📑 Linux/Unix: File Viewing, Redirection & Filters – View, Manipulate & Chain Output Like a Pro

🧲 Introduction – Master Shell Text Flow

In the Linux/Unix shell, managing text files and command output is a core daily task. Whether you’re reading logs, extracting columns, or redirecting output to a file, these tools make the shell powerful, scriptable, and flexible.

🎯 In this guide, you’ll learn:

• How to view file content using commands like cat, less, head, and tail
• How to extract/manipulate data using cut, paste, wc, and more
• How I/O redirection (>, <, >>, 2>) works
• How to use pipes and filters (|, tee, xargs, sort, uniq) to create workflows

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📘 Topics Covered

🔵 Subtopic	📖 Description
Viewing Tools (cat, less, head)	View content in full, page-by-page, or by line count
Text Utilities (cut, paste, nl)	Analyze, split, combine, and transform file content
I/O Redirection (>, >>, 2>, <)	Save output to files, capture errors, or read input from files
Pipes & Filters (`	, tee, xargs`)

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🔵 Linux/Unix: Viewing Tools (cat, less, more, head, tail, tac)

🔹 cat – View Entire File

cat file.txt

✅ Displays full content at once.

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🔹 less / more – Paginated View

less file.txt
more file.txt

✅ Scroll through large files one page at a time (q to quit).

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🔹 head / tail – View Start or End

head -n 5 file.txt    # First 5 lines
tail -n 10 file.txt   # Last 10 lines

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🔹 tac – Reverse Line Output

tac file.txt

✅ Displays file content in reverse line order.

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🔵 Linux/Unix: Text Utilities (nl, wc, cut, paste, split)

🔹 nl – Number Lines

nl file.txt

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🔹 wc – Word Count

wc -l file.txt     # Line count
wc -w file.txt     # Word count
wc -c file.txt     # Byte count

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🔹 cut – Extract Columns

cut -d ':' -f1 /etc/passwd

✅ Extracts usernames from /etc/passwd.

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🔹 paste – Merge Line-by-Line

paste file1.txt file2.txt

✅ Horizontally joins lines from two files.

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🔹 split – Divide File into Chunks

split -l 100 largefile.txt part_

✅ Splits into 100-line chunks.

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🔵 Linux/Unix: I/O Redirection (>, <, >>, 2>)

Symbol	Meaning	Example
>	Redirect output (overwrite)	echo Hello > file.txt
>>	Append output	echo Again >> file.txt
2>	Redirect stderr	ls nofile 2> error.log
<	Use file as input	sort < data.txt

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🔹 Combine Output and Error

command > out.txt 2>&1

✅ Sends both stdout and stderr to the same file.

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🔵 Linux/Unix: Pipes & Filters (|, tee, xargs, sort, uniq)

🔹 | – Pipe Output to Another Command

ls | grep txt

✅ Lists only .txt files.

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🔹 tee – Output to File and Screen

ls | tee list.txt

✅ Saves and shows output simultaneously.

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🔹 sort and uniq

sort names.txt | uniq

✅ Sorts and removes duplicates.

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🔹 xargs – Feed Arguments to Commands

ls *.txt | xargs rm

✅ Deletes all .txt files (safe alternative to rm *.txt in scripts).

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📌 Summary – Recap & Next Steps

Mastering file viewing, I/O redirection, and filters gives you the ability to read, process, and transform data directly in the terminal. These tools are the heart of every shell script and sysadmin task.

🔍 Key Takeaways:

• Use cat, less, head, tail, and tac to explore files
• Use cut, nl, wc, and paste to manipulate content
• Redirect input/output with >, >>, 2>, and <
• Chain commands using |, and manipulate streams with xargs, tee, sort, and uniq

⚙️ Practical Applications:

• Build scripts that log and monitor command outputs
• Quickly sort, filter, and deduplicate log files
• Combine commands for advanced text processing pipelines

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❓ Frequently Asked Questions

❓ How do I view a file in reverse order?
✅ Use tac file.txt to display lines in reverse.

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❓ How do I get line and word count of a file?
✅ Use:

wc -l file.txt     # lines
wc -w file.txt     # words

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❓ What does 2>&1 mean?
✅ It redirects stderr (2) to the same location as stdout (1).

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❓ How can I combine grep, sort, and uniq?
✅ Example:

cat logs.txt | grep error | sort | uniq -c

Counts unique error lines from a log file.

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