Linux tac Command: Print Lines in Reverse Order

The tac command prints records from the last record to the first. Because a newline is the default record separator, it is useful for reviewing a text log from its newest line backward or reversing the current line order of a file.

This guide explains the basic syntax, key options, practical examples, related commands, common cautions, and frequently asked questions for tac.

What Is the tac Command?

Its name is cat written backward. It reverses records rather than individual bytes or characters. When several files are provided, it processes the files in operand order while reversing the records within each file.

Basic Syntax

tac [OPTION]... [FILE]...

Options and edge cases can vary by implementation and version. Check tac --help and man tac on the target system before relying on implementation-specific behavior.

Basic tac Usage

Reverse the lines of a file

The final line becomes the first line of output.

tac events.log

Process multiple files

Each input is reversed independently while the file operand order remains unchanged.

tac january.log february.log

Reverse standard input

A pipeline can provide the input, but tac must reach the end before it can emit the first reversed record.

printf 'one\ntwo\nthree\n' | tac

Result of reversing the line order

tac does not reverse characters, but outputs the lines in reverse order. The order of characters within each line remains the same.

printf 'first\nsecond\nthird\n' | tac

Example output:

third
second
first

It is useful when reading logs that accumulate in chronological order from the most recent line, but it is not the same as sorting based on timestamps.

Key Options

Practical Examples

Browse a log from newest to oldest

Combine tac with a pager so the reversed output remains searchable.

tac application.log | less

Use a custom record separator

GNU tac can reverse records delimited by a string other than newline.

tac -s '---' records.txt

Find the most recent matching line

Reverse the log and stop grep after its first match.

tac access.log | grep -m 1 'ERROR'

tac and Related Commands

Important Cautions

It is not a sorting command

The command does not interpret timestamps or values; it only reverses the current record sequence.

Unbounded streams cannot be reversed promptly

The end of input must be known, so tac is not suitable for a stream that never ends.

Check separator portability

Custom separator and regular-expression behavior described here follows GNU tac and may differ elsewhere.

FAQ

Are tac and sort -r equivalent?

No. tac reverses the current order, while sort -r compares values and sorts them.

How do I reverse only the last ten lines?

Use tail -n 10 FILE | tac.

Can tac follow a live log?

Not usefully, because it needs the end of input. Use tail -f for live monitoring.

Does tac merge all input files before reversing?

Normally it processes each file in operand order and reverses records within that file.

Related Resources

• Essential Linux Commands: A Practical Reference by Task
• GNU Coreutils tac documentation