Cat, Grep, and Go: Leveling Up Your Text Manipulation

Welcome to the first post of the Textual Healing series, where we’re diving deep into the world of command-line text manipulation. Today, we’re focusing on three core tools: cat, grep, and the magic of piping and redirection.

If you’ve spent any time in the terminal, you’ve probably encountered these commands. But have you unlocked their full potential? Whether you’re just getting started or want to level up your command-line skills, we’ll break down these text manipulation wizards and show you how to combine them like a pro.

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1. cat – The Simple Viewer with Hidden Power

cat is often the first tool you’ll use to view the contents of a file. It’s short for concatenate, but most people just use it to output text to the terminal. Simple, right?

cat file.txt

That’s your basic use case—display the contents of file.txt. But cat can do more:

• Concatenate files: You can combine multiple files into one:

  cat file1.txt file2.txt > combined.txt

  This will combine the contents of file1.txt and file2.txt into combined.txt.

• Number lines: Want to see line numbers? Just add -n:

  cat -n file.txt

Pro Tip: While cat is great for small files, when you’re working with larger ones, you might want to consider less or more so you can scroll through them more easily.

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2. grep – The Search Master

Next up is grep, the go-to tool for searching through text. It’s like the Ctrl+F of the command line, but with way more power.

Want to find every line in a file that contains a specific word? Easy:

grep "word" file.txt

This will return all the lines in file.txt that contain the word “word.” Want to search across all .txt files in a directory? No problem:

grep "word" *.txt

Pro Tip: Use --color to Highlight Matches

When searching through long files with lots of text or code, it can be difficult to spot exactly where the match is. That’s where the --color flag comes in handy. It highlights your search term, making it easier to pick out from the rest of the output:

grep --color "word" file.txt

Now, every instance of “word” will be brightly colored in the output, helping you locate it faster—especially useful when you’re dealing with files that have hundreds or thousands of lines.

Advanced grep: You can combine grep with regular expressions to search for patterns, not just words. For example, this searches for lines that start with “error”:

grep "^error" file.txt

Combining cat and grep: Now here’s where the fun begins. You can combine cat and grep using pipes to take text from one command and pass it as input to another.

cat log.txt | grep --color "error"

This outputs the contents of log.txt and highlights all the lines containing “error.”

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3. Piping and Redirection: Connecting the Dots

Pipes (|) allow you to take the output of one command and use it as input for another, creating a chain of actions.

For example, if you have a log file and want to find all lines with “error” and then count how many times it appears, you can chain grep with wc -l (word count):

cat log.txt | grep "error" | wc -l

This command prints the number of lines containing “error”. Boom—instantly more powerful!

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4. Redirection (>, >>)

Redirection lets you send the output of a command to a file instead of displaying it in the terminal.

• Overwrite with >: This will take the output of your command and overwrite the contents of a file.

  echo "This will overwrite the file" > output.txt

• Append with >>: If you don’t want to overwrite, use >> to append the output to the file.

  echo "This will add to the file" >> output.txt

You can combine redirection with other commands, too. For example, find all instances of “error” in a log file and save them to a new file:

grep "error" log.txt > errors.txt

If you run this command again and want to add more results to errors.txt instead of overwriting it:

grep "warning" log.txt >> errors.txt

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5. Combining Everything Together

Now that you know how to use cat, grep, and combine them with pipes and redirection, let’s look at a more advanced example. Suppose you want to:

1. Search for all error lines in multiple log files.
2. Save the results to a file while still viewing the output in the terminal.

Here’s the magic one-liner:

cat *.log | grep "error" > errors.txt && cat errors.txt

Let’s break this down:

• grep "error": We’re searching for the word “error” across multiple .log files.
• > errors.txt: This uses redirection to save the output of grep to the file errors.txt.
• && cat errors.txt: The && ensures that the file is created first, then we use cat to display the contents of errors.txt back to the terminal, effectively letting you both see the results and save them at the same time.

This command allows you to search through all .log files for “error,” save the results to errors.txt, and then view them immediately.

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Coming Soon: A sed Deep Dive

As promised, the next post will be all about sed—a powerful stream editor for transforming and manipulating text. We’ll cover everything from basic substitutions to advanced text transformations. Stay tuned!

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