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.
The rev command is part of the util-linux package and is available from Linux Kernel Archive.