Linux and Unix tac command
tac (which is "cat" backwards) concatenates each FILE to standard output just like the cat command, but in reverse: line-by-line, printing the last line first. This is useful (for instance) for examining a chronological log file in which the last line of the file contains the most recent information.
If no FILE is specified, or if the FILE is specified as "-", tac reverses the contents of standard input.
tac [OPTION] ... [FILE] ...
|-b, --before||attach the line separator before each line of output instead of after.|
|-r, --regex||interpret the line separator as a regular expression (useful with the -s option, see below).|
|-s, --seperator=STRING||use STRING as the line separator instead of a newline.|
|--help||display command help and exit.|
|--version||output version information and exit.|
Prints the lines of file1.txt in reverse, from last line to first.