Bash: subtracting 10 mins from a given time

Question

In a bash script, if I have a number that represents a time, in the form hhmmss (or hmmss), what is the best way of subtracting 10 minutes?

ie, 90000 -> 85000

Answer 1

Since you have a 5 or 6 digit number, you have to pad it before doing string manipulation:

$ t=90100
$ while [ ${#t} -lt 6 ]; do t=0$t; done
$ echo $t
090100
$ date +%H%M%S --utc -d"today ${t:0:2}:${t:2:2}:${t:4:2} UTC - 10 minutes"
085100

Note both --utc and UTC are required to make sure the system's timezone doesn't affect the results.

For math within bash (i.e. $(( and ((), leading zeros will cause the number to be interpreted as octal. However, your data is more string-like (with a special format) than number-like, anyway. I've used a while loop above because it sounds like you're treating it as a number and thus might get 100 for 12:01 am.