Convert a time span in seconds to formatted time in shell

Here’s a fun hacky way to do exactly what you are looking for =)

date -u -d @${i} +"%T"

Explanation:

• The date utility allows you to specify a time, from string, in seconds since 1970-01-01 00:00:00 UTC, and output it in whatever format you specify.
• The -u option is to display UTC time, so it doesn’t factor in timezone offsets (since start time from 1970 is in UTC)
• The following parts are GNU date-specific (Linux):
  • The -d part tells date to accept the time information from string instead of using now
  • The @${i} part is how you tell date that $i is in seconds
• The +"%T" is for formatting your output. From the man date page: %T time; same as %H:%M:%S. Since we only care about the HH:MM:SS part, this fits!