Cat
The cat command is one of the most versatile and frequently used commands in Linux. It stands for „concatenate,“ and its primary function is to display the contents of a file on the screen.
What does Cat do?
When you use cat with a filename as an argument, it displays the entire contents of that file. If no arguments are provided, cat will simply sit there waiting for input, allowing you to enter text which will then be displayed back to you. The command is useful for viewing text files, reading system messages or log files, and even creating new ones by piping input into it.
Use Cases
- Viewing Files:
cat filename.txtis used to view the contents of a file namedfilename.txt. - Reading System Messages: After installing software, you might use
cat /var/log/packages/softwarenameto see if there were any issues during installation. - Creating New Text Files: By piping text into
cat, like this:echo "Hello World!" | cat > newfile.txt, you can create a new file namednewfile.txt.
Special Hacks and Tricks
- Piping Input to Cat: As shown in the previous example, you can use
echoor any other command that outputs text as input forcat. This allows you to generate new content or concatenate output from different commands. - Redirecting Output with Cat: When creating a file like
newfile.txt, using the pipe (|) and redirection symbols (>) is key. The syntax is usually something like this:command | cat > filename. - Viewing Multiple Files at Once: If you have several files that are not too large, you can use
catto display their contents one after another by listing them on the command line, separated by spaces.
Experience Level Required
This command is foundational and should be familiar to anyone starting with Linux. Beginners (Level 1) should understand how cat works right away, making it a great command for new users to learn first. As users progress through levels (Level 2-3), they’ll appreciate the nuances of using cat in various contexts, from simple text viewing to more complex piping scenarios.