if, elif, else statement issues in Bash

[ is a command (or a builtin in some shells). It must be separated by whitespace from the preceding statement:

elif [

There is a space missing between elif and [:

elif[ "$seconds" -gt 0 ]

should be

elif [ "$seconds" -gt 0 ]

All together, the syntax to follow is:

if [ conditions ]; then
   # Things
elif [ other_conditions ]; then
   # Other things
else
   # In case none of the above occurs
fi

---

As I see this question is getting a lot of views, it is important to indicate that the syntax to follow is:

if [ conditions ]
# ^ ^          ^

meaning that spaces are needed around the brackets. Otherwise, it won't work. This is because [ itself is a command.

The reason why you are not seeing something like elif[: command not found (or similar) is that after seeing if and then, the shell is looking for either elif, else, or fi. However it finds another then (after the mis-formatted elif[). Only after having parsed the statement it would be executed (and an error message like elif[: command not found would be output).

You have some syntax issues with your script. Here is a fixed version:

#!/bin/bash

if [ "$seconds" -eq 0 ]; then
   timezone_string="Z"
elif [ "$seconds" -gt 0 ]; then
   timezone_string=$(printf "%02d:%02d" $((seconds/3600)) $(((seconds / 60) % 60)))
else
   echo "Unknown parameter"
fi