Table of Contents

The cat command concatenates files and writes their contents to standard output. It is commonly used to quickly inspect files, combine multiple files, and feed content into pipelines.

Overview

Use cat when you need to:

  • Display file contents directly in the terminal
  • Concatenate several files into one output stream
  • Create small files from terminal input
  • Number lines or visualize non-printing characters
  • Bridge content between files and pipelines

Syntax

cat [options] [file...]

Common forms:

# Display one file
cat notes.txt

# Concatenate multiple files in order
cat part1.txt part2.txt part3.txt

# Read from standard input explicitly
cat -

# Write output to a new file via redirection
cat source.txt > copy.txt

If no files are provided, cat reads from standard input.

Common Options

Option Purpose Example
-n Number all output lines cat -n file.txt
-b Number non-empty output lines cat -b file.txt
-s Squeeze repeated blank lines cat -s file.txt
-E Show $ at end of each line cat -E file.txt
-T Show tab characters as ^I cat -T file.txt
-v Show non-printing chars (except tab/newline) cat -v file.txt
-A Equivalent to -vET (show all) cat -A file.txt

Behavior Notes

  • Input files are processed left to right.
  • cat does not modify source files.
  • Binary files can produce unreadable terminal output; redirect when needed.
  • Large outputs may scroll quickly, so pair with less when reviewing.

Examples

# Display a configuration file
cat config.yaml

# Concatenate log segments into one file
cat app.log.1 app.log.2 app.log.3 > combined.log

# Number all lines
cat -n script.sh

# Number non-blank lines only
cat -b document.txt

# Visualize tabs and line endings
cat -A data.tsv

# Collapse repeated blank lines before viewing
cat -s notes.txt

# Create a file from terminal input (Ctrl+D to finish)
cat > quick-note.txt

# Append terminal input to existing file (Ctrl+D to finish)
cat >> quick-note.txt

# Combine with grep in a pipeline (usually grep can read files directly)
cat app.log | grep -i error

Here-Document Alternative

For multi-line file creation in scripts, a here-document is often clearer:

cat > app.env <<'EOF'
APP_NAME=sample
APP_ENV=dev
LOG_LEVEL=info
EOF

Safe Usage Guidelines

  1. Use less for long output: cat file | less.
  2. Be careful with redirection targets to avoid overwriting files accidentally.
  3. Use -A when debugging whitespace-sensitive files.
  4. Prefer direct command input over useless pipes when possible (grep pattern file instead of cat file | grep pattern).

Performance and Practical Tips

  1. For very large files, less, head, and tail are often more efficient for interactive inspection.
  2. Use cat to concatenate files; use paste/awk when column-aware merging is required.
  3. Avoid printing binary files directly to interactive terminals.

Troubleshooting

cat: file: No such file or directory

Cause: Wrong path or filename.

Fix: Verify path and current directory.

pwd
ls -la
cat ./relative/path/file.txt

Terminal appears garbled after viewing a file

Cause: Binary or control characters were written to the terminal.

Fixes:

  • Use cat -v or hexdump -C for inspection
  • Reset terminal state with reset

Unexpected overwrite when creating files with cat > file

Cause: > truncates destination before writing.

Fix: Use >> to append, or verify target path before running.

Notes

cat is simple and ubiquitous, but it is best paired with other tools for filtering, searching, and paging.

For command details and implementation-specific behavior, run:

man cat
cat --help
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