Writing to Files Using cat Command on Linux

Introduction

The cat command is a Unix tool used for manipulating and displaying file contents. The command gets its name from the word "concatenate" because it has, among other things, the ability to concatenate files.

In this article, we'll go through a few easy ways to use this command to write text to a file with examples. Using cat is very straightforward, so no prior programming or Unix experience is needed to follow along.

cat Command Basics

Starting, we'll just sum up the basics of the cat command to help you out if you've never used it before or need a brief overview.

Syntax

The syntax looks like this:

cat [OPTION]... [FILE]...

To quickly look up the syntax or command options, run cat with the help option:

$ cat --help

Or, you can use the manual pages:

$ man cat

These commands should display the following list of options:

  -A, --show-all           equivalent to -vET
  -b, --number-nonblank    number non empty 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

Displaying File Contents on the Standard Output

To print the contents of a file to the standard output just name the file you want to display:

$ cat filename1 

If the file is in a different directory, you'll have to navigate to it:

$ cat /dir1/dir2/filename1 

We'll be expecting to see the contents of this file, printed to the standard output, in this case - the terminal:

Content of filename1!

This is the most common use of the cat command since it makes it easy to peek into the contents of a file without opening a text editor.

Writing Text to a File Using cat

To redirect the output of the cat command from the standard output to a file, we can use the output redirection operator >:

$ cat filename1 > filename2

This will overwrite the contents of filename2 with the contents of filename1, so make sure that filename2 doesn't contain anything you would mind losing. Now, filename2 contains:

Content of filename1!

The output redirection operator will redirect the output of any command we call. For example, let's try it out with the pwd command, which prints the name of the current working directory:

$ pwd > testfile

If we take a look at the testfile now:

$ cat testfile

It contains the current working directory's path:

/home/kristina

If the file you are redirecting to doesn't exist, a file by that name will be created:

$ cat filename1 > newfilename

Concatenating Files with cat

Concatenating multiple files using cat is simple - just list the files in the desired order:

$ cat filename1 filename2 > outputfile
$ cat outputfile

This takes the filename1 and filename2 files, concatenates them, and outputs the into a new outputfile:

Content of filename1!
Content of filename2!

Standard Input Between Files

When the name of the input file isn't listed, cat starts reading from the standard input until it reaches EOF (end-of-file). The end-of-file signal is sent by the ctrl+d command line shortcut:

$ cat > outputfile
Hello
World
$ cat outputfile

This would output:

Hello 
World

We can even add text from the standard input in between files we wish to concatenate by using - to indicate where we expect standard input. If we have files such as filename1, filename2, and filename3, and we want some text from the standard input in between filename1 and filename2, we would write:

 $ cat filename1 - filename2 filename3 > output
 Text from standard input!
 $ cat output

Checking output, we'll see something along the lines of:

Content of filename1!
Text from standard input!
Content of filename2!
Content of filename3!

Appending Files with cat

In the previous examples, using the redirection operator discarded the previous contents of the output file. What if we want to append the new content to the old content? To append files we use the >> operator:

$ cat filename1 filename2 >> output
$ cat output

And that should result in:

Original output file contents.
Content of filename1!
Content of filename2!

Concatenating Contents of all Files Directory with cat

To concatenate all contents of all files in a directory, we use the wildcard *:

$ cat /dir1/dir2/* > output

To concatenate all contents of all files in the current working directory, we'd use:

$ cat * > output

* can also be used to concatenate all files with the same extension:

$ cat *.txt > output

Enumerating Line Numbers

Enumeration of all lines of output is done with the -n option:

$ cat -n filename1 filename2 filename3 > output
$ cat output

Which would write something along the lines of:

     1  Content of filename1!
     2  Content of filename2!
     3  Content of filename3!

Write $ at the End of Every Line

The -E option marks the end of every line in the file with the $ character:

$ cat -E filename1 filename2 filename3 > output
$ cat output

Sorting Lines of Concatenated Files by Piping

This one is a bit of a cheat. The cat command can't sort, but we can use piping to achieve that. The pipe command (|) is used to turn the output of one command into the input of another. To sort the lines of a file, we'll use both cat and another command, called sort:

$ cat filename2 filename3 filename1 | sort > output
$ cat output

This results in:

Content of filename1!
Content of filename2!
Content of filename3!

Conclusion

Cat is a simple, yet powerful Unix tool that comes preinstalled on most systems. It can be used on its own or in combination with other commands using pipes. Originally made by Ken Thompson and Dennis Ritchie in 1971, cat's easy to use and intuitive functionalities stand the test of time.

In this article, we've explored some of the possibilities of using the cat command to write text to files, check contents, concatenate and append files as well as enumerate lines and sort them.