Viewing Files¶

Commands for reading and examining file contents without editing.

cat - Concatenate and Print¶

The simplest way to view file contents:

cat file.txt                    # Print entire file
cat file1.txt file2.txt         # Concatenate multiple files
cat -n file.txt                 # Number all lines
cat -b file.txt                 # Number non-empty lines
cat -s file.txt                 # Squeeze blank lines
cat -A file.txt                 # Show all (tabs, line endings)

Showing Special Characters¶

cat -A file.txt

Shows:

^I for tabs
$ at end of lines
^M for Windows carriage returns

Useful for debugging whitespace issues.

Creating Files with cat¶

cat > newfile.txt << 'EOF'
Line 1
Line 2
EOF

For files too large to fit on screen:

less file.txt

Key	Action
`Space` / `f`	Forward one page
`b`	Back one page
`d`	Forward half page
`u`	Back half page
`g`	Go to beginning
`G`	Go to end
`q`	Quit

Searching¶

Key	Action
`/pattern`	Search forward
`?pattern`	Search backward
`n`	Next match
`N`	Previous match
`&pattern`	Show only matching lines

Useful Options¶

less -N file.txt        # Show line numbers
less -S file.txt        # Don't wrap long lines
less -i file.txt        # Case-insensitive search
less +F file.txt        # Follow mode (like tail -f)
less +/pattern file.txt # Start at first match

Less vs More¶

more is older and less capable. Always prefer less:

# less has:
# - Backward navigation
# - Better search
# - No screen clear on exit (with -X)
less -X file.txt

head - View Beginning¶

head file.txt           # First 10 lines (default)
head -n 20 file.txt     # First 20 lines
head -n -5 file.txt     # All but last 5 lines
head -c 100 file.txt    # First 100 bytes

Multiple files:

head -n 5 *.txt

==> file1.txt <==
Line 1
...

==> file2.txt <==
Line 1
...

tail - View End¶

tail file.txt           # Last 10 lines (default)
tail -n 20 file.txt     # Last 20 lines
tail -n +5 file.txt     # Starting from line 5
tail -c 100 file.txt    # Last 100 bytes

Follow Mode¶

Watch a file for changes (essential for logs):

tail -f /var/log/system.log     # Follow file
tail -F /var/log/system.log     # Follow, handles rotation
tail -f -n 50 logfile           # Last 50 lines, then follow

Exit follow mode with Ctrl+C.

Multiple Files¶

tail -f log1.txt log2.txt

Combining head and tail¶

View lines 10-20:

head -n 20 file.txt | tail -n 11

Or with sed:

sed -n '10,20p' file.txt

wc - Word Count¶

wc file.txt             # lines, words, bytes
wc -l file.txt          # Lines only
wc -w file.txt          # Words only
wc -c file.txt          # Bytes only
wc -m file.txt          # Characters only

Example:

wc -l *.py

   45 main.py
   23 utils.py
   68 total

file - Identify File Type¶

Determine what a file contains:

file image.png

image.png: PNG image data, 800 x 600, 8-bit/color RGB, non-interlaced

file mystery_file

mystery_file: gzip compressed data, was "backup.tar", from Unix

file script.sh

script.sh: Bourne-Again shell script, ASCII text executable

Useful for:

Files without extensions
Verifying file types
Detecting encoding issues

MIME Type¶

file --mime-type document.pdf

document.pdf: application/pdf

Modern Alternative: bat¶

bat is a cat replacement with syntax highlighting:

brew install bat

bat file.py             # Syntax highlighting
bat -n file.txt         # Line numbers only (no header)
bat -p file.txt         # Plain (no decorations)
bat -A file.txt         # Show non-printable characters
bat --diff file.txt     # Show git changes

Features:

Automatic syntax detection
Git integration
Line highlighting
Paging built-in

See Modern Replacements.

Viewing Specific Parts¶

Extract Lines by Number¶

With sed:

sed -n '5p' file.txt            # Line 5 only
sed -n '5,10p' file.txt         # Lines 5-10
sed -n '5p;10p;15p' file.txt    # Lines 5, 10, and 15

With awk:

awk 'NR==5' file.txt            # Line 5
awk 'NR>=5 && NR<=10' file.txt  # Lines 5-10

Extract Columns¶

cut -d',' -f1,3 data.csv        # Fields 1 and 3
cut -c1-10 file.txt             # Characters 1-10

Viewing Binary Files¶

hexdump / xxd¶

hexdump -C file.bin | head      # Hex + ASCII
xxd file.bin | head             # Alternative format
xxd -b file.bin | head          # Binary format

strings¶

Extract readable text from binary:

strings binary_file
strings -n 10 binary_file       # Minimum 10 characters

Comparing Files¶

diff¶

diff file1.txt file2.txt        # Show differences
diff -u file1.txt file2.txt     # Unified format (patch style)
diff -y file1.txt file2.txt     # Side by side
diff -q file1.txt file2.txt     # Just report if different

comm¶

Compare sorted files:

comm file1.txt file2.txt

Output columns:

Lines only in file1
Lines only in file2
Lines in both

Practical Examples¶

Check Log Errors¶

tail -f /var/log/app.log | grep -i error

Quick File Preview¶

head -n 50 file.txt

Count Lines of Code¶

wc -l src/*.py

View CSV Header¶

head -n 1 data.csv

Check File Encoding¶

file -i document.txt

document.txt: text/plain; charset=utf-8

Try It¶

Create a test file:
```
seq 1 100 > /tmp/numbers.txt
```

Practice viewing:

cat /tmp/numbers.txt
head -n 5 /tmp/numbers.txt
tail -n 5 /tmp/numbers.txt
head -n 20 /tmp/numbers.txt | tail -n 10

Use less:

less /tmp/numbers.txt
# Try: /50, n, g, G, q

Count and identify:

wc -l /tmp/numbers.txt
file /tmp/numbers.txt

Clean up:
```
rm /tmp/numbers.txt
```

Summary¶

Command	Purpose
`cat`	Print entire file
`less`	Page through file
`head`	View beginning
`tail`	View end
`tail -f`	Follow file updates
`wc`	Count lines/words/bytes
`file`	Identify file type
`diff`	Compare files
`bat`	Syntax-highlighted viewing