Basic file manipulation with the cat command

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Here we'll use the cat command to display, create, and concatenate files. This command is useful when you do complex data wrangling.

The cat command is one of the most basic yet useful commands for working with files. It has three main functions when it comes to text files: display their content, create new files, and combine copies.

Display content

The cat command is most commonly used to read the content of files. Its default output destination, which is also known as a standard output, or stdout, is the monitor screen. To display the content of a file, you would enter cat <filename>.

• Run the cat command to display the content of the NASA-software-API.txt file:

  cat NASA-software-API.txt
  

  Your output should list each line in the file, and end with these lines:

  ...
  SSC-00424 SSC 2013-09-06T00:00:00.000 "General Public" "SSC Site Status Mobile Application"
  GSC-14732-1 GSFC 2004-06-09T00:00:00.000 "Open Source" "Tool For Interactive Plotting, Sonification, And 3D Orbit Display (TIPSOD)"
  GSC-14730-1 GSFC 2004-06-09T00:00:00.000 "Open Source" "Space Physics Data Facility Web Services"
  GSC-14726-1 GSFC 2004-06-09T00:00:00.000 "Open Source" "Earth Observing System (EOS) Clearinghouse (ECHO)"
  

Create files

You can use the cat command to create files. Using this command is often easier than using a text editor or vi (short for visual) editor for small files.

We create new files by redirecting the standard output of the cat command with the output redirect operator (>) followed by the name of the file to create. After you add text, you use Ctrl + D to return to the prompt.

1. Run the cat command with the greater than (>) operator and the name of the file to create, such as file1:

   cat > file1
   

2. Add the following text, and then press Enter to move the cursor to a new line.

   NASA headquarters
   

3. Press Ctrl + D to end the concatenation action and restore the prompt.

4. To append a line to the file without overwriting the existing content, use the double greater than (>>) operator with the cat command:

   cat >> file1
   

5. Add the following text, and then press Enter to move the cursor to a new line.

   Goddard Space Flight Center
   

6. Press Ctrl + D to end the concatenation action and restore the prompt.

7. If you run the ls command, you'll see the new file in your directory:

   file1   NASA-logs-1995.txt   NASA-software-API.txt
   

8. Use the basic cat command to show the content of the updated file:

   cat file1
   

   You should see this output:

   NASA headquarters
   Goddard Space Flight Center
   

Concatenate files

The third use of the cat command is to concatenate copies of the contents of files. Because the concatenation occurs only to the copies, there's no effect on the original files.

1. Run the following command to concatenate two copies of file1 to stdout:

   cat file1 file1
   

   Your output should look something like this:

   NASA headquarters
   Goddard Space Flight Center
   NASA headquarters
   Goddard Space Flight Center
   

2. If you run the ls command now, you'll notice no new file has been added to your directory:

   file1   NASA-logs-1995.txt   NASA-software-API.txt
   

   The content of each file was displayed to stdout starting on a new line and in the order that the files were named in the command. You can also redirect concatenation output to a file.

3. Run the cat command with the greater than (>) operator to save the output in a new file, such as file2:

   cat file1 file1 > file2
   

4. Examine the contents of the new file:

   cat file2
   

   The output should look like this:

   NASA headquarters
   Goddard Space Flight Center
   NASA headquarters
   Goddard Space Flight Center
   

5. If you run the ls command again, you'll see the new file in your directory:

   file1   file2   NASA-logs-1995.txt   NASA-software-API.txt