Non-fiction: ed (text editor)

Overview
ed is the original Unix line editor written by Ken Thompson in 1969. It operates without a visual display, letting users edit text one line at a time with terse commands and line addresses. Intended as a compact, scriptable tool, ed became the standard editor bundled with early Unix systems and anchored text editing in the emerging Unix philosophy.

Origins and purpose
ed was created to provide a reliable, simple editing facility on the limited hardware available to early Unix developers. Memory and terminal capabilities were constrained, so a non-visual, low-overhead editor fit the environment. The design prioritized minimalism, predictability, and the ability to be driven from both interactive sessions and automated scripts.

Design and interface
ed's interaction model centers on addressing lines and applying single-letter commands. Users specify ranges with numbers, use punctuation like "." and "$" to mean the current and last lines, and issue commands to print, delete, substitute, or append text. The interface is intentionally austere: feedback is minimal, error messages are brief, and commands are composed succinctly to keep the editor compact and efficient.

Innovations: regular expressions and scripting
One of ed's most influential features is its support for regular-expression search and substitution, an approach Ken Thompson refined and integrated into the editor. This facility allowed powerful, pattern-based text transformations and made ed a potent tool for programmatic editing. Its command-oriented design also encouraged non-interactive use, enabling pipelines and shell scripts to perform automated edits and forming a model for later stream editors.

Role in the Unix toolchain
ed served as both a user-facing editor and a conceptual building block for other Unix utilities. Tools like sed and various file-processing utilities drew on ed's command and pattern conventions, while later editors such as ex and vi evolved from the same line-editor roots into visual environments. ed's presence on early Unix systems made it the baseline editor assumed in documentation, scripts, and installation environments.

Limitations and criticisms
The same qualities that made ed practical on early machines, minimalism and terse feedback, also produced a steep learning curve and a poor fit for interactive, visual editing on modern terminals. Users accustomed to screen editors find ed's line-oriented model awkward for complex, exploratory editing tasks. Its terse error reporting and lack of on-screen context can be frustrating for newcomers.

Enduring legacy
Despite its limitations, ed's influence persists. Its regular-expression conventions and line-oriented command structure shaped a generation of text-processing tools, and its emphasis on small, composable utilities helped codify the Unix philosophy. ed remains present in many Unix-like systems as a standard, minimal editor and a reminder of the pragmatic engineering that guided early Unix development.