On Thu, Aug 08, 2024 at 08:31:10 +0200, Thomas Schmitt wrote:
> Greg Wooledge wrote:
> > * TIMEFORMAT=... time foo will invoke /usr/bin/time, but
> > TIMEFORMAT=... eval time foo will use the builtin.
(Yeah, oops, time is a "shell keyword", not a "builtin".)
> I wonder about the formal reason for this.
>
> Is it because "time" is not listed in the man page under
> SHELL BUILTIN COMMANDS but mentioned as "reserved word" under Pipelines ?
> Does the variable assignment before "time" cause the rest of the line
> to be regarded as Simple Command rather than as Pipeline ?
There are some mysteries in bash that I'm content simply to write off as
"here be magic". I note that such mysteries exist, I try to find the
best workarounds that I can, and I document them if I feel other users
are likely to find my workarounds helpful.
Honestly, I don't fully know. You could ask on the help-bash mailing
list, if you really want to find out. Or go source-diving in bash
yourself. But I suspect it's simply that the grammar for "time" was
never extended to allow temporary variable assignments.
Shell keywords (such as "time", and also "if", "while", "case", "[["
and several others) have special grammar/parsing rules. If one of these
keywords is involved, you're no longer looking at a Simple Command.
You've got something different.
In the specific case of "time", the syntax is
time [-p] pipeline
If your code says
time echo hi | sleep 2
then you aren't simply timing the echo command. You're timing the whole
*pipeline*, including the sleep command. This wouldn't be possible
without "time" flexing its keyword powers and bending the parsing rules.
You can't do that with /usr/bin/time, unless you invoke a whole new shell:
/usr/bin/time sh -c 'echo hi | sleep 2'
which of course distorts your timing. (Also note that "time echo" runs
the shell's builtin echo, while "/usr/bin/time echo" would be forced
to run /usr/bin/echo, since /usr/bin/time doesn't have access to the
shell's builtins.)
