sed

sed (stream editor) is a Unix utility that parses and transforms text, using a simple, compact programming language. sed was developed from 1973 to 1974 by Lee E. McMahon of Bell Labs,and is available today for most operating systems. sed was based on the scripting features of the interactive editor ed ('editor', 1971) and the earlier qed ('quick editor', 1965–66). sed was one of the earliest tools to support regular expressions, and remains in use for text processing, most notably with the substitution command. Popular alternative tools for plaintext string manipulation and 'stream editing' include AWK and Perl. First appearing in Version 7 Unix, sed is one of the early Unix commands built for command line processing of data files. It evolved as the natural successor to the popular grep command. The original motivation was an analogue of grep (g/re/p) for substitution, hence 'g/re/s'. Foreseeing that further special-purpose programs for each command would also arise, such as g/re/d, McMahon wrote a general-purpose line-oriented stream editor, which became sed. The syntax for sed, notably the use of / for pattern matching, and s/// for substitution, originated with ed, the precursor to sed, which was in common use at the time, and the regular expression syntax has influenced other languages, notably ECMAScript and Perl. Later, the more powerful language AWK developed, and these functioned as cousins, allowing powerful text processing to be done by shell scripts. sed and AWK are often cited as progenitors and inspiration for Perl, and influenced Perl's syntax and semantics, notably in the matching and substitution operators. GNU sed added several new features, including in-place editing of files. Super-sed is an extended version of sed that includes regular expressions compatible with Perl. Another variant of sed is minised, originally reverse-engineered from 4.1BSD sed by Eric S. Raymond and currently maintained by René Rebe. minised was used by the GNU Project until the GNU Project wrote a new version of sed based on the new GNU regular expression library. The current minised contains some extensions to BSD sed but is not as feature-rich as GNU sed. Its advantage is that it is very fast and uses little memory. It is used on embedded systems and is the version of sed provided with Minix. sed is a line-oriented text processing utility: it reads text, line by line, from an input stream or file, into an internal buffer called the pattern space. Each line read starts a cycle. To the pattern space, sed applies one or more operations which have been specified via a sed script. sed implements a programming language with about 25 commands that specify the operations on the text. For each input line, after running the script, sed ordinarily outputs the pattern space (the line as modified by the script) and begins the cycle again with the next line. Other end-of-script behaviors are available through sed options and script commands, e.g. d to delete the pattern space, q to quit, N to add the next line to the pattern space immediately, and so on. Thus a sed script corresponds to the body of a loop that iterates through the lines of a stream, where the loop itself and the loop variable (the current line number) are implicit and maintained by sed. The sed script can either be specified on the command line (-e option) or read from a separate file (-f option). Commands in the sed script may take an optional address, in terms of line numbers or regular expressions. The address determines when the command is run. For example, 2d would only run the d (delete) command on the second input line (printing all lines but the second), while /^ /d would delete all lines beginning with a space. A separate special buffer, the hold space, may be used by a few sed commands to hold and accumulate text between cycles. sed's command language has only two variables (the 'hold space' and the 'pattern space') and GOTO-like branching functionality; nevertheless, the language is Turing-complete, and esoteric sed scripts exist for games such as sokoban, arkanoid, chess, and tetris. A main loop executes for each line of the input stream, evaluating the sed script on each line of the input. Lines of a sed script are each a pattern-action pair, indicating what pattern to match and which action to perform, which can be recast as a conditional statement. Because the main loop, working variables (pattern space and hold space), input and output streams, and default actions (copy line to pattern space, print pattern space) are implicit, it is possible to write terse one-liner programs. For example, the sed program given by: