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1. Introduction

Sometimes, we need to do some operations that require using multiple files at the same time. This can be something as common as searching for some text in multiple files or merging multiple files into a new one.

In this quick tutorial, we'll show some useful operations that can make our life easier when concatenating files in Linux.

2. The cat Command

The most frequently used command to concatenate files in Linux is probably cat, whose name comes from concatenate.

The command syntax follows the form:

cat [options] [files]

In the next sections, we'll dig deeper into the command and the options we can use.

3. Displaying a File

Let's first go quickly over the basics of the cat command. The most straightforward operation we can do is to display a file:

cat myfile

This displays myfile in the standard output:

This is a text file.

4. Creating a File

We can also use cat to create new files without a text editor.

It's as easy as using the redirection operator:

cat > newfile

After that, we can start typing what we want to add to the file:

creating a new file.

When we want to save the file, we have to press CTRL+D. Notice that if the file exists, it will be overwritten.

5. Concatenating Files

One of the most common functions of the cat command is to concatenate files, as its name suggests.

The most simple concatenation is to display multiple files in the standard output:

cat file1 file2

The command above displays the files sequentially:

My file 1 
My file 2

We can also use wildcards to display all the files that match a common pattern:

cat file*

So far, we've been displaying the files in the standard output, but we can write the output into a new file:

cat file1 file2 > file3

Also, we can append a file to an existing file:

cat file1 >> file2

Another useful option is to read from the standard input, which we represent by using ‘-‘ :

cat - file1 > file2

Then, we can type the text we want to concatenate before file1:

text from standard input

Now, if we type cat file2 to display the file, we can see the text we've introduced concatenated with file1:

text from standard input
My file 1

Also, we could append the standard input after the file instead of before:

cat file1 - > file2

If we go a bit further, we can also concatenate the output of any other command to cat:

ls -la | cat > file1

Finally, we can pipe cat output to other utilities to create more powerful commands:

cat file1 file2 file3 | sort > file4

In this case, we've concatenated three files, sorted the result of the concatenation, and written the sorted output to a new file called file4.

6. Other Options

In the help of the cat command, we can find some other useful options that we can add to our commands:

cat --help
Usage: cat [OPTION]... [FILE]...
Concatenate FILE(s) to standard output.

With no FILE, or when FILE is -, read standard input.

  -A, --show-all           equivalent to -vET
  -b, --number-nonblank    number nonempty output lines, overrides -n
  -e                       equivalent to -vE
  -E, --show-ends          display $ at end of each line
  -n, --number             number all output lines
  -s, --squeeze-blank      suppress repeated empty output lines
  -t                       equivalent to -vT
  -T, --show-tabs          display TAB characters as ^I
  -u                       (ignored)
  -v, --show-nonprinting   use ^ and M- notation, except for LFD and TAB
      --help     display this help and exit
      --version  output version information and exit

For instance, we can use the -n option:

cat -n myfile

That displays the number of each line:

1 This is a test file. 
2 It contains multiple lines.

Alternatively, we can use -e:

cat -e myfile

In this case, it shows a at the end of each line:

This is a test file.$ 
It contains multiple lines.$

These are just some quick examples that show how to use these options.

7. Conclusion

In this quick tutorial, we've shown some examples of how to use the cat command in Linux.

We covered the basics pretty quickly to focus later on the file concatenation.

And, we've also seen that cat can be handy when combined with other commands and can be used in many different situations.

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