cat

What it does

cat (short for "concatenate") reads one or more files in sequence and writes their contents to standard output. With a single file it simply prints it; with multiple files it prints them one after another, effectively joining them. It's also commonly used with shell redirection to build files from typed input or to combine several files into one.

Beginner examples

• cat file.txt — print a file's contents to the terminal
• cat file1.txt file2.txt — print two files back to back
• cat -n file.txt — print with line numbers
• cat > newfile.txt — type content interactively, saved with Ctrl+D

cat /etc/os-release

Advanced examples

• Join multiple files into a new one: cat part1.log part2.log part3.log > full.log
• Append rather than overwrite when combining: cat extra.log >> full.log
• Number only non-blank lines, useful for reviewing scripts: cat -b script.sh
• Reveal hidden whitespace issues (tabs as ^I, line ends as $): cat -A file.txt
• Squeeze repeated blank lines when reading a messy file: cat -s report.txt

cat header.txt body.txt footer.txt > document.txt

Common mistakes

• Using cat file | grep pattern when grep pattern file does the same thing with one fewer process (the classic "useless use of cat").
• Redirecting a file into itself, e.g. cat file.txt > file.txt, which truncates the file to empty before cat ever reads it, destroying the content.
• Running plain cat on a large binary file and flooding the terminal with garbage characters, sometimes even breaking the terminal's display settings.
• Forgetting that cat > file.txt overwrites an existing file entirely; use >> to append instead.

Tips

• If your terminal display gets garbled after accidentally cat-ing a binary file, run reset to restore it.
• Use cat -e or cat -A when debugging files with mysterious formatting issues, such as trailing whitespace or Windows-style line endings (^M$).
• For quickly viewing large files, prefer less — cat dumps everything at once with no way to scroll back.

Best practices

• Avoid the "useless use of cat" anti-pattern in scripts and pipelines; pass the filename directly to the next command (grep pattern file instead of cat file | grep pattern) when only one file is involved.
• Use cat for genuinely concatenating multiple files or dumping small files quickly, not as a general-purpose file viewer for large files.
• When building files programmatically, prefer heredocs (cat <<EOF) over interactive cat > for scripts, since heredocs are non-interactive and reproducible.

Real-world use cases

• Combining log rotation parts into a single file for analysis: cat access.log.1 access.log.2 > combined.log.
• Quickly creating a small config file without opening an editor: cat > .env.local.
• Dumping a script's contents with line numbers to reference specific lines in a bug report: cat -n deploy.sh.

Common interview questions

• What does "useless use of cat" mean? It refers to piping cat file | command when command could read the file directly (command file), adding an unnecessary extra process.
• How do you combine multiple files into one with cat? cat file1 file2 file3 > combined.txt — each file's contents are written to stdout in order and redirected into the new file.
• What happens if you run cat file.txt > file.txt? The shell truncates file.txt to empty as soon as it opens the redirection target, so cat reads an already-empty file and you lose the original contents.