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:

1. Preview changes (no file modified):

   sed 's/canda/canada/g' userinfo.txt
   

2. 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.

Extracting Fields with cut

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

• 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

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

Links and References

• GNU Coreutils Manual
• sed – An Introduction and Tutorial
• Linux diff Tutorial