Process Text Streams Using Filters

In Linux, nearly every interaction—SSH sessions, command outputs, system logs, and configuration files—is plain text. Mastering text filters allows you to view, transform, and compare these streams efficiently at the command line.

Viewing Files with `cat`, `tac`, `head`, and `tail`

Displaying Entire and Reversed Files

Use cat for quick, on-screen dumps of small files:

cat /home/users.txt
# Output:
# user1
# user2
# user3
# user4
# user5
# user6

To flip the order (last line first), pipe through tac:

tac /home/users.txt
# Output:
# user6
# user5
# user4
# user3
# user2
# user1

Inspecting the Start or End of Large Logs

Log files can grow huge. Quickly grab the first or last N lines:

Last 10 lines (default):
tail /var/log/dnf.log
Last 20 lines:
tail -n 20 /var/log/dnf.log
First 20 lines:
head -n 20 /var/log/dnf.log

These let you preview recent errors or initial startup messages without opening the full file.

Automating In-File Replacements with `sed`

The stream editor sed excels at find-and-replace tasks:

Preview changes (no file modified):
sed 's/canda/canada/g' userinfo.txt

Apply in-place (-i) substitutions:

sed -i 's/canda/canada/g' userinfo.txt
cat userinfo.txt

s/pattern/replacement/g replaces all occurrences on each line.
The -i flag edits the file directly.

Always preview your sed commands without -i first. To keep a backup, use -i.bak (e.g., sed -i.bak 's/old/new/g' file).

Extracting Fields with `cut`

When working with delimited data (spaces, commas, or tabs), cut slices out columns:

The image shows a terminal interface with a command prompt on the left and a text file named "userinfo.txt" on the right, containing a list of names, cities, countries, and numbers.

By space delimiter: extract the first field (name)
cut -d ' ' -f 1 userinfo.txt
By comma delimiter: extract the third field (country) and save
cut -d ',' -f 3 userinfo.txt > countries.txt

Listing Unique Entries with `sort` and `uniq`

The uniq filter only removes adjacent duplicates—sort first to catch all duplicates:

sort countries.txt | uniq

If your file isn’t sorted, uniq may leave non-adjacent duplicates. Always sort before uniq for a full cleanse.

Comparing Files with `diff`

Spot differences between configuration files using:

Basic side-by-side:
diff file1 file2
Unified context (-c):
diff -c file1 file2
Two-column view (-y):
diff -y file1 file2

This helps pinpoint changes before editing or deploying configurations.

Quick Reference: Linux Text Filters

Command	Purpose	Basic Usage
`cat`	Dump entire file	`cat file.txt`
`tac`	Reverse file order	`tac file.txt`
`head`	Show first N lines	`head -n 20 file.log`
`tail`	Show last N lines	`tail -n 20 file.log`
`sed`	Stream editor (find & replace)	`sed -i 's/old/new/g' file.txt`
`cut`	Extract columns from delimited streams	`cut -d',' -f3 file.csv`
`sort`	Sort lines alphabetically or numerically	`sort file.txt`
`uniq`	Remove adjacent duplicates	`sort file.txt \| uniq`
`diff`	Compare files line by line	`diff -y file1 file2`

Introduction

System Architecture

Linux Installation and Package Management

GNU and Unix Commands

Devices, Linux Filesystems, Filesystem Hierarchy Standard

Conclusion and Mock Exam

Process Text Streams Using Filters

Viewing Files with `cat`, `tac`, `head`, and `tail`

Displaying Entire and Reversed Files

Inspecting the Start or End of Large Logs

Automating In-File Replacements with `sed`

Extracting Fields with `cut`

Listing Unique Entries with `sort` and `uniq`

Comparing Files with `diff`

Quick Reference: Linux Text Filters

Links and References

Watch Video

​Viewing Files with cat, tac, head, and tail

​Displaying Entire and Reversed Files

​Inspecting the Start or End of Large Logs

​Automating In-File Replacements with sed

​Extracting Fields with cut

​Listing Unique Entries with sort and uniq

​Comparing Files with diff

​Quick Reference: Linux Text Filters

​Links and References

Watch Video

Viewing Files with `cat`, `tac`, `head`, and `tail`

Displaying Entire and Reversed Files

Inspecting the Start or End of Large Logs

Automating In-File Replacements with `sed`

Extracting Fields with `cut`

Listing Unique Entries with `sort` and `uniq`

Comparing Files with `diff`

Quick Reference: Linux Text Filters

Links and References