Question

Consider a this string containing an integer

nanoseconds=$(date +%s%N)

when I want to strip off the last six characters, what would be semantically better?

Stripping just the characters off the string

nanoseconds=$(date +%s%N)

milliseconds=${nanoseconds%??????}

or dividing the value by 1000000

milliseconds=$((nanoseconds / 1000000))

EDIT

Sry for not being clear. It’s basically for doing a conversion from nanoseconds to milliseconds. I think I answered my own question…

Answer 1

Both are equivalent, but in general I would consider the former method to be safer. The first method is explicit and does precisely what you want to do: to remove a substring from the back of the string.

The other one is a mathematical operation that relies on correct rounding. Although I cannot imagine where it would fail, I would prefer the first method.

Unless, of course, what you really want is not stripping the last three characters but dividing by 1000 🙂

Post scriptum: hah, of course I know where it would fail. Let value=”123″. ${value%???} strips the last three digits, as intended, leaving an empty string. $(( value / 1000 )) results in value equal to "0" (a string of length of 1).

EDIT: since we know now that it is not about stripping characters, but rounding, clearly dividing by 1000 is the correct way of approaching the problem 🙂