How to use the command 'tac' (with examples)

The tac command is a Linux utility that is part of the GNU Coreutils package. It is used to display and concatenate files in reversed order, line by line. Unlike the cat command, which presents file content in the natural order from top to bottom, tac reverses that order, making it useful in scenarios where reversed content display is desirable. It provides various options to manage how lines are reversed, using line separators, regular expressions for segmentation, and more.

Use case 1: Concatenate specific files in reversed order

Code:

tac path/to/file1 path/to/file2 ...

Motivation:

Imagine you have several log files that record events sequentially over time, with the most recent events at the end of the respective files. Reviewing these logs in reverse order can be crucial for quickly identifying the latest occurrences across multiple files. Using tac, you can easily concatenate these files, displaying their content from bottom to top, providing immediate insights from the most recent entries.

Explanation:

• tac: This is the command used to reverse the order of the lines in the given files.
• path/to/file1 path/to/file2 ...: These are the file paths of the files you want to concatenate. tac processes them in the order you supply but outputs their content in reverse line order.

Example Output:

File2 Line3
File2 Line2
File2 Line1
File1 Line4
File1 Line3
File1 Line2
File1 Line1

Use case 2: Display stdin in reversed order

Code:

cat path/to/file | tac

Motivation:

This use case is particularly useful in environments where you are dealing with output piped from another command. Instead of dealing with an intermediary file, you can directly reverse the order of the line outputs from a command, useful in interactive shell scripting or when analyzing output step-by-step backward.

Explanation:

• cat path/to/file: This part of the command reads the content of the file and sends it to stdout.
• |: The pipe operator takes the output from cat and directs it as input to the next command.
• tac: The tac command then reverses the order of the lines from the input it receives.

Example Output:

Last Line
...
Second Line
First Line

Use case 3: Use a specific separator

Code:

tac --separator , path/to/file1 path/to/file2 ...

Motivation:

In cases where you are dealing with CSV files or text files with custom delimiters, it may be necessary to reverse sections based on those delimiters. This can be particularly valuable for data analysis where sections are structurally defined by these separators.

Explanation:

• tac: This begins the command to reverse the content.
• --separator ,: This option specifies a comma as the separator between lines, allowing tac to treat each comma-separated section as a block to be reversed.
• path/to/file1 path/to/file2 ...: These represent file paths that tac processes, reversing each section defined by the separator.

Example Output:

segment3,segment2,segment1

Use case 4: Use a specific regex as a separator

Code:

tac --regex --separator [,;] path/to/file1 path/to/file2 ...

Motivation:

If you have files that use multiple delimiters, such as a combination of commas and semicolons, you can leverage regex patterns to ensure tac reverses content where either delimiter appears. This is ideal for complex data manipulation where multiple sectioning criteria exist.

Explanation:

• tac: The base command to reverse content.
• --regex: Instructs tac to interpret the separator as a regular expression.
• --separator [,;]: Specifies the regex pattern, matching either commas or semicolons as separators.
• path/to/file1 path/to/file2 ...: File paths to be reversed using the regex-based separation.

Example Output:

subsection3;subsection2,subsection1

Use case 5: Use a separator before each file

Code:

tac --before path/to/file1 path/to/file2 ...

Motivation:

When handling files that need prefix-based operations — adding headers or divisions at the start of content per file — using the --before option ensures that separators precede file content, offering cleaner and more organized output when reversing content.

Explanation:

• tac: Initiates the command to reverse lines.
• --before: Specifies that the separator should appear before content from each file rather than between.
• path/to/file1 path/to/file2 ...: This refers to the file paths whose content is processed and separated.

Example Output:

**File2 Begin**
Last Line2
...
First Line2
**File1 Begin**
Last Line1
...
First Line1

Conclusion:

The tac command is a versatile tool for reversing lines, making it invaluable for users who require backward content processing for analysis or data manipulation. Its varied options for separators and input types allow for adaptiveness across numerous scenarios, showcasing the flexibility and depth of its utility in everyday Linux tasks.