Despite how it sounds, if you happen to be most comfortable working in a desktop environment with a graphical user interface, the cut command doesn’t fill your clipboard. Instead, it cuts out pieces of standard input or a file and spills them out on your screen. Now you are bourne-again.

As it happens, the cut command is a powerful tool that helps you navigate through the complexities of text formatted documents and get things done in the command line and bash scripts like a boss.

Here we will focus on examples, getting our hands dirty as we dive deeper into the bash cut command. Read on.

When to use the cut command?

Use the cut command when manipulating field delimited text files such as CSV, log files, any text file with a format. For example, you may want to reduce the number of columns in a file instead of using other commands like awk. You may also want to retrieve the first section of text found inside parenthesis without using other commands like grep.

Cut command example in bash, Single cut examples

Example) Some random cut

Here is a quick cut example where the delimiter is set but the selected field is variable showing how to use the cut command dynamically.

Commands

Output

Example) Cutting out IP addresses from nslookup output

nslookup is a helpful command-line utility for looking up a host IPs and names that you will find in commonly used DNS tools. It may be old but gets the job done. It comes with an output that is to my best guess standard across most systems.

For example, consider the command that follows.

Command

Output

Now suppose that we want to reduce the nslookup output into a single ip by cutting, here is a snippet showing how to cut out nslookup ip values in bash. Note that we assume the lookup always returns with success just to make our example work. You may implement a more robust version as an exercise.

Commands

Output

Example) Cutting out IP from dig output

Dig is a command-line utility included in a package call Bind 9 like nslookup that I just came across recently. I guess I really should have read advanced Linux networking commands. It is particularly helpful when trying to lookup large batches of host ips. Here is what the corresponding command line output would look like.

Command

Output

Notes on format

(1) ;; indicate that which follows is a comment
(2) sections are separated by blank lines

Now suppose we want to implement the same function as the previous example using dig, here is how it may look.

Commands

Output

Notes

(1) In the example immediately above, our cut delimiter is the default, tab character
(2) In the grep command preceding cut, we filter out formatted lines discussed in Notes on format

Example) Convert CSV to TSV using cut

You are tasked to convert a CSV file to TSV. There is a hoard of ways to accomplish this task. However, we are going to use cut because we can. Here’s how.

Commands

Output

Note

(1) The input delimiter we use is ,
(2) We set the output delimiter to the tab character
(3) -f1- means to output all fields

Double cut examples

Some formats require a double cut to get the fields we are looking for. The following examples show a few cases that you are likely to find in the wild.

Example) Cutting out the apache access log path info

In this next example, we are going to dig into some apache access logs and retrieve the path from the URL part. If you are not sure what that means, it’s the part that comes after the domain name in the URL. I’ll color it.

Example apache log line (above) from Loggly Apache Logging Basics

Also, here are some log format used in apache logs. Note that it is common that the request field shows up before other composite fields in double quotations. We will use this knowledge to cut out what we need from apache logs.

Source: Apache Module mod_log_config

Here is how the code would look as a boilerplate.

Boilerplate commands

Now if we feed the above commands into the terminal or source from a script, you would be able to call the paths function. Initially, it doesn’t do anything but once first-cut and second-cut have been implemented, it will.

The following assumes that the boilerplate commands (above) are loading into context.

In the first-cut, we will need to implement a function to select what is in the first set of double-quotes. Implementation follows.

Commands

Notes on above commands

(1) We expect the input to be piped in. That is where the – comes into play at the end of cut. You can get away without it but I think it is easy to read and more explicitly so we’ll use it.

(2) The input delimiter is ”

(3) The 2nd field is selected

Just to exemplify how first-cut works, let’s throw together a quick example.

Commands

Output

Okay. It works! Moving on.

In the second-cut we will need to implement a function to select what comes second in a line delimited by the space character.

Commands

Notes on above commands

(1) second-cut is identical to first-cut except the input delimiter is the space character instead of a double quote

Just so that we are sure it works, here is a quick example.

Commands

Output

Now that we know everything work, let’s try rerunning paths.

Commands

Output

Wrapping things up, let’s complete the boilerplate with a fully implemented version of first-cut and second-cut.

Commands

Multiple cut examples

When it comes to command-line voodoo, it doesn’t get much harder than multiple cuts. At this point you should be asking yourself, should I be using cut for everything? Why not. Nevertheless, you will be tempted to cut your way through Linux if it works.

Example) Cut: The Game

Trust me. It says cut.

Truth is that while thinking up bash cut command examples, I ran out of ideas. Why not make a game? Now that’s a good idea! How?

Dog ate my script. So, I guess I will have to write it from scratch. Hopefully, it comes out better than before.

Source: cut-the-game.sh

Commands

Bottom line

The cut command is not going anywhere. That is to say, familiarity with its usage makes a great addition to your command line toolbox. I hope the above example helped improve your understanding of cut.