What is tac?

tac is a command-line utility available on Unix-like operating systems, including Linux and macOS. It is the reverse of the cat command; tac concatenates and prints files in reverse order.

• Functionality: The primary function of tac is to print the contents of a file, or files, to standard output, with the last line being printed first and the first line being printed last. This is achieved by reversing the order of the lines within each file. See: File Reversal

• Usage: Basic usage involves providing one or more file names as arguments: tac file1.txt file2.txt. If no file names are provided, tac reads from standard input.

• Options:

  • -b, --before: Attach the separator before, not after. See: Separator
  • -r, --regex: Interpret the separator as a regular expression. See: Regular%20Expression
  • -s, --separator=STRING: Use STRING as the separator instead of newline. See: String
  • --help: Display help information. See: Help
  • --version: Display version information. See: Version

• Typical Use Cases: Common use cases include reversing log files to view the most recent entries first or reversing the order of entries in any text-based file.

• Example: If a file named example.txt contains the lines "line 1", "line 2", and "line 3", running tac example.txt will output:

  line 3
  line 2
  line 1