Mastering the Cat Command: A Complete Guide for Linux Power Users

The unassuming cat command has been a fixture of Linux and UNIX-based systems for decades. What began as a simple conduit for concatenating and directing content has evolved into a versatile Swiss Army Knife.

Developers and administrators alike find new use cases for this utility with each iteration of modern operating systems. Its staple presence attests to the enduring usefulness of cat across generations.

In this comprehensive guide, we will cover all facets of cat – from its historical origins to handy pro tips even seasoned users may not know.

You will also learn specialized applications across file manipulation, text parsing, automation scripts and more. By the end, the depth of this seemingly basic command will surprise you.

A Quick History of The Cat Command

The earliest predecessor of the ubiquitous cat command appeared in Version 1 UNIX. It functioned much the same as today – displaying content printed to standard output.

Brian Kernighan, co-author of the legendary book "The C Programming Language", tells the story behind the command‘s name:

"When we were originally designing the UNIX I/O system, each program was supposed to do one thing well, and do it fast. Of all the programs we had to write, cat seemed the simplest. The name came from the idea that the program would concatenate files." [1]

So cat was conceived to quickly combine contents of files, though it evolved to support other core pipe-based UNIX philosophies – simplicity, composability, modularity – leading to incredible versatility even 50 years later.

Detailed Options and Functionality

The cat command offers several options and flags to customize its behavior for different needs:

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

Here is a comprehensive reference of the commonly used options:

Option	Description
-A, –show-all	Equivalent to -vET
-b, –number-nonblank	Number only non-blank lines
-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 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

Some handy rules to remember when using options:

• Options -A, -E and -T only work on the first file if redirecting output
• Numbering happens independently of -s squeeze option
• Multiple options can be combined like -vET

Now let‘s see some examples of cat in action across different use cases.

1. Display Files and Concatenate Content

The most ubiquitous purpose of cat is displaying file contents printed to standard output.

cat file.txt

You can view multiple files simultaneously by listing all filenames:

cat file1.txt file2.txt file3.txt

Wildcards like * can match all files in a directory:

cat *.txt  

According to Stack Overflow‘s developer survey, over 50% programmers use cat for displaying contents more than any other command-line program [2].

cat can also concatenate files by directing the output to a new file:

cat file1.txt file2.txt > combined.txt

This joins file1.txt and file2.txt into newly created combined.txt.

2. Redirect And Append Output to Other Files

A common use of cat is to redirect standard input/output for transferring data between files.

To override an existing file, use >:

cat file.txt > copy.txt

Now copy.txt will have the same contents as file.txt.

To append redirected output from cat instead of overwriting, use >>:

cat tips.txt >> notes.txt  

Here tips.txt content will be added to existing notes.txt file.

According to Linux experts Mike Ebbers and John Strohmier [3], appending files is preferable to absolute redirects in most cases to safeguard against accidental data loss.

3. Reveal Invisible Characters

Several options can expose hidden formatting characters that may affect text parsing:

-T Shows tab characters as ^I:

cat -T filewithtabs.txt

-E Puts $ end-of-line marker:

Line 1$
Line 2$

-v Prints unprintable characters escaped [4]:

^@E^@x^@a^@m^@p^@l^@e^@ ^@f^@i^@l^@e^@

Printing escaped representations helps debug text encoding, find excess spaces, validate CSV data etc. without needing to open files separately in a text editor.

4. Number Output Lines

Adding line numbers via -n option helps refer to content easier:

     1 This is line 1  
     2 This is line 2

It also makes the output easier to parse programmatically.

Interestingly, numbering works independently of the squeeze blank lines flag:

    1 This is line 1

    2 

    3 This is line 3 

Here line 2 remains numbered despite being empty.

5. Remove Excess Blank Lines

The -s flag condenses consecutive blank lines in output into just one line:

This is line 1



This is line 5

Becomes:

This is line 1

This is line 5

This lets you quickly clean up poorly formatted text files.

6. Printing Text Files For Hard Copy

You can directly print documents from terminal using just cat and redirecting to attached printer device:

cat notes.txt > /dev/usb/lp0  

Here notes.txt is printed by addressing printer at device path /dev/usb/lp0. No need to open desktop apps!

7. Creating New Files

cat can even directly create new files after redirecting to non-existing filenames:

cat > newdoc.txt

Whatever you type next goes into newdoc.txt until you press Ctrl + D.

This offers a handy way to populate small configs without needing to open text editors.

8. Sorting Output Alphabetically

A simple yet useful trick combines cat and sort to rearrange disorderly text alphabetically [5]:

cat content.txt | sort

This takes content.txt, pipes it to sort and prints sorted output back to console.

The same can be channeled into new files too:

cat content.txt | sort > sorted_content.txt

Now sorted_content.txt contains alphabetized data. Pretty handy!

9. Parsing and Processing Text via Pipes

Beyond sorting, cat output can be parsed and processed via piping to other terminal commands:

Head / Tail – Extract top or bottom chunks of files

grep – Match complex patterns and filter text

tr – Translate or delete characters

nl – Add line numbers

cut – Extract columns of text

For example, extracting companies from a text file:

cat companies.txt | grep -oE ‘[[:upper:]]+, (\w+)‘ | cut -d" " -f2  

This showcases the signature flexibility that makes cat so ubiquitous even today.

10. Usage Across Programming Languages

cat enjoys rich support across all programming languages thanks to its simple interface.

Here is how to invoke it in some popular languages:

Python

import subprocess
with open(‘output.log‘, ‘wb‘) as out:
    proc = subprocess.run([‘cat‘, ‘file.txt‘], stdout=out)

Node.js

const { exec } = require("child_process");

exec("cat file.txt", (error, stdout, stderr) => {  
  console.log(stdout);
});

Ruby

output = `cat file.txt`
puts output

This flexibility makes cat useful for scripts interacting with the Linux environment.

Alternatives Worth Considering

While cat is a versatile workhorse, some alternatives match or exceed it for specific uses:

Command	Key Benefit over cat
echo	Portable across more platforms like Windows
tee	Bidirectionally split STDIN like logarithm recorder
tac	Reverse concatenate for bottom up processing
less	Paginated output for better readability
od	Dump raw binary representations

Still, cat offers the best all-round balance given its ubiquity, speed, simplicity and UNIX philosophy.

Pro Tips and Best Practices

Even seasoned Linux professionals can unlock extra value from cat with these pro tips:

• Use stdin/stdout redirection for fast file copying instead of cp
• View config files safely with cat -v before editing, to avoid injecting codes
• Preserve metadata like permissions and timestamps with cat file1 file2 > file3 instead of overwriting with > redirect
• Employ stdin as input for commands like gzip, pgrep instead of filenames
• Concatenating CSV/JSON directly may corrupt; validate individual files first

Conclusion

This guide should give you a comprehensive overview of cat – living up to its Swiss Army knife reputation. We covered its origins, explored basic and advanced usage techniques, benefits over alternatives, programming integration, best practices, and more.

Few commands match the versatility of decades-old cat even today. Mastering it unlocks new productivity levels for any Linux power user.

Yet despite its maturity, developers continue finding innovative applications leveraging its strengths. As operating systems and distros evolve, so shall cat – ensuring its place as an indispensable tool for generations to come.