How To Time Command Execution with the time Command

Introduction

There are times you’ll want to know how long it took for a command or script to execute. You could track the start and end times and calculate the difference to get the duration. You could even grab your trusty stopwatch app and track things that way. Or, you could save yourself all that trouble by leveraging the time command.

The command-line is full of a bunch of small, single-purpose utilities that can help eliminate the need to write any additional code yourself. The time command is one such command. The time command is in no way related to the date command, which provides the system’s date and time. Instead, it times a program or script’s execution and tells you how long it took.

In this tutorial, you’ll use the time command and explore its output.

The examples in this tutorial (unless otherwise noted) will be using time that’s built into the Bash shell on Linux.

Step 1 — Timing Command Execution

To time the execution duration of a command, you prefix your command with time.

However, how you execute time depends on your operating system. time exists both as a built-in command for some shells, like bash and zsh, as well as a stand-alone command, known as GNU time which has different arguments than the time built into shells.

Use the following command to see how time works on your system:

type -a time

You’ll see output like the following:

Output
time is a shell keyword
time is /usr/bin/time

In this case, there’s both a built-in shell command called time, and a version of time installed at /usr/bin/time.

If you want to use the GNU version of time instead, you can prefix it with a backslash:

\time

If you don’t, your shell will use its built-in version instead.

Both methods serve the same purpose, although the GNU version has more advanced formatting options.

Let’s explore timing program execution by using the tree command on the root / of your file system, which will list out a visual tree of all of your files and directories.

The tree command is not always installed by default on many systems, but you can install it on Ubuntu and Debian systems with apt:

sudo apt install tree

On macOS, you can install tree with Homebrew:

brew install tree

Now that the tree command is installed, use it to look at all the files on the system, but prefix it with time to see how long it takes:

time tree /

You’ll see the file information scroll through. Eventually, it’ll stop and present you with the time taken:

Output
# The output of tree will scroll by here.

166590 directories, 1568127 files
tree /  12.24s user 10.37s system 66% cpu 33.827 total

Notice the command you executed, tree /, is echoed back by the time command as well. The output shows a few pieces of info, but for now, focus on the total and the number just prior:

Output
tree /  12.24s user 10.37s system 66% cpu 33.827 total

That’s how long the command took to execute in seconds.

The time command also works if you happen to cancel a command with CTRL+C. If you were to run time tree / again and quickly hit CTRL+C, the tree command will stop scrolling by, and you’ll be presented with the time results for the time the command was executing before you stopped it.

Step 2 — Understanding Resource Usage

The output from time includes three values, in addition to the total duration.

Output
tree /  12.24s user 10.37s system 66% cpu 33.827 total

The first is the total amount of time (in CPU-seconds) that the command spent in user mode. That’s the value with user after it.

The next, suffixed by system, is the amount of time (again, in CPU-seconds) that the command spent in system, or kernel mode.

Finally, the percentage of the CPU that was allocated to the command, suffixed with cpu.

The difference between user and system time is that CPU usage is broken down by access levels. When code is executed in user mode, it doesn’t have direct access to hardware or reference memory and must rely on APIs of the system for delegation. This is how most code runs on your system, and due to its isolation, crashes are always recoverable.

Kernel mode, on the other hand, is when code being executed has unrestricted access to the system hardware. This mode is pretty much reserved for the most trusted functions of the operating system. Because of having complete access, when things crash in kernel mode, they crash bad and tend to take the system down with them.

Conclusion

In this tutorial, you explored how to use the time command to see how long commands and scripts take to run, and where that time goes. Your shell’s time command gives you quick access to timing the execution duration of a command without any additional scripting.