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In Linux, nearly everything—from system logs to SSH sessions and configuration files—is plain text. Mastering file viewing, editing, transformation, and comparison tools is essential for effective system administration and DevOps workflows.

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Viewing Files on Linux

When troubleshooting or inspecting data, these commands will help you quickly access file contents.

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Basic Commands: cat and tac

  • cat: Dump the entire file to your terminal.
  • tac: Display a file in reverse line order.
# Show all lines in order
$ cat /home/users.txt
user1
user2
user3
user4
user5
user6

# Show lines in reverse order
$ tac /home/users.txt

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Inspecting Beginnings and Endings: head and tail

  • head -n N: Display the first N lines.
  • tail -n N: Display the last N lines (default is 10).
# First 20 lines of a log
$ head -n 20 /var/log/dnf.log

# Last 20 lines of a log
$ tail -n 20 /var/log/dnf.log

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Quick Reference: File Viewing Commands

CommandDescriptionExample
cat fileDump whole filecat /etc/hosts
tac fileDump file in reversetac /home/users.txt
head -n NFirst N lineshead -n 10 /var/log/syslog
tail -n NLast N linestail -n 50 /var/log/auth.log

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Transforming Text with sed

The image shows a terminal interface with a text file named "userinfo.txt" containing names, cities, countries, and numbers. The text appears to be part of a tutorial on transforming text using the "sed" command.
Imagine userinfo.txt contains a typo—“canda” instead of “canada.” You can preview a global replacement without altering the file: 
$ sed 's/canda/canada/g' userinfo.txt
Breakdown of the substitute command (s):
  • Pattern delimiters wrapped in single quotes prevent shell expansion.
  • First token (canda) is the search string.
  • Second token (canada) is the replacement.
  • g (global) applies to every match on each line.
  • Specify the file at the end.
By default, omitting g (i.e., sed 's/canda/canada/') replaces only the first occurrence per line.
Once you’re ready to edit the file in place, add -i: 
$ sed -i 's/canda/canada/g' userinfo.txt
Verify with: 
$ cat userinfo.txt

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Extracting Columns with cut

Field extraction is straightforward using cut. For space-delimited data, grab usernames from userinfo.txt: 
$ cut -d ' ' -f 1 userinfo.txt
ravi
mark
john
ravi
mary
  • -d ' ' specifies a space delimiter.
  • -f 1 selects the first field.
For comma-separated files, pull the third field (country) and save it: 
$ cut -d ',' -f 3 userinfo.txt > countries.txt
Now countries.txt holds all the country names, including duplicates.

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Filtering Unique Entries with uniq

The uniq command collapses only adjacent duplicate lines. To get a sorted list of unique countries: 
# Collapse adjacent duplicates
$ uniq countries.txt

# Sort and then collapse duplicates
$ sort countries.txt | uniq
canada
usa
Always sort before piping to uniq if you want to remove all duplicate entries, not just adjacent ones.

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Comparing Files with diff

When a package upgrade modifies a configuration, diff helps you spot exactly what’s changed.

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Summary of diff Options

OptionDescriptionExample
(none)Plain diffdiff file1 file2
-cContext diff (shows surrounding lines)diff -c file1 file2
-y or sdiffSide-by-side diff alignmentdiff -y file1 file2

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1. Plain diff

$ diff file1 file2
1c1
< only exists in file 1
---
> only exists in file 2
4c4
< only exists in file 1
---
> only exists in file 2
  • 1c1: Change at line 1.
  • <: Content from the first file.
  • >: Content from the second file.

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2. Context Diff (-c)

Shows a few lines of context around each change, marked with !: 
$ diff -c file1 file2
*** file1
--- file2
***************
*** 1,4 ****
! only exists in file 1
  identical line 2
  identical line 3
! only exists in file 1
--- 1,4 ----
! only exists in file 2
  identical line 2
  identical line 3
! only exists in file 2

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3. Side-by-Side Diff (-y / sdiff)

Aligns both files in columns for easy scanning: 
$ diff -y file1 file2
only exists in file 1    | only exists in file 2
identical line 2         | identical line 2
identical line 3         | identical line 3
only exists in file 1    | only exists in file 2

$ sdiff file1 file2
only exists in file 1    | only exists in file 2
identical line 2         unchanged
identical line 3         unchanged
only exists in file 1    | only exists in file 2
Here, | separates differing lines; identical lines may appear without markers.

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