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

The ’tac’ command is a command-line utility in Linux that is used to display and concatenate files with lines in reverse order. It is similar to the ‘cat’ command, but instead of displaying the lines in the order they appear in the file, it displays them in reverse order. This can be useful when you want to view or concatenate files in a bottom-up fashion.

Use case 1: Concatenate specific files in reversed order

Code:

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

Motivation: In some cases, you may need to combine multiple files into a single file while preserving the original order of lines within each file. Using the ’tac’ command, you can concatenate specific files in reversed order, ensuring that the lines from each file are displayed in reverse order while maintaining the order of the files themselves.

Explanation: The command ’tac’ is followed by the path(s) to the files that you want to concatenate. You can specify multiple files by adding their paths separated by a space. The ’tac’ command will read the files in the order you specify and display the lines in each file in reverse order.

Example output:

$ tac file1.txt file2.txt
This is line 3 from file2.
This is line 2 from file2.
This is line 1 from file2.
This is line 3 from file1.
This is line 2 from file1.
This is line 1 from file1.

Use case 2: Display ‘stdin’ in reversed order

Code:

cat path/to/file | tac

Motivation: Sometimes, you may want to view the contents of a file in reverse order without modifying the file itself. The ’tac’ command, when combined with the ‘cat’ command, allows you to achieve this by displaying standard input (stdin) in reverse order.

Explanation: In this use case, the ‘cat’ command is used to read the contents of a file and display them as standard output. The ‘|’ symbol, known as a pipe, is then used to redirect the output of the ‘cat’ command to the input of the ’tac’ command. The ’tac’ command will read the standard input and display the lines in reverse order.

Example output:

$ cat file.txt | tac
This is line 3.
This is line 2.
This is line 1.

Use case 3: Use a specific separator

Code:

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

Motivation: In some cases, you may want to use a specific separator to distinguish between lines in the reversed order output. This can be useful when dealing with files that have a specific format or structure. The ‘-s’ option in the ’tac’ command allows you to specify a separator.

Explanation: In this use case, the ‘-s’ option is followed by the separator that you want to use. The ’tac’ command will use the specified separator to separate lines in the output and display them in reverse order.

Example output:

$ tac -s ";" file.txt
This is line 2;This is line 1

Use case 4: Use a specific regex as a separator

Code:

tac -r -s separator path/to/file1 path/to/file2 ...

Motivation: When dealing with files that have complex structure or require pattern matching, it can be useful to use a regular expression as a separator to organize the reversed lines. The ‘-r’ and ‘-s’ options in the ’tac’ command allow you to specify a regular expression as a separator.

Explanation: In this use case, the ‘-r’ option is included to indicate that a regular expression separator will be used. The ‘-s’ option is followed by the separator, which can be a regular expression. The ’tac’ command will use the regular expression separator to separate lines in the output and display them in reverse order.

Example output:

$ tac -r -s "[0-9]+" file.txt
This is line 3
This is line 2
This is line 1

Use case 5: Use a separator before each file

Code:

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

Motivation: Sometimes, you may want to have a separator line between the lines of each file in the reverse order output. This can be useful when you need to distinguish the lines of each file in the concatenated output. The ‘-b’ option in the ’tac’ command allows you to include a separator before each file.

Explanation: In this use case, the ‘-b’ option is included to indicate that a separator line will be added before each file. The ’tac’ command will read each file in reverse order and display the lines in each file in reverse order, with a separator line between each file.

Example output:

$ tac -b file1.txt file2.txt
This is line 3 from file2.
This is line 2 from file2.
This is line 1 from file2.
--
This is line 3 from file1.
This is line 2 from file1.
This is line 1 from file1.

Conclusion:

The ’tac’ command provides a convenient way to display and concatenate files with lines in reverse order. By using the different options and arguments available with the command, you can customize the output according to your requirements. Whether you need to reverse the order of lines within a file or concatenate multiple files in reverse order, the ’tac’ command can help you achieve these tasks efficiently.