I want to measure execution time of commands and get (to me) strange behavior of the time command.

I call time like it is mentioned in the examples of the manpages with the -f or --format option and get a command not found error. Seems like it thinks -f is the command to measure the execution time of...

user@laptop:~$ time -f '%e' sleep 1
-f: Befehl nicht gefunden.

Then I call the same line again, just that I specify the path to time. Result changes, works as intended (wtf?):

user@laptop:~$ /usr/bin/time -f '%e' sleep 1

Now I'm confused... even which can't help me:

user@pc:~$ which time

And... if I don't use options it works fine, even without full path:

user@laptop:~$ time sleep 1

real    0m1.003s
user    0m0.001s
sys 0m0.002s

How do I use this command correctly if I want to specify options? Using the full path seems to work, but looks more like an unreliable side effect of something buggy (or something I don't understand)?

Edit: I'm using Ubuntu 14.04 LTS (trusty)


2 Answers 2


Bash has a built in command called time. So if you just type time it will use the shell built-in which has no options like -f. But man time (which talks about -f) will give the manpage of the /usr/bin/time program. So if you want to use options described in the manpage you should ensure you call the program, not the shell built-in. One way achieving this is using the full path.

(IMHO, this is really ugly in bash)

For more info, see the links below the question.

  • 1
    Actually time isn't a shell builtin it's a reserved word.
    – user591
    Mar 1, 2016 at 6:16
  • Do you have a reference for that? unix.stackexchange.com/questions/183745/… Similar to a builtin, a keyword is hard-coded into Bash, but unlike a builtin, a keyword is not in itself a command, but a subunit of a command construct. So are you saying time is a subunit of a command construct ? Mar 1, 2016 at 12:52
  • It is in POSIX, The reason: time "needs" to be transparent to pipes and redirections. The "needs": for practical common uses and historical reasons. EDIT: POSIX uses "times", but the reason is the same. Mar 1, 2016 at 14:11

Just thought I would add a way to show the info for Boris's answer.

As was just puzzled by time again.

In bash type is your friend. I should really remember to use it more. Old csh habits die hard.

which -a time  

type time 
time is a shell keyword

type times  
times is a shell builtin

type nslookup  
nslookup is /usr/bin/nslookup

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