1. Core Purpose & Concepts

• Core Purpose: To use a series of specialized commands to read, merge, reverse, and extract file content, all operations are performed in the command line without the need to launch a full text editor, thus achieving efficient file content preview and processing.
• Core Terminology:
• <span>cat</span> (concatenate): Its core function is **”to connect and print”**. It can read the content of one or more files, concatenate them, and then output to standard output (usually the screen).
• <span>tac</span> (cat backwards):<span>cat</span> in reverse, its function is to vertically reverse the line order of the file (the last line is displayed first).
• <span>rev</span> (reverse): Horizontally reverses the character order of each line in the file (the last character is displayed first).
• <span>less</span> (Pager): A **”pager”** program used to view long files or long command outputs interactively, page by page, avoiding the problem of massive text scrolling.
• <span>head</span>: A command used to view the beginning part of a file or data stream, defaulting to display the first 10 lines.
• <span>tail</span>: A command used to view the end part of a file or data stream, defaulting to display the last 10 lines.

2. Key Command & Options

Overview of Core Commands

<span>less</span> Common Keys in the Pager

• <span>q</span>: Quit, return to the command line.
• Arrow <span>↑</span> / <span>↓</span>: Scroll up/down one line.
• <span>PageUp</span> / <span>PageDown</span>: Scroll up/down one page.
• <span>/</span> + <span>keyword</span>: Search down for the keyword.
• <span>?</span> + <span>keyword</span>: Search up for the keyword.
• <span>n</span>: Jump to the next search match.

Classic Command Examples

Bash

# Example 1: Merge three part files into a complete filecat part1.txt part2.txt part3.txt > story.txt
# Example 2: Safely view a potentially long system log file using lessless /var/log/syslog
# Example 3: View the result of a long command output, avoiding scrollingfind /etc -type f | less
# Example 4: Quickly view the first 20 lines of a configuration filehead -n 20 /etc/ssh/sshd_config
# Example 5: Monitor the latest dynamics of a log file in real-time (one of the most common usages) # -f option means "follow", which continuously displays new content added to the end of the filetail -f /var/log/nginx/access.log
# Example 6: Extract lines 11 to 20 of a file # First use head to get the first 20 lines, then use tail to get the last 10 from those 20 lineshead -n 20 some_file.txt | tail -n 10

3. Practical Use Cases

• Quick Preview: Use <span>cat</span> to quickly view short configuration files, scripts, or notes to confirm the content is correct.
• Reading Large Files:<span>less</span> is the standard way to read large log files, code files, or any long text, as it does not occupy the terminal screen and allows for easy searching and navigation.
• Log Analysis: Use <span>head</span> to view the initial configuration information of log files, and use <span>tail</span> to view the latest errors or access records. <span>tail -f</span> is a powerful tool for real-time monitoring of service status.
• Merging Documents: Use the <span>cat</span> command to sequentially merge scattered text fragments (such as various chapters of a report) into a final document.
• Pipeline Filtering: In complex command pipelines, use <span>head</span> or <span>tail</span> to extract the parts of data you care about for further processing. For example, <span>sort -r data.txt | head -n 5</span> (find the top 5 largest values).

4. Common Pitfalls

• Using <span>cat</span> on Large Files: This is the classic beginner’s mistake. Using <span>cat</span> on a log file of several hundred megabytes will instantly fill the terminal with massive text, potentially causing it to freeze. The correct approach is to always use <span>less</span> for files of unknown size.
• Confusing <span>tac</span> and <span>rev</span>:<span>tac</span> is the reverse of <span>cat</span>, dealing with the order of “lines” (vertical reversal); <span>rev</span> deals with the order of “characters” within lines (horizontal reversal).
• Forgetting to Exit <span>less</span>: After viewing a file, the user remains in the interactive interface of <span>less</span>, and must press <span>q</span> to return to the normal shell prompt.
• Redirecting and Overwriting Files: Using <span>cat a.txt b.txt > c.txt</span> will overwrite the content of <span>c.txt</span> if it already exists, without mercy. If your intention is to append content, you should use <span>>></span>, i.e., <span>cat a.txt b.txt >> c.txt</span>.