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sed is the stream editor: it reads input line by line into the pattern space, applies a script to each line, and prints the result. It never needs the whole file in memory, so it happily edits gigabytes.

sed [options] 'script' [file...]

ℹ️ Examples here use GNU sed (Linux). BSD/macOS differs on -i, -E and the a/i/c commands — see the callouts. brew install gnu-sed gives you gsed on macOS.

⚙️ Options

Option	Description
`-n`	Do not print automatically — pair it with `p`
`-e <script>`	Add a script; repeat for several
`-f <file>`	Read the script from a file
`-i[SUFFIX]`	Edit files in place, optionally keeping a backup
`-E` (or `-r`)	Extended regular expressions — no backslashes before `(`, `+`, `?`
`-s`	Treat each file separately instead of one long stream
`-z`	Split on NUL instead of newline (pairs with `find -print0`)
`--debug`	Show how sed executes the script (GNU 4.6+)

📍 Addressing

An address decides which lines a command applies to. Without one, the command applies to every line.

Address	Matches
`5`	Line 5
`$`	The last line
`/regex/`	Every line matching the pattern
`\%regex%`	Same, with `%` as the delimiter (handy for paths)
`2,7`	Lines 2 to 7
`2,$`	Line 2 to the end
`/start/,/end/`	From the first `start` to the next `end`, inclusive
`2,+3`	Line 2 and the three lines after it
`0~3`	Every third line (GNU)
`2~5`	Line 2, then every fifth line (GNU)
`/regex/!`	Invert — every line that does not match

# print line 5
sed -n '5p' file

# print lines 3 to 5
sed -n '3,5p' file

# print everything between two markers
sed -n '/BEGIN/,/END/p' file

# delete every line except the ones matching
sed -n '/keep/p' file
sed '/keep/!d' file

# count the lines (like wc -l)
sed -n '$=' file

🔁 Substitution

sed 's/pattern/replacement/flags' file

Flag	Effect
none	Replace the first match on each line
`g`	Replace every match on the line
`i`	Case-insensitive matching
`p`	Print the line when a replacement happened
`2`	Replace only the second match — any number works
`2g`	Replace from the second match to the end of the line
`w f`	Write the changed lines to file `f`
`e`	Run the result as a shell command (GNU)

# first match on each line
sed 's/foo/bar/' file

# every match on the line
sed 's/foo/bar/g' file

# only the second match
sed 's/foo/bar/2' file

# every match, ignoring case
sed 's/foo/bar/gI' file

# & stands for the whole match — wrap every number in brackets
sed 's/[0-9]\+/[&]/g' file

# capture groups: \( \) in basic regex, ( ) with -E
sed 's/\(foo\)bar/\1baz/' file
sed -E 's/(\w+)@(\w+)/\2 at \1/' file

# any character can be the delimiter — pick one that is not in the text
sed 's|/usr/bin|/usr/local/bin|' file
sed 's#http://#https://#' file

# GNU case conversion: upper-case the first word
sed 's/\w\+/\U&/' file

# ...and lower-case a whole line
sed -E 's/(.*)/\L\1/' file

# only substitute on lines that match an address
sed '/^deb /s/http:/https:/' sources.list

✂️ Command Reference

Command	Does
`p`	Print the pattern space
`d`	Delete it and start the next cycle
`s`	Substitute
`y`	Transliterate characters, like `tr`
`a text`	Append a line after the current one
`i text`	Insert a line before the current one
`c text`	Replace the matched line(s)
`q`	Quit — `q5` also sets the exit status
`Q`	Quit without printing the current line
`r file`	Read a file in after the current line
`w file`	Write the pattern space to a file
`=`	Print the current line number
`n` / `N`	Load the next line, replacing / appending to the pattern space
`D` / `P`	Delete / print up to the first newline of the pattern space
`h` `H`	Copy / append the pattern space into the hold space
`g` `G`	Copy / append the hold space into the pattern space
`x`	Swap the pattern and hold spaces
`b` `t` `T` `:label`	Branching — jump, jump if a substitution happened, jump if none

🗑 Deleting & Printing

# by line number
sed '2d' file
sed '5,10d' file
sed '$d' file

# lines containing a word
sed '/error/d' file

# blank lines
sed '/^$/d' file

# comment lines
sed '/^#/d' file

# blank or whitespace-only lines
sed '/^\s*$/d' file

# strip comments and blank lines from a config
sed -e '/^#/d' -e '/^$/d' /etc/ssh/sshd_config

# from the top up to the first match
sed '1,/pattern/d' file

# everything between two markers
sed '/start/,/end/d' file

# keep only what matches
sed -n '/pattern/p' file

🧵 Insert, Append & Change

# GNU one-line form
sed '/pattern/a appended line' file
sed '/pattern/i inserted line' file
sed '/pattern/c replacement line' file

# portable form (works on BSD/macOS too)
sed '/pattern/a\
appended line' file

# add a line at the very top or bottom
sed '1i #!/bin/bash' file
sed '$a # end of file' file

# insert the contents of another file after a marker
sed '/INCLUDE HERE/r snippet.txt' file

🧠 Multi-line & Hold Space

The hold space is a second buffer that survives between lines — that is what makes sed more than a line-at-a-time filter.

# join every pair of lines
sed 'N;s/\n/ /' file

# reverse the file (what tac does)
sed -n '1!G;h;$p' file

# print the line before each match
sed -n '/pattern/{x;p;x};h' file

# print the line after each match
sed -n '/pattern/{n;p}' file

# squeeze runs of blank lines into one
sed '/^$/{N;/^\n$/D}' file

# delete the last two lines
sed 'N;$!P;$!D;$d' file

🧨 In-Place Editing

# GNU: edit in place
sed -i 's/foo/bar/g' file

# keep a backup as file.bak
sed -i.bak 's/foo/bar/g' file

# several files at once
sed -i 's/foo/bar/g' *.conf

# every matching file in a tree
find . -name '*.py' -exec sed -i 's/old_api/new_api/g' {} +

⚠️ BSD/macOS sed requires an argument to -i. sed -i '' 's/foo/bar/' file edits without a backup there, while the same command on GNU sed reads '' as the script. Write sed -i.bak for something that works on both, or use gsed.
💡 Always run the command without -i first and read the output. -i has no undo.

🧰 Recipes

# trim leading and trailing whitespace
sed 's/^[[:space:]]*//; s/[[:space:]]*$//' file

# collapse repeated spaces
sed 's/  */ /g' file

# tabs to four spaces
sed 's/\t/    /g' file

# strip Windows carriage returns
sed 's/\r$//' file

# strip HTML tags
sed -e 's/<[^>]*>//g' page.html

# number the lines
sed = file | sed 'N;s/\n/\t/'

# print a specific line and quit early (fast on huge files)
sed -n '1000{p;q}' file

# extract the value of a key from a config file
sed -n 's/^Port[[:space:]]*//p' /etc/ssh/sshd_config

# replace only in lines between two markers
sed '/BEGIN/,/END/ s/old/new/g' file

# change a value only on the first match, then stop
sed '0,/version:/s//version: 2/' file

# comment out a line
sed '/^ExecStart/s/^/#/' unit.service

# uncomment a line
sed '/^#ExecStart/s/^#//' unit.service