tac

Additional Notes

tac is cat spelled backwards, and that is exactly what it does: it prints files with lines in reverse order. The last line of each file is printed first, and the first line is printed last. For multiple files, each file is reversed independently, then concatenated.

This is useful for viewing log files chronologically backwards (most recent entries first), reversing the output of other commands in pipelines, or any situation where reverse line-order is needed.

Syntax

tac [options] [file...]

Parameters

• file: One or more files to reverse and concatenate. If omitted, reads from standard input.

Common Options

• -b, --before: Attach the separator before each record instead of after.
• -r, --regex: Treat the separator as a regular expression.
• -s separator, --separator=separator: Use separator as the record delimiter instead of newline.

Examples

tac file.txt

Print file.txt with lines in reverse order.

tac access.log | head -n 20

View the last 20 lines of a log file (most recent entries first).

tac part1.txt part2.txt

Reverse each file independently, then concatenate.

tac -s , data.csv

Reverse records delimited by commas instead of newlines.

echo -e "line1\nline2\nline3" | tac

Reverse piped input.

tac -r -s '\.' code.pl

Reverse a Perl script using periods as record separators (via regex).

Practical Notes

• tac is part of GNU coreutils and is available on all Linux systems.
• For viewing reverse chronological logs, tail -r (BSD) or tac are both options.
• The -s option is useful for reversing records in non-line-delimited data.
• tac reads the entire input into memory before outputting, so very large files may use significant memory.
• For binary files, tac may produce unexpected results since it operates on lines (newline-separated).