Shell execution: time vs. /usr/bin/time

time is builtin in both zsh and bash. However, which is only built-in to zsh. In bash, when you use which it runs /usr/bin/which which has no idea about shell built-ins.

So in bash, you should use:

$ type timetime is a shell keyword

The reason time -l ... doesn't work is that the time syntax doesn't include the -l flag.

In both cases, it's not really correct to say that time is a built-in function. Calling it a "reserved word" or "shell keyword" is more accurate, because it applies to an entire pipeline; it cannot be implemented as a function or external command. In that sense, it is similar to other syntactic elements like if and while.

Another way to know whether a command is a bash builtin command is to use the help builtin command. The side effect is to get the information about said command (and supported command line arguments.)

bash$ help timeReport time consumed by pipeline's execution.Execute PIPELINE and print a summary of the real time, user CPU time,and system CPU time spent executing PIPELINE when it terminates.Options:  -p    print the timing summary in the portable Posix formatThe value of the TIMEFORMAT variable is used as the output format.Exit Status:The return status is the return status of PIPELINE.

For non-builtin commands, help says it does not know about it.

bash$ help lsbash: help: no help topics match `ls'.  Try `help help' or `man -k ls' or `info ls'.

time is a builtin function in zsh. It is not in bash. If you want to use the /usr/bin/time version you need to supply the full path when using zsh.

It is also possible to disable this behavior using the "disable -r" command in zsh.