Ed is a line-oriented text editor.
A restricted version of `ed', `red', can only edit files in the current directory and cannot execute shell commands. `ed' is the `standard' text editor in the sense that it is the original editor for Unix, and thus widely available. For most purposes, however, it is superseded by full-screen editors such as Emacs.
Ed is a GNU ed is an 8-bit clean, POSIX-compliant line editor. Extensions to and deviations from the POSIX are described in the file `POSIX'.
For a description of the ed algorithm, see Kernighan and Plauger's book "Software Tools in Pascal," Addison-Wesley, 1981.