What format to pass to printf to convert seconds to hh:mm:ss?

Question

I'm looking at an instruction of the form:

$ printf [format] $elapsed

with $elapsed expressed in seconds. I want it to output 'hh:mm:ss'. I couldn't figure out format from the man. Would someone please make a suggestion?

Answer 1

With the printf builtin of the ksh93 shell, and assuming $elapsed is less than 86400 (one day) that would be:

TZ=UTC0 printf '%(%T)T\n' "#$elapsed"

Or with the printf builtin of recent versions of the bash shell (which has copied a subset of ksh93's feature in an incompatible way):

TZ=UTC0 printf '%(%T)T\n' "$elapsed"

%(strftime-format)T is a ksh93 extension that consumes one argument which is interpreted as a timestamp and formats it as per the strftime() format specification, where %T short for %H:%M:%S gives the time of day in HH:MM:SS 24-hour format. ksh93 handles all sorts of different timestamp formats including #number for Unix epochtime while bash handles only numbers which are interpreted as Unix epoch time except for -1 interpreted as now and -2 interpreted as the time the shell was invoked.

With the zsh shell and its strftime builtin:

zmodload zsh/datetime
TZ=UTC0 strftime %T $elapsed

With GNU or busybox date:

date -ud "@$elapsed" +%T

Those basically interpret $elapsed as an Unix epoch time, the number of Unix seconds since 1970-01-01 00:00:00 +0000, hence the TZ=UTC0 and -u to get the corresponding UTC time on that January 1st 1970.

BTW, the man page you link to seems to try (and fail) to document a mix between the GNU printf implementation and the printf builtin of bash, the GNU shell (two not fully compatible implementations of the standard printf utility), I wouldn't trust that site.

Run man printf on your system to get the man page of your standalone printf utility¹, or info bash printf or info zsh printf or the man page of your shell if neither zsh nor bash for the documentation of its printf builtin if it has one (most shells do).

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^{¹ Beware though that if run from within the fish shell, man printf will give you the man page for fish's printf builtin.}