The cat command in Linux concatenate files and displays the output to the standard output (usually, the shell).
One of the most common uses of cat is to display a file to the screen and also to create a file on the fly and allow basic editing straight at the terminal.
How to Create a File Using 'cat'
To create a file using the cat command enter the following in the terminal window:
cat > filename
When you create a file in this manner, the cursor will be left on a new line, and you can start typing. This technique offers a great way to start a text file. To finish editing the file, press Ctrl+D. The file saves with whatever you used for filename.
Test that the process worked by typing the ls command:
ls -lt
You should see your new file, and the size should be greater than zero.
How to Display a File Using 'cat'
The cat command displays a file to the screen as well. All you need to do is eliminate the greater than symbol as follows:
cat <nameoffile>
To view the file page by page use the more command:
cat <nameoffile> | more
Alternatively, you can use the less command as well:
cat <nameoffile> | less
How to Show Line Numbers
For all the non-empty lines in a file use the following command:
cat -b <nameoffile>
If there are lines with no characters at all they won't be numbered. To show numbers for all the lines regardless as to whether they are blank, type the following command:
cat -n <nameoffile>
How to Show the End of Each Line
Sometimes when parsing data files, programmers discover problems because there are hidden characters at the end of lines that they weren't expecting — such as spaces. This error prevents their parsers from working correctly.
To show the dollar as an end of line character enter the following command:
cat -E <nameoffile>
As an example look at the following line of text
the cat sat on the mat
When you run this with the cat -E command you receive the following output:
the cat sat on the mat$
Reducing Blank Lines
When you show the contents of a file using the cat command you probably don't want to see when there are loads of consecutive blank lines. Use the -s switch to condense all blank lines into a single blank line:
cat -s <nameoffile>
How to Show Tabs
When you display a file that uses tab delimiters, you won't ordinarily see the tabs.
The following command shows ^I instead of the tab, which makes it easy to see them:
cat -T <nameoffile>
Concatenate Multiple Files
The whole point of cat is concatenation. Concatenate several files to the screen with the following command:
cat <nameoffile1> <nameoffile2>
To concatenate the files and create a new file use the following command:
cat <nameoffile1> <nameoffile2> > <newfile>
Showing Files in Reverse Order
Show a file in reverse order by using the following command:
tac <nameoffile>
Technically this isn't the cat command, it is the tac command, but it essentially does the same thing but in reverse.