How to use Linux cat and tac Commands

Teams cat and tac display the contents of text files, but there is more to it than meets the eye. Dive a little deeper and learn some useful Linux command line tricks.

These are two simple little commands, often ignored as too simple to actually use. But once you know how they can be used in different ways, you will realize that they are quite capable of doing their hard work when it comes to working with files.

cat team

cat used to check the contents of text files and merge parts of files into one big file.

Back in the dial-up era, binaries were often split into several smaller files to make downloading easier. Instead of uploading one large file, you returned each smaller file. If any file failed to download correctly, you will simply get that file again.

Of course, then you needed a way to restore a collection of small files back into a single working binary. This process has been called unification. And that’s where it came from cat and where he got his name from.

Broadband and fiber optic connections have made this special need disappear — just like the sounds of screeching dialing — so what’s left to do cat today? Quite a lot actually.

Text file display

To cat list the contents of a text file into a terminal window, use the following command.

Make sure the file is a text file. If you try to display the contents of a binary file in a terminal window, the results will be unpredictable. You could end up with a locked terminal session or worse.

  кот poem1.txt 

The contents of the poem1.txt file are displayed in the terminal window.

This is only half of the famous poem. Where is the rest? There is another file here called poem2.txt. We can do it cat list the contents of multiple files with a single command. All we have to do is list the files in order on the command line.

  кот poem1.txt poem2.txt 

It looks better; we have a whole poem now.

Use of a cat at a lower cost

The whole poem is there, but it flew past the window too quickly to read the first few verses. We can redirect output from cat in less and scroll down the text at your own pace.

  кошка poem1.txt poem2.txt |  Меньше 

Now we can move forward and backward through the text in one stream, even if it is contained in two separate text files.

Line numbering in a file

We can specify the line number in the file as it is displayed. For this we use the option -n (number).

  cat -n poem1.txt 

The lines are numbered as they appear in the terminal window.

Do not number empty lines

We managed to number the lines with cat but empty lines between verses are also counted. To have text line numbering but ignore empty lines, use the option -b (number-nonblank).

  cat -b poem1.txt 

Now text lines are numbered and blank lines are skipped.

Don’t show multiple blank lines

If the file has sections of consecutive empty lines, we can ask cat ignore all but one empty line. Look at this file.

The following command will force cat display only one empty line from each group of empty lines. The option we need for this is the option -s (squeeze-blank).

  cat -s poem1.txt 

This does not affect the contents of the file in any way; it only changes the way cat displays the file.

Tab display

If you want to know if spaces are caused by spaces or tabs, you can find out using the option -T (show-tabs).

  cat -T poem1.txt 

Tabs are represented by the symbols «^I».

Line end display

You can check for spaces using the option -E (show-end).

  кот -E poem1.txt 

Line ends are represented by the «$» symbol.

Combining files

It does not make sense to save the poem in two files, one half in each. Let’s merge them together and create a new file with the entire poem.

  cat poem1.txt poem2.txt> jabberwocky.txt 

Our new file contains the contents of two other files.

Adding text to an existing file

It’s better, but it’s not really the whole poem. The last verse is missing. The last verse in Jabberwocky is the same as the first verse.

If we have the first verse in the file, we can add it to the end of the jabberwocky.txt file and we have the complete verse.

In this next command we have to use >> not just > . If we use one > we overwrite jabberwocky.txt. We don’t want to do this. We want add text to the end.

  cat first_verse.txt >> jabberwocky.txt 

And finally, all parts of the poem together.

Redirecting standard input

You can redirect keyboard input to a file using cat . Whatever you type is redirected to a file until you press Ctrl+D. Please note that we are using single > because we want to create the file (or overwrite it if it exists).

  cat> my_poem.txt 

This sound is like a distant turbine, probably Lewis Carroll spinning at high speed in the grave.

Tac Command

tac look like cat but lists the contents of the files in reverse order.

Let’s see what:

  tac my_poem.txt 

And the file is displayed in the terminal window in reverse order. In this case, it does not affect its literary merit.