Bash cat Command

It's primarily used to display the content of files right in your terminal and to combine (concatenate) multiple files into a single output or new file.

Basic Usage: Displaying File Content

The most common use of cat is to simply show what's inside a file.

bash
cat myfile.txt

Remove Extra Empty Lines

If your file has multiple consecutive blank lines, the -s option can "squeeze" them down, replacing any sequence of two or more blank lines with a single blank line.

Example: Imagine you have a file, myfile.txt, that looks like this:

file_type_powershellterminal
This is the first line.
 
 
And this is the third.
 
 
 
 
Last line here.

if you were to run cat myfile.txt without any options, you would see all those blank lines exactly as they are in the file

But when you use cat -s myfile.txt, the -s option tells cat: "If you find two or more blank lines in a row, replace that entire block of blank lines with just one blank line."

Output with -s:

file_type_powershellterminal
This is the first line.
 
And this is the third.
 
Last line here.

Using cat with Piping

The cat command is frequently used with pipes (|). A pipe takes the standard output of one command and feeds it as the standard input to another command. This is incredibly useful for processing text data with other command-line tools.

bash
cat myfile.txt | grep "world"

Output:

file_type_powershellterminal
world

How it works:

  • cat myfile.txt displays the entire content of myfile.txt.
  • The pipe | sends this content as input to grep "world".
  • grep "world" then filters this input and only displays lines that contain the word "world".

Combining Files into a New File

This is the most frequent way cat is used for concatenation. You list the files you want to combine, and then use the redirection operator (>) to send the combined output to a new file.

bash
cat file1.txt file2.txt > combined_file.txt

  • file1.txt, file2.txt: These are the source files whose contents you want to combine. You can list as many files as you need, separated by spaces.

  • >: This is the output redirection operator. It tells your shell to take the result of the cat command (which is the combined content of file1.txt and file2.txt) and write it into combined_file.txt.