SED(1plan9)

💖 SED(1plan9) — stream editor 💖

okay bestie, sit down, hydrate, because sed is the stream editor and you're gonna be obsessed. this is the Plan 9 manual, section 1, and I'm giving you the whole thing, no skips, no "you get the idea" energy. every flag, every command, every example, fr fr. 🦋

real talk first 🙈 — a few single characters fell out of this page somewhere between the printing press and me. I did not guess at them, because guessing is not the vibe. anywhere the original has a hole, you'll see [gap]. everything else is exactly what the page says, character for character. no cap.

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✨ NAME ✨

sed — stream editor

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🌸 SYNOPSIS 🌸

sed [ -gln ] [ -e script ] [ -f sfile ] [ file ...  ]

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🎀 DESCRIPTION 🎀

sed copies the named files (standard input is the default, so if you name nothing, it reads stdin) to the standard output, edited according to a script of commands. ✨

The -f option causes the script to be taken from the file sfile; these options accumulate — stack as many as you want. If there is just one -e option and no -f's, the flag -e may be omitted entirely. 💅

• -n — suppresses the default output.
• -g — causes all substitutions to be global, as if suffixed with g.
• -l — causes sed to flush its output buffer after every newline.

🧚 how a script is shaped 🧚

A script consists of editing commands, one per line, of the following form:

[address [, address] ] function [argument ...]

In normal operation sed cyclically copies a line of input into a pattern space (unless there is something left after a command), applies in sequence all commands whose addresses select that pattern space, and at the end of the script copies the pattern space to the standard output (except under -n) and deletes the pattern space. that's the whole loop, that's the main character arc. 🔁

🌈 addresses, explained 🌈

An address is one of these three things:

• a decimal number that counts input lines cumulatively across files,
• a [gap] that addresses the last line of input,
• or a context address, /regular-expression/, in the style of regexp(7), with the added convention that it matches a newline embedded in the pattern space.

Here's how many addresses you get and what it means:

• A command line with no addresses selects every pattern space.
• A command line with one address selects each pattern space that matches the address.
• A command line with two addresses selects the inclusive range from the first pattern space that matches the first address through the next pattern space that matches the second. (If the second address is a number less than or equal to the line number first selected, only one line is selected.) Thereafter the process is repeated, looking again for the first address.

Editing commands can be applied to non-selected pattern spaces by use of the negation function (below). 👀

🍓 the text argument 🍓

An argument denoted text consists of one or more lines, all but the last of which end with [gap] to hide the newline. Backslashes in text are treated like backslashes in the replacement string of an [gap] command, and may be used to protect initial blanks and tabs against the stripping that is done on every script line. 🫶

🌟 the rfile / wfile argument 🌟

An argument denoted rfile or wfile must terminate the command line and must be preceded by exactly one blank. Each wfile is created before processing begins. There can be at most 120 distinct wfile arguments. (yes, 120. that's the ceiling. respect it. 👑)

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💅 the COMMANDS 💅

• a\ text — Append. Place text on the output before reading the next input line.
• b label — Branch to the : command bearing the label. If label is empty, branch to the end of the script.
• c\ text — Change. Delete the pattern space. With 0 or 1 address or at the end of a 2-address range, place text on the output. Start the next cycle.
• d — Delete the pattern space. Start the next cycle.
• D — Delete the initial segment of the pattern space through the first newline. Start the next cycle.
• g — Replace the contents of the pattern space by the contents of the hold space.
• G — Append the contents of the hold space to the pattern space.
• h — Replace the contents of the hold space by the contents of the pattern space.
• H — Append the contents of the pattern space to the hold space.
• i\ text — Insert. Place text on the standard output.
• n — Copy the pattern space to the standard output. Replace the pattern space with the next line of input.
• N — Append the next line of input to the pattern space with an embedded newline. (The current line number changes.)
• p — Print. Copy the pattern space to the standard output.
• P — Copy the initial segment of the pattern space through the first newline to the standard output.
• q — Quit. Branch to the end of the script. Do not start a new cycle.
• r rfile — Read the contents of rfile. Place them on the output before reading the next input line.
• s/regular-expression/replacement/flags — Substitute the replacement string for instances of the regular-expression in the pattern space. Any character may be used instead of [gap]. For a fuller description see regexp(7). flags is zero or more of:
  • g — Global. Substitute for all non-overlapping instances of the regular expression rather than just the first one.
  • p — Print the pattern space if a replacement was made.
  • w wfile — Write. Append the pattern space to wfile if a replacement was made.
• t label — Test. Branch to the command bearing the label if any substitutions have been made since the most recent reading of an input line or execution of a [gap]. If label is empty, branch to the end of the script.
• w wfile — Write. Append the pattern space to wfile.
• x — Exchange the contents of the pattern and hold spaces.
• y/string1/string2/ — Transform. Replace all occurrences of characters in string1 with the corresponding character in string2. The lengths of string1 and string2 must be equal.
• ! function — Don't. Apply the function (or group, if function is only [gap]) to lines not selected by the address(es).
• : label — This command does nothing; it bears a label for b and t commands to branch to.
• = — Place the current line number on the standard output as a line.
• { — Execute the following commands through a matching [gap] only when the pattern space is selected.

An empty command is ignored. 🫥

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🔥 EXAMPLES 🔥 (the fun part, obviously)

1. the classic

sed 10q file

Print the first 10 lines of the file. baby's first sed, iconic. 👑

2. bye bye blank lines

sed '/^$/d'

Delete empty lines from standard input.

3. glow-up a word

sed 's/UNIX/& system/g'

Replace every instance of [gap] by [gap].

4. the big one — one word per line

sed 's/ *$//   drop trailing blanks
/^$/d               drop empty lines
s/  */\        replace blanks by newlines
/g
/^$/d' chapter*

Print the files chapter1, chapter2, etc. one word to a line.

5. the manuscript one, aka the flex

nroff -ms manuscript | sed '
${
     /^$/p          if last line of file is empty, print it
}
//N            if current line is empty, append next line
/^\n$/D'       if two lines are empty, delete the first

Delete all but one of each group of empty lines from a formatted manuscript. 😭 this one is genuinely so pretty.

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💀 SOURCE 💀

/src/cmd/sed.c

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🌈 SEE ALSO 🌈

ed(1), grep(1), awk(1), lex(1), sam(1), regexp(7)

L. E. McMahon, `SED — A Non-interactive Text Editor', Unix Research System Programmer's Manual, Volume 2.

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💀 BUGS 💀

If input is from a pipe, buffering may consume characters beyond a line on which a command is executed. ⚠️ like, it will simply read past where you think it stopped. be aware.

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and that's the whole page, bestie — SED(1plan9), Plan 9 manual, section 1, from /src/cmd/sed.c. go forth and substitute. 💖✨🎀🦋