Linux 13 🐧 uniq, head, tail and tee commands - KandZ

These four commands handle specific parts of file processing: finding duplicates, viewing the start or end of files, and simultaneously displaying and saving output.

Overview

Command	Purpose	Best For
`uniq`	Filter duplicate lines	Removing/finding duplicates in sorted files
`head`	Show beginning of file	Previewing files
`tail`	Show end of file	Viewing recent log entries
`tee`	Write to file AND display	Logging while watching output

The `uniq` Command

Filters out duplicate lines from a sorted file.

cat data.txt
uniq data.txt
uniq -c data.txt
uniq -d data.txt
uniq -u data.txt

⚠️ Important: uniq only removes adjacent duplicates. Sort the file first!

Option	Description
(none)	Remove adjacent duplicate lines
`-c`	Count the number of times each line appears
`-d`	Show only duplicate lines
`-u`	Show only unique lines
`-i`	Ignore case
`-f N`	Skip first N fields

Examples

Sample file:

$ cat data.txt
apple
apple
banana
banana
banana
cherry
cherry
date

Basic uniq (removes adjacent duplicates):

$ uniq data.txt
apple
banana
cherry
date

Count occurrences (-c):

$ uniq -c data.txt
      2 apple
      3 banana
      2 cherry
      1 date

Only duplicates (-d):

$ uniq -d data.txt
apple
banana
cherry

Only unique (-u):

$ uniq -u data.txt
date

The `uniq` + `sort` Pipeline

This is the classic pattern — sort first, then uniq:

$ cat unsorted.txt
banana
apple
banana
cherry
apple
banana

$ sort unsorted.txt | uniq
apple
banana
cherry

$ sort unsorted.txt | uniq -c
      2 apple
      3 banana
      1 cherry

$ sort unsorted.txt | uniq -c | sort -rn
      3 banana
      2 apple
      1 cherry

Word frequency — a common interview task:

$ cat file.txt | tr ' ' '\n' | sort | uniq -c | sort -rn | head -10

The `head` Command

Displays the beginning of a file. Default: 10 lines.

head -n 2 data.txt
head -c 3 data.txt
cat data.txt | head -n 2

Option	Description
(none)	Show first 10 lines
`-n N`	Show first N lines
`-c N`	Show first N bytes
`-q`	Quiet (no headers with multiple files)
`-v`	Always show headers

Examples

First 10 lines (default):

$ head data.txt
# Shows first 10 lines

First N lines:

$ head -n 2 data.txt
Line 1
Line 2

$ head -n 5 log.txt
# First 5 lines

First N bytes:

$ head -c 3 data.txt
Lin

$ head -c 10 data.txt
Line 1
Li

Multiple files:

$ head -n 2 file1.txt file2.txt
==> file1.txt <==
Line 1
Line 2

==> file2.txt <==
Alpha
Beta

With pipe:

$ cat data.txt | head -n 2
Line 1
Line 2

$ ls -l | head -n 5
# First 5 lines of directory listing

Negative numbers (GNU extension):

$ head -n -5 data.txt
# Shows all lines EXCEPT the last 5

The `tail` Command

Displays the end of a file. Default: 10 lines.

tail -n 2 data.txt
cat data.txt | tail -n 2

Option	Description
(none)	Show last 10 lines
`-n N`	Show last N lines
`-c N`	Show last N bytes
`-f`	Follow — show new lines as they’re added
`-F`	Follow even if file is rotated

Examples

Last 10 lines (default):

$ tail data.txt
# Shows last 10 lines

Last N lines:

$ tail -n 2 data.txt
Line 8
Line 9

$ tail -n 5 log.txt
# Last 5 lines

Last N bytes:

$ tail -c 3 data.txt
ine

The -f flag — follow a log in real-time:

$ tail -f /var/log/syslog
# Displays new lines as they appear
# Ctrl+C to stop

Combine with grep for monitoring:

$ tail -f /var/log/syslog | grep "ERROR"
# Only shows new ERROR lines

Multiple files:

$ tail -n 2 file1.txt file2.txt
==> file1.txt <==
Line 8
Line 9

==> file2.txt <==
Gamma
Delta

Starting from line N:

$ tail -n +3 data.txt
# Starts from line 3 and shows the rest

The `tee` Command

Reads from stdin, writes to stdout AND a file at the same time.

echo "kronos" | tee data2.txt
echo "kronos2" | tee -a data2.txt
echo "kronos2" | tee data2.txt data3.txt

Visual:

        ┌──────────────────┐
stdin ──│  tee  │──stdout──│  Terminal
        └───────┴──────────┘
                │
                └──file──→ File

Option	Description
(none)	Write to stdout and file (overwrite)
`-a`	Append to file instead of overwriting
`-i`	Ignore interrupt signals
Multiple files	Write to all files at once

Examples

Basic tee:

$ echo "kronos" | tee data2.txt
kronos                          # ← displayed on terminal
$ cat data2.txt
kronos                          # ← also saved to file

Append mode (-a):

$ echo "kronos2" | tee -a data2.txt
kronos2
$ cat data2.txt
kronos
kronos2                         # ← both lines saved

Write to multiple files:

$ echo "kronos2" | tee data2.txt data3.txt
kronos2
$ cat data2.txt
kronos2
$ cat data3.txt
kronos2                         # ← saved to both files

Save AND view command output:

$ ls -la | tee file-list.txt
# Displays listing AND saves to file-list.txt

Log while watching:

$ ./script.sh | tee -a script.log
# See live output AND append to log

Privileged write (sudo tee):

$ echo "127.0.0.1 mysite.local" | sudo tee -a /etc/hosts
# Use tee when you need sudo to write
# (sudo redirect ">" doesn't work as expected)

Complete Example Session

# ============================================
# PART 1: UNIQ
# ============================================

# Create a file with duplicates
$ cat > data.txt << EOF
apple
apple
banana
banana
banana
cherry
cherry
date
EOF

# Remove adjacent duplicates
$ uniq data.txt
apple
banana
cherry
date

# Count occurrences
$ uniq -c data.txt
      2 apple
      3 banana
      2 cherry
      1 date

# Only duplicates
$ uniq -d data.txt
apple
banana
cherry

# Only unique
$ uniq -u data.txt
date

# Classic sort | uniq pipeline
$ sort unsorted.txt | uniq -c | sort -rn

# ============================================
# PART 2: HEAD
# ============================================

# First 10 lines (default)
$ head data.txt

# First 2 lines
$ head -n 2 data.txt
apple
apple

# First 3 bytes
$ head -c 3 data.txt
app

# Multiple files
$ head -n 2 file1.txt file2.txt
==> file1.txt <==
...

# With pipe
$ cat data.txt | head -n 2

# ============================================
# PART 3: TAIL
# ============================================

# Last 10 lines (default)
$ tail data.txt

# Last 2 lines
$ tail -n 2 data.txt
cherry
date

# Last 3 bytes
$ tail -c 3 data.txt
ate

# Real-time log monitoring
$ tail -f /var/log/syslog

# Filter followed output
$ tail -f /var/log/syslog | grep "ERROR"

# ============================================
# PART 4: TEE
# ============================================

# Write and display
$ echo "kronos" | tee data2.txt
kronos

# Append
$ echo "kronos2" | tee -a data2.txt
kronos2

# Multiple files
$ echo "kronos2" | tee data2.txt data3.txt
kronos2

# Save command output
$ ls -la | tee file-list.txt

# Using tee with sudo
$ echo "127.0.0.1 myapp.local" | sudo tee -a /etc/hosts

# ============================================
# PART 5: COMBINING THEM
# ============================================

# Top 5 most frequent lines
$ sort file.txt | uniq -c | sort -rn | head -5

# View the beginning and end
$ head -n 5 data.txt && tail -n 5 data.txt

# Watch a log AND save it
$ tail -f /var/log/syslog | tee -a syslog-backup.txt

# Find duplicate IPs in a log
$ grep -oE "[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+" access.log | sort | uniq -c | sort -rn | head -10

# First and last lines of file
$ { head -n 1 file.txt; tail -n 1 file.txt; }

# Save AND display grep results
$ grep "ERROR" log.txt | tee errors.txt | wc -l

# ============================================
# PART 6: PRACTICAL EXAMPLES
# ============================================

# Analyze top visitors
$ awk '{print $1}' access.log | sort | uniq -c | sort -rn | head -10

# Watch deployment log with backup
$ tail -f deploy.log | tee -a deploy-backup.txt

# Check first and last entries in a CSV
$ head -n 2 data.csv
$ tail -n 2 data.csv

# Find duplicate filenames
$ ls | sort | uniq -d

# Unique lines only
$ cat file.txt | sort -u

# Recent errors from syslog
$ grep "ERROR" /var/log/syslog | tail -n 20

# Save terminal session
$ script -c "bash" session.log

Quick Reference

`uniq` Options

Option	Description
(none)	Remove adjacent duplicates
`-c`	Count occurrences
`-d`	Duplicates only
`-u`	Unique only
`-i`	Ignore case

`head` Options

Option	Description
(none)	First 10 lines
`-n N`	First N lines
`-c N`	First N bytes
`-n -N`	All except last N lines

`tail` Options

Option	Description
(none)	Last 10 lines
`-n N`	Last N lines
`-c N`	Last N bytes
`-f`	Follow (real-time)
`-n +N`	Start from line N

`tee` Options

Option	Description
(none)	Write (overwrite)
`-a`	Append
`-i`	Ignore interrupts
`file1 file2`	Multiple files

Best Practices

✅ Do This:

# Always sort before uniq
sort file.txt | uniq

# Use uniq -c for frequency counting
sort file.txt | uniq -c

# Use head to preview before processing
head -n 5 bigfile.txt

# Use tail -f to monitor logs
tail -f /var/log/syslog

# Use tee to see AND save
command | tee output.txt

# Use sudo tee for privileged writes
echo "content" | sudo tee /etc/important.conf

# Combine head and tail to see both ends
head -n 5 file.txt; echo "..."; tail -n 5 file.txt

❌ Don’t Do This:

# Don't use uniq on unsorted files
uniq unsorted.txt          # ❌ Only removes ADJACENT duplicates
sort unsorted.txt | uniq   # ✅ Correct

# Don't forget the -a flag with tee
echo "line 2" | tee file.txt      # ❌ Overwrites!
echo "line 2" | tee -a file.txt   # ✅ Appends

# Don't use > with sudo (misleading)
sudo echo "x" > /etc/file  # ❌ Redirect runs as user, not root
sudo tee /etc/file <<< "x" # ✅ tee runs as root

# Don't tail -f without a way to stop
tail -f log.txt            # Ctrl+C to stop
# (it runs forever, watching the file)

# Don't use uniq for global duplicate removal
sort file.txt | uniq       # ✅ Always sort first

Common Pitfalls

Pitfall	Problem	Solution
`uniq` on unsorted file	Only adjacent duplicates removed	Use `sort \| uniq`
`tee` overwrites	Loses old data	Use `-a` to append
`sudo >` doesn’t work	Redirect runs as user	Use `sudo tee`
`tail -f` on deleted file	Keeps running	Use `-F` to follow rotations
`head`/`tail` without args	10 lines default	Add `-n N` for precision
Forgetting to sort	`uniq` misses duplicates	Always sort first

Real-World Examples

1. Find most common HTTP status codes

awk '{print $9}' access.log | sort | uniq -c | sort -rn

2. Monitor a log for errors in real time

tail -f /var/log/syslog | grep --line-buffered "ERROR"

3. Save and display a command’s output

find /etc -name "*.conf" 2> /dev/null | tee config-files.txt

4. Preview a large file

# First 5 lines and last 5 lines
head -n 5 huge.log; echo "..."; tail -n 5 huge.log

5. Write to a root-owned file

echo "nameserver 8.8.8.8" | sudo tee -a /etc/resolv.conf

6. Top 10 most frequent words in a file

cat book.txt | tr -s ' ' '\n' | sort | uniq -c | sort -rn | head -10

7. Find duplicate lines across multiple files

cat *.txt | sort | uniq -d

8. Count unique visitors from a log

awk '{print $1}' access.log | sort -u | wc -l

9. Watch a log while saving a copy

tail -f app.log | tee -a app-archive.log

10. Verify package installation order

grep "install " /var/log/dpkg.log | tail -n 20

Visual: Command Output

UNIQ:                              HEAD:           TAIL:
Sort removes duplicates            First N lines   Last N lines
                                   ┌──────────┐    ┌──────────┐
Input:     Output:                 │ Line 1   │    │ Line 18  │
apple                              │ Line 2   │    │ Line 19  │
apple    → apple                   │ Line 3   │    │ Line 20  │
banana   → banana                  │ ...      │    │ ...      │
banana   → cherry                  └──────────┘    └──────────┘
cherry   → date                    (head -n 3)     (tail -n 3)
cherry
date

TEE:
              ┌────────────────────┐
stdin  ──→  │    tee -a          │  ──→ stdout (terminal)
              └────────┬───────────┘
                       │
                       └──→ file (saved)

Summary

Command	Purpose	Example
`uniq`	Filter duplicates	`sort file \| uniq -c`
`head`	Show beginning	`head -n 5 file.txt`
`tail`	Show end	`tail -f log.txt`
`tee`	Save and display	`cmd \| tee -a log.txt`

Key takeaways:

uniq works on sorted input — always sort first for real deduplication
uniq -c is your friend for frequency counting
head and tail both default to 10 lines and support -n and -c
tail -f is the go-to way to monitor logs in real time
tee lets you see AND save — perfect for logging
tee -a appends instead of overwriting
sudo tee is the correct way to write to root-owned files

Remember: These commands are the precision tools of text processing. uniq for duplicates, head/tail for previewing, and tee for logging — combined with sort, grep, and pipes, they let you handle almost any file-processing task. And the most common pattern in the wild? sort | uniq -c | sort -rn | head — the go-to one-liner for “show me the top occurrences of anything”!

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Overview

The uniq Command

Examples

The uniq + sort Pipeline

The head Command

Examples

The tail Command

Examples

The tee Command

Examples

Complete Example Session

Quick Reference

uniq Options

head Options

tail Options

tee Options

Best Practices

Common Pitfalls

Real-World Examples

1. Find most common HTTP status codes

2. Monitor a log for errors in real time

3. Save and display a command’s output

4. Preview a large file

5. Write to a root-owned file

6. Top 10 most frequent words in a file

7. Find duplicate lines across multiple files

8. Count unique visitors from a log

9. Watch a log while saving a copy

10. Verify package installation order

Visual: Command Output

Summary

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The `uniq` Command

The `uniq` + `sort` Pipeline

The `head` Command

The `tail` Command

The `tee` Command

`uniq` Options

`head` Options

`tail` Options

`tee` Options