rev - reverse lines characterwise
rev [option] [file...]
The rev utility copies the specified files to standard output, reversing the order of characters in every line. If no files are specified, standard input is read.
This utility is a line-oriented tool and it uses in-memory allocated buffer for a whole wide-char line. If the input file is huge and without line breaks than allocate the memory for the file may be unsuccessful.
Display version information and exit.
Display help text and exit.
tac(1)
The rev command is part of the util-linux package and is available from Linux Kernel Archive.