Linux terminal running on an Ubuntu-themed desktop environment.
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The Linux look command races through a file and lists all the lines that begin with a particular word or phrase. But watch out! It behaves differently on different Linux distributions. This tutorial will show you how to use it.

Ubuntu’s look Command Behaves Differently

For a simple, but useful, command, look certainly gave me the runaround when I was researching this article. There were two problems: compatibility and documentation.

This article was checked using Ubuntu, Fedora, and Manjaro. look was bundled with each of those distributions, which was great. The problem was the behavior wasn’t the same across all three. The Ubuntu version was very different. According to the Ubuntu Manpages, the behavior should be the same.

I eventually figured it out. look traditionally uses a binary search, while Ubuntu look uses a linear search. The online Ubuntu man pages for Bionic Beaver (18.04), Cosmic Cuttlefish (18.10), and Disco Dingo (19.04) all say the Ubuntu version uses a binary search, which is not the case.

If we take a look at the local Ubuntu man page, we see it clearly states their look uses a linear search. There is a command-line option to force it to use a binary search. Neither of the versions in the other distributions has an option to choose between search methods.

man look

"man look" in a terminal window.

Scrolling down through the man page, we see the section that describes this version of  look using a linear instead of binary search.

Ubuntu look man page in a terminal window.

The moral of the story is to check the local man pages first.

Linear Search versus Binary Search

The binary search method is faster and more efficient than a linear search. Working with large files makes this very apparent. The downside to the binary search is your file must be sorted. If you don’t want to sort your file, sort a copy of it, and then use that with look.

We’ll demonstrate this elsewhere in this article. Just be aware that on Fedora, Manjaro, and I expect most of the rest of the Linux world, you’ll need to create a sorted copy of your file and work with that.

Installing words

look can work with any text file you choose, or it can work with the local dictionary file “words.”

On Manjaro you need to install the “words” file. Use this command:

sudo pacman -Syu words

"sudo pacman -Syu words" in a terminal window.

Using look

For this article, we’ll work with a text file of the Edward Lear poem “The Jumblies.”

Let’s look at its contents with this command:

less the-jumblies.txt

"less the-jumblies.txt" in a terminal window.

Here’s the first part of the poem. Note that we’re using Ubuntu, so the file remains unsorted. For Fedora and Manjaro, we’d work with a sorted copy of the file, which we’ll cover later in this article.

"The Jumblies" in less in a terminal window.

If we look for lines that start with the word, “They,” we’ll find out some of what the Jumblies did.

look They the-jumblies.txt

"look They the-jumblies.txt" in a terminal window.

look responds by listing these lines:

Output from "look They the-jumblies.txt" in a terminal window.

Ignoring Character Case

To make look ignore differences between upper- and lowercase, use the -f (ignore case) option. We’ve used “they” as the search word again, but this time, it’s in lowercase.

look -f they the-jumblies.txt

look -f they the-jumblies.txt in a terminal window.

This time, the results include an extra line.

Output from "look -f they the-jumblies.txt" in a terminal window.

The line that begins with “THEY” was missed in the last set of results because it’s in all uppercase and didn’t match our search term, “They.”

Ignoring case allows look to include it in the results.

Using look with a Sorted File

If your Linux distribution has a version of look that follows the traditional behavior of using a binary search, you must either sort your file or work with a sorted copy of it.

Let’s repeat the command to search for “They,” but this time on Manjaro.

look They the-jumblies.txt in a terminal window.

As you can see, no results were returned. But we know there are lines in the poem that start with the word, “They.”

Let’s make a sorted copy of the file. If you’re going to use the -f (ignore case) or -d (alphanumeric characters and spaces only) options with look, you must use them when you sort the file.

The -o (output) option allows you to specify the name of the file the sorted lines should be added to. In this example, it’s “sorted.txt.”

sort -f -d the-jumblies.txt -o sorted.txt

sort -f -d the-jumblies.txt -o sorted.txt in a terminal window.

Let’s use look on the sorted.txt file, and then use the -f and -d options.

sort -f -d the-jumblies.txt -o sorted.txt in a terminal window.

Now, we get the results we expected.

Only Consider Spaces and Alphanumerics

To make look ignore anything that isn’t an alphanumeric character or a space, use the -d (alphanumeric) option.

Let’s see if there are any words that start with, “Oh.”

look -f oh the-jumblies.txt

look -f oh the-jumblies.txt in a terminal window.

No results are returned by look.

Let’s try again and tell look to ignore anything other than alphanumeric characters and spaces. That means characters and symbols, such as punctuation, will be ignored.

look -f -d oh the-jumblies.txt

look -f -d oh the-jumblies.txt in a terminal window.

This time, we get a result. We didn’t find this line before because the quotation marks and exclamation point confused the search.

Output from "look -f -d oh the-jumblies.txt" in a terminal window.

Specifying the Terminating Character

You can tell look to use a specific character as the terminating character. Usually, spaces and end of lines are used as the terminating character.

The -t (terminate character) option allows us to specify the character we’d like to use. In this example, we’re going to use the apostrophe character. We need to quote it with a backward slash so that look knows we’re not opening a string.

We’re also quoting the search term because it includes a space. We’re searching for two words.

look -f -t \' "they call" the-jumblies.txt

look -f -t \' "they call" the-jumblies.txt in a terminal window.

The results match the search term, terminated by the apostrophe we used as the terminating character.

Output from "look -f -t \' "they call" the-jumblies.txt" in a terminal window.

Using look Without a File

If you don’t provide a filename on the command line, look uses the words file.

The command:

"look circle" in a terminal window.

gives these results:

"look circle" results in a terminal window.

These are all the words in the file that begin with the word “circle.”

look No Further

That’s all there is to look.

It’s pretty easy once you know there are different behaviors across different Linux distributions, and you’ve bottomed out whether your version uses a binary or linear search.

Dave McKay Dave McKay
Dave McKay first used computers when punched paper tape was in vogue, and he has been programming ever since. After over 30 years in the IT industry, he is now a full-time technology journalist. During his career, he has worked as a freelance programmer, manager of an international software development team, an IT services project manager, and, most recently, as a Data Protection Officer. Dave is a Linux evangelist and open source advocate.
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