cat / tac

cat [options] [file...]
tac [options] [file...]

Command / Option	Description
cat file	Prints the contents of a file as-is.
cat file1 file2	Concatenates multiple files and prints them together.
cat -n file	Prints each line with a line number, including blank lines.
cat -b file	Prints line numbers for non-blank lines only.
cat -A file	Shows tabs as ^I and line endings as $. Useful for inspecting invisible characters.
cat -s file	Squeezes consecutive blank lines into a single blank line.
cat > file	Writes standard input to a file (press Ctrl+D to finish).
cat >> file	Appends standard input to an existing file.
tac file	Prints the lines of a file in reverse order (last line first).

The following files are used in the examples below.

~/project/script.sh

#!/bin/bash

echo "Hello, World!"

~/project/numbers.txt

apple
banana
cherry

Print the contents of a file.

cat script.sh
#!/bin/bash

echo "Hello, World!"

Print with line numbers. Blank lines are also numbered.

cat -n script.sh
     1	#!/bin/bash
     2
     3	echo "Hello, World!"

Inspect invisible characters. If Windows-style line endings (\r) are present, they appear as ^M.

cat -A script.sh
#!/bin/bash$
$
echo "Hello, World!"$

Concatenate multiple files and write the result to a single file.

cat header.txt body.txt footer.txt > output.txt

Write multiple lines to a file at once using a here document.

cat > config.txt <<'EOF'
host=localhost
port=8080
debug=true
EOF

Print a file in reverse order with $ tac.

tac numbers.txt
cherry
banana
apple

A common use case is to show the most recent log entries first.

tac /var/log/app.log | head -20

Printing a large file with $ cat will flood the terminal. Use less when you want to scroll through the output. To inspect just the beginning or end of a file, head / tail is more convenient.

$ cat is commonly combined with redirects and pipes. See Redirect (>) for details.