How to Use the Command 'cat' (with Examples)

The cat command, short for “concatenate,” is a fundamental command in Unix and Unix-like operating systems. It is primarily used for displaying the contents of files, as well as for concatenating and creating files. This versatile command is often utilized for text processing, allowing users to quickly manipulate files from the command line. In this article, we will explore various use cases of the cat command to demonstrate its capability in handling different tasks effectively.

Use case 1: Print the contents of a file to stdout

Code:

cat path/to/file

Motivation:
The primary use of the cat command is to print the contents of a file to the standard output (stdout). This is especially useful when you want to quickly inspect the content of a file without the need for opening it in a text editor. Whether checking logs, configuration files, or simple text documents, using cat allows for a swift and straightforward review.

Explanation:

• The command cat is followed by the path to the file you wish to view. In this example, path/to/file is a placeholder indicating the location of your file.
• cat reads the file content and writes it to the standard output, typically the terminal.

Example output:
If path/to/file contains the text:

Hello, World!
This is a sample file.

Executing cat path/to/file would display:

Hello, World!
This is a sample file.

Use case 2: Concatenate several files into an output file

Code:

cat path/to/file1 path/to/file2 ... > path/to/output_file

Motivation:
Concatenating multiple files into a single output file is a common task when dealing with fragmentation or when collecting data from several sources. By using cat, you can easily merge contents of different files into a consolidated file, aiding in data organization or compilation efforts.

Explanation:

• cat command is supplied with multiple files (path/to/file1, path/to/file2, etc.) which it reads sequentially.
• The > operator redirects the combined output to another file specified (path/to/output_file). This overwrites the contents of the output file if it already exists.

Example output:
Assuming file1 contains:

Apple
Orange

And file2 contains:

Banana
Grape

After running cat path/to/file1 path/to/file2 > path/to/output_file, path/to/output_file would contain:

Apple
Orange
Banana
Grape

Use case 3: Append several files to an output file

Code:

cat path/to/file1 path/to/file2 ... >> path/to/output_file

Motivation:
Appending files rather than overwriting is ideal when you want to add content to an existing file without losing its current data. This is particularly beneficial in log management or when incrementally compiling information into a single file over time.

Explanation:

• Here, cat again operates over multiple files.
• The >> operator appends the output to path/to/output_file. If the file does not exist, it is created. Existing content in the file remains intact, and the new data is added at the end.

Example output:
If path/to/output_file initially contains:

Fish
Chips

And you run cat path/to/file1 path/to/file2 >> path/to/output_file, the path/to/output_file will now display:

Fish
Chips
Apple
Orange
Banana
Grape

Use case 4: Copy the contents of a file into an output file without buffering

Code:

cat -u /dev/tty12 > /dev/tty13

Motivation:
This use of cat is particularly useful when dealing with device files. By copying contents from one terminal device to another without buffering, any latency issues can be mitigated. It allows real-time data transfer, which might be crucial in synchronous communication between devices.

Explanation:

• -u is an option that tells cat to output the contents without buffering.
• /dev/tty12 and /dev/tty13 are device file placeholders. Data is read from /dev/tty12 and directly transferred to /dev/tty13.

Example output:
As this involves device files, there won’t be textual output visible. Instead, any keystrokes or data appearing on /dev/tty12 will be mirrored on /dev/tty13 instantaneously.

Use case 5: Write stdin to a file

Code:

cat - > path/to/file

Motivation:
Writing standard input to a file can be advantageous when capturing user input or piping data from another command into a file. This offers flexibility and control over input data, facilitating dynamic content addition.

Explanation:

• - signifies that cat should read data from standard input (stdin).
• The > operator redirects this input into path/to/file. This will overwrite the file’s content unless combined with >> for appending.

Example output:
When executed, the terminal will await user input. Enter text like:

User-generated content

After hitting CTRL+D to indicate EOF, path/to/file will contain:

User-generated content

Use case 6: Number all output lines

Code:

cat -n path/to/file

Motivation:
Numbering each line of a file can be indispensable in scenarios like code reviews, debugging, or data analysis, where identifying specific line references is essential. It improves readability and makes it easier to reference and edit sections.

Explanation:

• -n is an option that prompts cat to number each output line.
• path/to/file is the file whose contents are being printed with line numbers added.

Example output:
For a file containing:

First line
Second line
Third line

Executing cat -n path/to/file results in:

     1  First line
     2  Second line
     3  Third line

Use case 7: Display non-printable and whitespace characters

Code:

cat -v -t -e path/to/file

Motivation:
In situations where unseen characters such as tabs, line breaks, or non-printable characters might affect data processing or display, this command helps in identifying these hidden elements. It assists in pinpointing formatting issues or unusual characters that may cause errors in scripts or data files.

Explanation:

• -v shows non-printing characters visibly, using ^ and M- notation.
• -t displays tab characters as ^I.
• -e appends $ at the end of each line, making line endings distinctive.

Example output:
If path/to/file includes:

Tab	Space
LineEnd

cat -v -t -e path/to/file would output:

Tab^ISpace$
LineEnd$

Conclusion:

The cat command offers a powerful and flexible way to handle files and their contents directly from the terminal. By mastering the various use cases presented, users can effectively manage and manipulate data within their system, expanding their command-line toolkit for both simple and complex tasks. Understanding these utilities deepens your ability to diagnose, transform, and streamline file operations with minimal effort.