cat - Concatenate Files

Synopsis

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

Concatenate FILE(s) to standard output. With no FILE, or when FILE is -, read standard input.

Flags

Examples

# Display file contents
cat file.txt

# Concatenate multiple files
cat file1.txt file2.txt > combined.txt

# Number all lines
cat -n file.txt

# Number non-blank lines
cat -b file.txt

# Squeeze multiple blank lines into one
cat -s file.txt

# Show hidden characters (tabs, line endings)
cat -A file.txt

# Show tabs as ^I
cat -T file.txt

# Create a file from stdin (type content, Ctrl+D to end)
cat > newfile.txt

# Append to a file
cat >> existing.txt

# Concatenate with stdin in between
cat header.txt - footer.txt < body.txt

Gotchas

• Useless use of cat (UUOC): cat file | command is almost always better written as command < file or command file. Most commands accept files as arguments.
• cat overwrites the output file if you redirect to the same file:

  # WRONG: destroys file.txt
  cat file.txt | sort > file.txt
  # RIGHT: use a temp file or sponge
  sort file.txt > sorted.txt && mv sorted.txt file.txt

• For large files, cat loads and outputs everything. Use head, tail, less, or more for partial viewing.
• cat -v is useful for debugging files with hidden characters like carriage returns (^M) from Windows line endings.