Displaying Entire Files with cat

The cat (concatenate) command is the simplest way to display a file's contents. It reads the entire file and prints it to your terminal in one go. 

# Display the contents of a file
cat /etc/hostname

# Display with line numbers
cat -n /etc/hosts

# Display multiple files sequentially
cat file1.txt file2.txt

Example output with line numbers:

$ cat -n /etc/hosts
     1  127.0.0.1       localhost
     2  127.0.1.1       mycomputer
     3
     4  # IPv6
     5  ::1             localhost ip6-localhost ip6-loopback
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When to use cat

cat is best for small files -- configuration files, short scripts, or quick checks. For anything longer than a screenful of text, use less instead, because cat will flood your terminal and scroll past too fast to read.

Useful cat Variations

cat -n Number all output lines. Helpful for referencing specific lines.
cat -b Number only non-blank lines. Cleaner output for files with many empty lines.
cat -s Squeeze consecutive blank lines into a single blank line.

Scrolling Through Files with less

The less command opens a file in a scrollable viewer. Unlike cat, it does not dump everything to the screen at once. Instead, it shows one screenful at a time and lets you navigate forward and backward. 

# Open a file in less
less /var/log/syslog

# Open with line numbers
less -N /var/log/syslog

Once inside less, use these keys to navigate:

Space / Page Down Move forward one full screen.
b / Page Up Move backward one full screen.
j or Down Arrow Move forward one line.
k or Up Arrow Move backward one line.
g Jump to the beginning of the file.
G Jump to the end of the file.
/pattern Search forward for "pattern". Press n for next match, N for previous.
q Quit and return to the terminal.
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less is the go-to file viewer.

System administrators and developers use less constantly for reading log files, configuration files, and source code. The search feature (/pattern) is especially powerful -- you can search through thousands of lines in an instant.

Peeking at the Start with head

Sometimes you only need to see the beginning of a file. The head command prints the first lines of a file -- by default, the first 10 lines. 

# Show the first 10 lines (default)
head /etc/passwd

# Show the first 5 lines
head -n 5 /etc/passwd

# Show the first 20 lines
head -n 20 /var/log/syslog

head is commonly used to quickly check what a file contains without loading the whole thing. For example, you might use it to verify a CSV file has the right column headers, or to check the top of a log file for startup messages. 

# Check the header of a CSV file
head -n 1 data.csv

# Peek at a configuration file
head -n 15 /etc/nginx/nginx.conf

Watching the End with tail

The tail command is the opposite of head: it prints the last lines of a file. By default, it shows the last 10 lines. 

# Show the last 10 lines (default)
tail /var/log/syslog

# Show the last 20 lines
tail -n 20 /var/log/syslog

# Show the last 50 lines
tail -n 50 /var/log/auth.log

Live Log Monitoring with tail -f

The most powerful feature of tail is the -f (follow) flag. It keeps the file open and prints new lines as they are added in real time. This is essential for monitoring log files while troubleshooting. 

# Follow a log file in real time
tail -f /var/log/syslog

# Follow the last 50 lines and continue watching
tail -n 50 -f /var/log/auth.log

While tail -f is running, every new line written to the file appears on your screen instantly. Press Ctrl + C to stop following and return to the terminal prompt.

⚠️
Some log files require sudo.

System log files like /var/log/auth.log or /var/log/syslog may not be readable by regular users. If you get a "Permission denied" error, prepend the command with sudo: sudo tail -f /var/log/auth.log.

Counting with wc

The wc (word count) command tells you how many lines, words, and bytes are in a file. It is a quick way to gauge the size or length of text content. 

# Full count: lines, words, bytes
wc /etc/passwd
  45   68  2467 /etc/passwd

# Count only lines
wc -l /etc/passwd
45 /etc/passwd

# Count only words
wc -w readme.txt

# Count only characters
wc -c data.csv
wc -l Count lines. Most commonly used. Answers "how many entries/records?"
wc -w Count words (sequences of non-whitespace characters).
wc -c Count bytes. Useful for checking file sizes.
wc -m Count characters (handles multi-byte encodings like UTF-8 correctly).

A common use case is counting lines in a log file to see how many events were recorded, or counting lines of code in a project: 

# How many lines in the system log?
wc -l /var/log/syslog

# How many users on the system?
wc -l /etc/passwd

Summary

You now have a toolkit of commands for reading and inspecting files:

  • cat -- Display entire files (best for short files)
  • less -- Scroll through files interactively (best for large files)
  • head -- View the first N lines of a file
  • tail -- View the last N lines; use -f to follow live updates
  • wc -- Count lines, words, and characters in a file

Choosing the right tool depends on the situation: cat for quick peeks, less for reading, head / tail for targeted views, and wc for measuring.

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Nice work!

You can now read and inspect files without opening a text editor. Next up, you will learn about Linux file permissions -- what rwx means and how to control who can read, write, and execute your files.

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