Bash – Using the built-in “time” command in bash rather than the external command

bashshell-builtintime-utility

Working with the time command, I came across a situation where I should
use the built-in time rather than the external GNU time command /usr/bin/time. So, how can I do this? I saw somewhere that using enable and/or command would help, but they didn't.

This is a use case:

watch "time ls"

which uses the external /usr/bin/time command, which I don't want! This happens when time invokes the internal bash function when I run time ls on a terminal, like this:

$ time ls

Please note that the exact opposite request has been answered here:

Make bash use external `time` command rather than shell built-in

There is a lot of difference with two commands. The internal time is more precise (which I want), but the external command has more options (which I do not need).

Best Answer

By default, watch runs your command with /bin/sh -c '...' so the output you see is how /bin/sh interprets the time command. Your /bin/sh apparently doesn't have a builtin time.

To run the command with a different shell, use the -x option to get rid of the default, then add your own explicit invocation of the shell whose builtin you want.

watch -x bash -c 'time ls'
watch -x zsh -c 'time ls'

No matter how you run watch, the command you're watching is not a child of the shell that ran the watch command, so that shell's settings aren't directly relevant.

Related Solutions

Bash – Why bash time is more precise then GNU time

After some hardcore bash code examining I found out that bash time uses getrusage() and GNU time uses times(). getrusage() is far more precise because of microsecond resolution.

Bash – Make bash use external `time` command rather than shell built-in

You can use the command shell built-in to bypass the normal lookup process and run the given command as an external command regardless of any other possibilities (shell built-ins, aliases, etc.). This is often done in scripts which need to be portable across systems, although probably more commonly using the shorthand \ (as in \rm rather than command rm or rm, as especially the latter may be aliased to something not known like rm -i).

$ time

real    0m0.000s
user    0m0.000s
sys 0m0.000s
$ command time
Usage: time [-apvV] [-f format] [-o file] [--append] [--verbose]
       [--portability] [--format=format] [--output=file] [--version]
       [--quiet] [--help] command [arg...]
$

This can be used with an alias, like so:

$ alias time='command time'
$ time
Usage: time [-apvV] [-f format] [-o file] [--append] [--verbose]
       [--portability] [--format=format] [--output=file] [--version]
       [--quiet] [--help] command [arg...]
$

The advantage of this over e.g. alias time=/usr/bin/time is that you aren't specifying the full path to the time binary, but instead falling back to the usual path search mechanism.

The alias command itself can go into e.g. ~/.bashrc or /etc/bash.bashrc (the latter is global for all users on the system).

For the opposite case (forcing use of the shell built-in in case there's an alias defined), you'd use something like builtin time, which again overrides the usual search process and runs the named shell built-in. The bash man page mentions that this is often used in order to provide custom cd functionality with a function named cd, which in turn uses the builtin cd to do the real thing.

Best Answer

Related Solutions

Bash – Why bash time is more precise then GNU time

Bash – Make bash use external `time` command rather than shell built-in

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