[Linux] Bash tac Usage: Reverse file contents line by line

Overview

The tac command in Bash is used to concatenate and display files in reverse order, specifically reversing the order of lines. It is essentially the reverse of the cat command, which displays file contents in the standard order.

Usage

The basic syntax of the tac command is as follows:

tac [options] [arguments]

Common Options

• -b, --before: Place a delimiter before the line instead of after it.
• -r, --regex: Treat the delimiter as a regular expression.
• -s, --separator: Specify a custom separator instead of the default newline.

Common Examples

Example 1: Reverse a Single File

To reverse the contents of a file named example.txt:

tac example.txt

Example 2: Reverse Multiple Files

To reverse the contents of multiple files, file1.txt and file2.txt, and display them one after the other:

tac file1.txt file2.txt

Example 3: Using a Custom Separator

To reverse the contents of a file and use a custom separator, such as a comma:

tac -s ',' example.txt

Example 4: Reverse Output with a Delimiter

To reverse lines and place a delimiter before each line:

tac -b example.txt

Tips

• Use tac in combination with other commands like grep or sort to manipulate data more effectively.

• Redirect the output of tac to a new file if you want to save the reversed content:

  tac example.txt > reversed_example.txt
  

• Remember that tac reads the entire file into memory, so it may not be suitable for very large files.