cat Command Cheat Sheet

cat (short for concatenate) is one of the most frequently used commands in Unix-like operating systems. It reads data from files and outputs their contents. It is commonly used to display files, combine copies of files, and create new files through redirection.

Synopsis

cat [OPTION]... [FILE]...

Description

The cat command reads files sequentially, writing them to standard output. If no file is specified, or if - is used as a filename, cat reads from standard input.

Basic Usage

Display File Contents

cat filename.txt

Prints the contents of filename.txt to the terminal.

Concatenate Multiple Files

cat file1.txt file2.txt

Prints content of file1.txt followed by file2.txt.

Create a New File

cat > newfile.txt

Type your text, then press Ctrl+D to save and exit.

Append to Existing File

cat >> existing.txt

Appends typed text to the end of existing.txt. Press Ctrl+D to finish.

Options

-n, --number

Number all output lines (starting at 1).

cat -n script.sh

-b, --number-nonblank

Number nonempty output lines, overrides -n.

cat -b script.sh

-s, --squeeze-blank

Suppress repeated empty output lines.

cat -s wide_spaced_file.txt

Reduces multiple adjacent blank lines to a single blank line.

-v, --show-nonprinting

Display non-printing characters (except LFD and TAB).

cat -v binary_file

Useful for debugging weird characters in text files. - Control characters printed as ^C - Meta characters printed as M-

-E, --show-ends

Display $ at end of each line.

cat -E file.txt

Helps identify trailing whitespace.

-T, --show-tabs

Display TAB characters as ^I.

cat -T Makefile

Essential for debugging Makefiles where tabs vs spaces matter.

-A, --show-all

Equivalent to -vET. Shows all non-printing characters, tabs, and line endings.

cat -A suspicious_file.txt

Advanced Usage

Merging Files

Combine multiple files into one:

cat part1.txt part2.txt part3.txt > complete_book.txt

Displaying Large Files

While cat dumps the whole file, for large files use pager:

cat large_file.log | less

Using Standard Input

Read from keyboard (stdin) and a file:

cat - file.txt

Type some text, press Ctrl+D, then it prints file.txt.

Numbering Lines in Stream

ls -1 | cat -n

Heredocs (Here Documents)

A powerful way to create files in scripts using cat.

Basic Heredoc

cat <<EOF > output.txt
This is line 1.
This is line 2.
Variables like $HOME are expanded.
EOF

Append with Heredoc

cat <<EOF >> output.txt
Appended line.
EOF

Suppress Variable Expansion

Use quotes around the delimiter to prevent expansion:

cat <<'EOF' > script.sh
echo "Current directory is \$PWD"
# $PWD is NOT expanded here
EOF

Indented Heredoc (Tabs)

Use <<- to strip leading tabs (but not spaces):

cat <<-EOF
    This line has leading tabs stripped.
    This one too.
EOF

Practical Examples

Concatenate Log Files

Merge daily logs into a monthly log:

cat access.log.* > access_month.log

Check for Hidden Characters

If a script is failing due to Windows line endings (\r\n):

cat -v script.sh
# Output might show: #!/bin/bash^M

^M indicates carriage return (CR).

Reverse File Content

Use tac (reverse cat):

tac file.txt

Prints lines in reverse order (last line first).

Create Dummy File

cat /dev/urandom | head -c 100 > random_bytes.bin

Copy File Content to Clipboard (macOS/Linux)

# macOS
cat file.txt | pbcopy

# Linux (with xclip)
cat file.txt | xclip -selection clipboard

Display File with Line Numbers for Code Review

cat -n code.py

Alternatives

tac

Reverse concatenate (lines).

tac file.txt

rev

Reverse characters in each line.

rev file.txt

head / tail

View beginning or end of file.

head -n 5 file.txt
tail -f file.txt

less / more

Interactive pagers.

less file.txt

bat

A modern cat clone with syntax highlighting and Git integration.

bat file.txt

Common Mistakes

Useless Use of Cat (UUoC)

Piping cat into a command that accepts file arguments is redundant.

Bad:

cat file.txt | grep "pattern"

Good:

grep "pattern" file.txt

Bad:

cat file.txt | awk '{print $1}'

Good:

awk '{print $1}' file.txt

Exception: When you want to construct a stream from multiple sources.

cat header.txt body.txt footer.txt | grep "TODO"

Overwriting Input File

Dangerous:

cat file.txt > file.txt

This truncates file.txt to zero bytes before reading it! The file becomes empty.

Safe way:

cat file.txt > temp.txt && mv temp.txt file.txt
# Or use sponge from moreutils
cat file.txt | sponge file.txt

Exit Status

Tips and Best Practices

1. Use Redirection - > for new/overwrite, >> for append
2. Watch Out for Large Files - cat dumps everything; Ctrl+C to stop
3. Use -v for Debugging - Finds hidden control characters
4. Heredocs are Awesome - Use them in scripts for config generation
5. Standard Input - cat without args reads from keyboard; useful for testing pipes
6. Binaries - Don't cat binary files to terminal (can mess up display encoding)
7. Reset Terminal - If you cat a binary and terminal garbles, type reset