digits before passing it to date -d:

$ date -d @1455086371603
Tue Nov  7 02:46:43 PST 48079     #Incorrect

This gives incorrect data. Remove the last three digits.

$ date -d @1455086371
Tue Feb  9 22:39:31 PST 2016      #Correct after removing the last three digits. You may remove and round off the last digit too.