Linux/CLI/cat

• cat command

  • Overview

    • cat (concatenate) is a standard Unix utility that reads files sequentially and writes them to standard output.
    • Defined by the POSIX standard (POSIX.1-2017), meaning it's included in compliant Unix-like systems.

  • Basic Usage

    • cat [OPTION]... [FILE]...
    • Displays the contents of one or more files to standard output.
    • Without any FILE arguments, reads from standard input.

  • Common Options

    • -A, --show-all - equivalent to -vET, shows all non-printing characters
    • -b, --number-nonblank - number non-empty output lines
    • -E, --show-ends - display $ at end of each line
    • -n, --number - number all output lines
    • -s, --squeeze-blank - suppress repeated empty output lines
    • -T, --show-tabs - display TAB characters as ^I
    • -v, --show-nonprinting - use ^ and M- notation, except for LFD and TAB

  • Examples

    • Display file contents

      • cat filename.txt - display contents of a single file
      • cat file1.txt file2.txt - concatenate and display multiple files

    • Create a new file

      • cat > newfile.txt - create a new file (type content, press Ctrl+D to save)

    • Append to a file

      • cat >> existing.txt - append content to an existing file

    • Display with line numbers

      • cat -n filename.txt - display file with line numbers

    • Display non-printing characters

      • cat -A filename.txt - show all non-printing characters

  • Common Use Cases

    • Viewing small text files
    • Concatenating multiple files
    • Creating simple text files
    • Redirecting output to other commands via pipes

  • Related Commands

    • less - view files with pagination
    • more - similar to less but with fewer features
    • head - display the beginning of a file
    • tail - display the end of a file
    • tac - display files in reverse order (opposite of cat)