sed

Sed is a stream editor for filtering and transforming text in a pipeline. It reads input, line by line, applies a script of editing commands, and writes the result to standard output. Unlike interactive editors, sed operates non-interactively and is designed for batch processing of text streams. It is a core utility on Unix-like systems and a standard component of the POSIX environment.

History and purpose: Sed was developed in the early 1970s by Lee E. McMahon as part of the Unix toolkit. The acronym stands for stream editor. It was designed to perform simple, automated text transformations without opening an editor, making it suitable for scripting and data processing tasks.

How it works and syntax: sed [options] 'script' [file ...]. A script comprises editing commands and addresses that select which lines to operate on. Core commands include s/pat/repl/flags for substitution, d to delete, i\ / a\ / c\ to insert or replace text, and p to print. Addresses can be a line number, a pattern, or a range; -n suppresses automatic printing, -e adds scripts, -f reads scripts from a file. In GNU sed, -i enables in-place editing of files, with optional backup suffix.

Examples: sed 's/foo/bar/g' file replaces all occurrences of foo with bar in the file. sed -n '/pattern/p' file prints only lines matching pattern. sed '2,5d' file deletes lines 2 through 5. sed -i 's/old/new/g' file performs an in-place substitution.

Variants and limits: Sed implementations differ (GNU sed, BSD sed) in options and in-place editing behavior. Sed is line-oriented and not a full text editor; for complex or multi-line processing, other tools such as awk or perl may be more appropriate. Nonetheless, sed remains a foundational tool for simple, fast text transformations in scripts and pipelines.