Normally the cat command is applied for basically three things in particular:

• Display the contents of a file
• Copy text files to another text file
• Append a text file’s contents to another text file

Being a UNIX command it is ported to all GNU/LINUX distributions and of course, it has options to expand and vary the power of the command itself.  Let’s see the output at the terminal if we do the following command:

cat --help

The command shows us a quick help on the cat command including its options.

The basic usage syntax is:

cat [OPTION] [FILE]…

And the list of commands is shown in this table:

Don’t worry; we’ll examine the most useful options with several examples.

Display Content of File

If we want to show the content of a text file, the syntax would be:

cat path_of_the_archive

I’m going to show the content of a file called file1.txt located in the Example folder. Run below command to display file1.txt content.

cat file1.txt

And there we have the content of the file shown on the terminal in above image.

If you would like to display the contents of several files, simply place them next. Run below command to display contents of two files file1.txt and file2.txt.

cat file1.txt file2.txt

The exit at the terminal would be as follows:

You can see the contents of both files one by one in above image.

Identify Specific File Content

As we can see in above image, it simply shows the contents of file 1 and when it ends, it immediately shows the content of file 2 as if they were “concatenated”. If you would like to know where the content of each file ends, then use the -e option to show a $ at the end of the file.

cat –e file1.txt file2.txt

Add Number of Lines in Output

If you want to display the number of lines along with the file content then you can show it by adding -n option. Run below command to display the line numbers while displaying the file content.

cat –n file1.txt file2.txt

Copy Contents of One file to Another

We can also copy the content of one text file to another using CAT command. Run below command to copy content of file1.txt to file4.txt.

cat file1.txt > file4.txt

In this case, the command reads the contents of file1.txt and overwrites the contents of the new file. If it does not exist, it is simply created. Now we can validate above command whether contents have been copied to not. Now we proceed to show the contents of file4.txt to verify that it made the “copy”. Display content of file4.txt by running below command.

cat file4.txt

We can see the content is same as file1.txt in above image.

Combine or Append the Content of One File to Another

The cat command also allows us to combine or append a text file data to another one, thanks to the operator >>

In this case, we will combine the content of file1.txt with that of file2.txt

cat file1.txt >> file2.txt

And then we check the contents of the file2.txt file to validate it.

cat file2.txt

We can see contents have been combined in to this file. Basically the command takes the content of file1.txt and places it at the end of the file2.txt file

As a conclusion we can say that the cat command is a useful and important command to work with text files in GNU/LINUX and everything about the terminal. It is very useful for managing service configuration files on a server.

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LIST FILES IN LINUX: “ls” COMMAND 

As it was said, the command ls allows you to list files and folders of a specific path, is easy to use and is very useful and used in the daily work. The command has several options to improve the listings. In a terminal, we can invoke the help of the command ls in this way.

ls --help 

Above command will display all options available for ls command to list the files and folders. In this way we get the syntax of usage and options as output per terminal.

As all options will not cover in one single screenshot so i have scrolled the above screen and taken another screenshots to cover all options in screenshots that has been displayed below.

As we can see the documentation is extensive, but don’t worry we will examine the most useful and used options. The basic syntax of the command is:

ls [options] [file|dir]

Although it is possible to use ls from the most basic form only by typing:

#List files from your given directory.
ls

With this we only list the files that are in the directory where we are located in the terminal. For example, if we wanted to list everything in the root directory of our GNU/LINUX distribution it would be with the command:

#List everything in the root directory
ls /

It was easy, wasn’t it?

Most Important Options of the Command “ls”

ls as well as all GNU/LINUX commands have options that modify the basic command execution result. Here is a list of the most outstanding ones for this command.

 Examples of Command “ls”

Here i have given different examples of command ls that you can use to fulfill different requirements.

List Hidden Files 

We can use option -a if we want to list the hidden files of a specific folder, it would be enough to write:

ls –a path

In my case it would be something like that:

ls –a /home/angelo/

You can see all hidden files as well in above screenshot. We can identify hidden files by looking at the output as such files or folders start with dot (.).

Display Files with their Index Number

We can list files with their index number by running below command. To show files/folders with their index number we will use the option –i along with command ls. You can see i have used below command and output is showing in below image.

ls -i /home/angelo/
 

List Files with full Details (Permissions, Index no, Owner, Create Date etc.)

We can use ls command to display information such as permissions and owner of the folder or file. We will be using option -I to show complete information about files and folders as shown in below screenshot.

ls -l /home/angelo/

This output shows us more specific information about what we have in the folder: First it shows the permissions on the folder, its index number, the owner of the folder, who created the folder, its size in kb, the date it was modified and the folder name.

List Files or Folders in a Reverse Order

ls command also gives an option to sort files and folders in reverse order. We can use -r option to show files and folders in a reverse order as shown in below screenshot after running below command.

ls –r  /home/angelo/

List Subdirectories Recursively

We can List subdirectories recursively by adding the -R option with ls command.  It will display the contents of the directory subfolders. Run below command to display the result.

ls –R /home/angelo/

 Display list of Directories Only

We can use -d and an asterisk option to list and show only the directories. Have a look at below command that has displayed the list of directories under /home/angelo.

ls –d */home/angelo/

Display Files Sort by their File Size

Run below command to display files sort by file size. It is possible with the option –S.

ls -S /home/angelo/

Sort by Date/Time

We can option -t to display files sort by date/time. Run below command to see the output that is also showing in below screenshot.

ls –t /home/angelo/

Display Files as they were Created

Use option -U to show folders and files in no order at all. It will display just as they are created.

ls –U /home/angelo/

Sort by File Extension

We can use option -X with ls command to sort files by their extension names. You can see this in below screenshot.

ls –X /home/angelo/

Sort by Modified Files

We can use combination of options -ltr to show last modification file or directory date. Simply run below command to display the output.

ls -lrt

ls -lrt [path]

In conclusion we can say that the ls command, even when it is relatively easy to use, is one of the tools most used by system administrators.

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