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What program is being executed upon time command?

Louis Coulet

9/22/23, 7:49 PM

In my terminal in Ubuntu, I want to execute the time program with option -v, but it fails:

$ time -v ls
-v: command not found

However if I specify the path /usr/bin/time of the program as such, it works:

$ /usr/bin/time -v ls
foo  bar  baz
        Command being timed: "ls"
        User time (seconds): 0.00
        ...

So it seems that time and /usr/bin/time are different executables. But to my surprise, when trying to identify time with which, it tells that it is the same:

$ which time
/usr/bin/time

I am puzzled, can someone explain what is happening?
Is the result of which somehow not correct?

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bash

command-line-interface

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symcbean

9/22/23, 7:58 PM

Try running type time - it will probably tell you that on your system time is a shell keyword (compare this with orher comands like echo, ls and nano).

If you explicitly want to run /usr/bin/time, then type /usr/bin/time -v ls

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Louis Coulet

9/22/23, 8:02 PM

Yes indeed ```time``` is a shell keyword, I didn't know about those, that's treacherous!

Jörg W Mittag

9/23/23, 6:01 AM

It's not a keyword, it's a builtin. There is no such thing as a "keyword" in a POSIX shell. There are the *reserved words* `!`, `{`, `}`, `case`, `do`, `done`, `elif`, `else`, `esac`, `fi`, `for`,`if`, `in`, `then`, `until`, and `while` which are reserved in all shells and `[[`, `]]`, `function`, and `select` which are reserved in some shells. As you will notice, `time` is not one of those.

Gordon Davisson

9/23/23, 8:21 AM

@JörgWMittag In bash, it's a keyword. See the [Reserved Words section of the bash manual](https://www.gnu.org/software/bash/manual/bash.html#Reserved-Words). This allows things like applying `time` to [an entire pipeline](https://www.gnu.org/software/bash/manual/bash.html#Pipelines) (though this is disabled in POSIX mode).

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vidarlo

9/22/23, 7:57 PM

It's both a binary in /usr/bin/time and a Shell builtin.

$ type time
time is a shell keyword

Shell builtins have precedence over binaries on disk. To use the binary, you have to specify the path, e.g. /usr/bin/time -v ls

The Wikipedia Article has some more information on the topic.

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Louis Coulet

9/22/23, 8:03 PM