cat

Updated

September 4, 2026

Overview

cat (concatenate) reads files sequentially and writes them to standard output. Use it to print small files, join files, and write heredocs. For large files, prefer a pager (less). For syntax-highlighted viewing, consider bat. For following growth, use tail -f / journalctl -f.

Classic Unix joke still holds: cat file | something is often better as something < file or something file.

Syntax

cat [options] [file ...]

With no file (or -), reads standard input.

Common Options

Option	Description
`-n`	Number all output lines
`-b`	Number non-blank lines
`-s`	Squeeze multiple blank lines
`-A`	Show all non-printing (`-vET`)
`-E`	Show `$` at end of lines
`-T`	Show tabs as `^I`
`-v`	Show non-printing characters
`-u`	Unbuffered (historical; often default)
`--`	End of options

Key Use Cases

Print small config/text files
Concatenate parts into one file
Create files via heredoc
Reveal invisible characters (-A)
Quick copy via redirect

Examples with Explanations

Print files

cat file.txt
cat /etc/os-release
cat -n file.txt              # number lines
cat -A file.txt              # show tabs/line endings

Concatenate

cat part1 part2 part3 > whole
cat part1 part2 >> whole     # append
cat header.json body.json > combined.json

Heredocs

cat <<'EOF' > greeting.txt
hello
world
EOF

cat <<EOF >> /etc/hosts
# added by bootstrap
10.0.0.5 app.local
EOF

Quoted 'EOF' disables expansion; unquoted EOF expands $vars and command substitutions.

Here-string and stdin

cat <<< 'single line'
cat - <<'EOF' | ssh host 'cat > /tmp/x'
content
EOF

Show non-printing / DOS endings

cat -A dosfile.txt
# CRLF shows as ^M$

Useless use of cat — avoid

# avoid
cat file | grep pattern
# prefer
grep pattern file
grep pattern < file

Binary caution

cat binary.dat                # can mess up terminal
cat binary.dat | xxd | less   # better

Multiple files with separators (manual)

for f in *.conf; do
  echo "===== $f ====="
  cat "$f"
done | less

Notes / Pitfalls

Large files: use less, tail, or streaming tools; don’t dump multi-GB logs with cat.
cat file1 file2 > file1 truncates file1 first — data loss. Write to a new name.
Terminal corruption from binary output: reset or tput reset.
Order matters for concatenation; globs are sorted by shell locale.
BusyBox cat may support fewer flags.

2026-relevant notes

Prefer bat for interactive reading; keep cat for scripts and POSIX pipelines.
systemd-cat sends stdin to the journal — different tool for logging.
For cloud-init / config blobs, heredoc with cat <<'EOF' remains standard in shell provisioning.

Additional Resources

man cat