cat command

cat - Concatenate and display file content.

cat command in Linux (short for "concatenate") is used to display file contents, combine files, or create small files directly from the terminal. It’s a versatile and widely-used tool for handling text files.

Usage: cat [OPTION]... [FILE]...

• OPTION: Flags which enhances the cat abilities.
• FILE: The file(s) you want to work with (optional; reads from input if omitted).

Examples

• Displaying a file's contents

  The simplest use of cat is to print a file’s contents to the terminal.

  $ cat notes.txt
  • Displays the entire contents of notes.txt.
  • If the file doesn’t exist, cat outputs an error No such file or directory.

• Concatenating multiple files

  cat can combine multiple files and display them as one continuous output.

  $ cat file1.txt file2.txt
  • Prints the contents of file1.txt followed by file2.txt.

  Save to a new file by redirecting the output to create a combined file:

  $ cat file1.txt file2.txt > combined.txt
  • Creates combined.txt with the contents of both files.

• Creating a file

  You can use cat with input redirection to create a file interactively.

  $ cat > notes.txt
  Hello this is the content for notes
  $ cat notes.txt
  Hello this is the content for notes
  • Type the text to place in the file and press Ctrl+D or command+D to save and exit
  • notes.txt now contains your input.

• Appending to a file

  Use >> to append content to an existing file instead of overwriting it.

  $ cat >> notes.txt
  Reminder to do things today
  $ cat notes.txt
  Hello this is the content for notes
  Reminder to do things today
  • Add new text and press Ctrl+D or command+D to save and exit

• Numbering lines

  Add line numbers to the output with -n (all lines) or -b (non-blank lines only).

  $ cat -n greet.txt
    1   Hello
    2   Hola
    3
    4
    5   Ciao
    6
    7   Bonjour
    8
    9   Marhabaan
  • Displays greet.txt with line numbers

  Non-Blank Lines:

  $ cat -b greet.txt
    1   Hello
    2   Hola
  
  
    3   Ciao
  
    4   Bonjour
  
    5   Marhabaan
  • Numbers only non-empty lines.

• Showing End of Lines

  Use -E to display a $ at the end of each line, useful for spotting hidden line breaks.

  $ cat -E greet.txt
    Hello$
    Hola$
    $
    $
    Ciao$
    $
    Bonjour$
    $
    Marhabaan$

• Squeezing Blank Lines

  Remove repeated empty lines with -s (squeeze).

  $ cat -ns greet.txt
    1  Hello
    2  Hola
    3
    4  Ciao
    5
    6  Bonjour
    7
    8  Marhabaan
  • Reduces multiple blank lines to a single blank line.

• Displaying Tabs and Non-Printable Characters

  Use -T to show tab characters as ^I.

  $ cat -T config.txt
  • Tabs appear as ^I in the output.
  • Combine with -v for other non-printable characters:

  $ cat -vT strange.txt

To get help related to the cat command use --help option

$ cat --help
Usage: cat [OPTION]... [FILE]...
Concatenate FILE(s) to standard output.

With no FILE, or when FILE is -, read standard input.

  -A, --show-all           equivalent to -vET
  -b, --number-nonblank    number nonempty output lines, overrides -n
  -e                       equivalent to -vE
  -E, --show-ends          display $ at end of each line
  -n, --number             number all output lines
  -s, --squeeze-blank      suppress repeated empty output lines
  -t                       equivalent to -vT
  -T, --show-tabs          display TAB characters as ^I
  -u                       (ignored)
  -v, --show-nonprinting   use ^ and M- notation, except for LFD and TAB
      --help     display this help and exit
      --version  output version information and exit

Examples:
  cat f - g  Output f's contents, then standard input, then g's contents.
  cat        Copy standard input to standard output.