What does the -n flag do in sed?

By default sed prints every line after processing. The -n flag suppresses that automatic printing, so output appears only where you explicitly use the p command or the p flag on a substitution. It is the basis of sed -n '5p' to print one line.

How do I replace all occurrences on a line, not just the first?

Add the g flag to the substitute command: sed 's/old/new/g'. Without g, sed replaces only the first match on each line. You can also target the N-th match with a number flag, or N-th onward with Ng.

How do I edit a file in place?

Use the -i flag. GNU sed accepts an optional backup suffix as -i.bak, while BSD/macOS sed requires an explicit argument such as -i '' for no backup. Always test without -i first, since it overwrites the file.

What are the hold space and pattern space?

The pattern space is the buffer holding the current line being processed. The hold space is a second buffer you can copy to and from using h, H, g, G and x, letting you carry text across lines for tasks like reversing or joining.

What is the difference between basic and extended regex in sed?

Basic regex (the default) requires backslashes before grouping and alternation, as in $...$ and \|. The -E or -r flag enables extended regex where (), +, ? and | work unescaped, which is usually easier to read.

What is the sed Commands Reference?

Searchable sed command reference covering line and regex addressing, the substitute command and its g/N/p/i flags, append/insert/change/delete editing, hold-space and branching commands, plus command-line flags like -n, -i and -E. It runs free in your browser on Gera Tools, with nothing uploaded.

sed Commands Reference

Name: sed Commands Reference
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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sed — stream editor reference

sed is a non-interactive stream editor: it reads input line by line into a buffer called the pattern space, applies a script of editing commands, and writes the result out. It excels at find-and-replace, deleting or printing selected lines, and multi-line transforms via the hold space. This page is a searchable, offline reference to sed’s addresses, commands, substitute flags and command-line options, each with a working example.

How it works

Every sed command may be prefixed with an address that selects which lines it applies to:

Addresses can be a line number 3, the last line $, a regex /re/, a range 2,5 or /a/,/b/, GNU step form 1~2, or a negated form addr!.
The substitute command s/regex/replacement/flags is the workhorse. Flags include g (all matches), a number N (the N-th match), p (print on change), i (case-insensitive) and m (multi-line). In the replacement, & is the whole match and \1..\9 are captured groups.
Editing commands add (a), insert (i), change (c), delete (d), print (p), transliterate (y) and quit (q).
Advanced commands move text between the pattern and hold spaces (h, H, g, G, x) and branch with labels (:label, b, t, T) for loops.

The output model matters: with no -n, sed prints the pattern space at the end of each cycle; with -n it prints nothing unless you ask.

Practical examples

Delete blank lines and comment lines:

sed '/^$/d; /^#/d' config

Replace every tab with a comma across the whole file, in place with a backup:

sed -i.bak 's/\t/,/g' data.tsv

Print only lines 10 through 20:

sed -n '10,20p' file

Join all lines into one using the hold-space loop idiom:

sed ':a; N; $!ba; s/\n/ /g' file

Extract lines between two patterns (inclusive):

sed -n '/START/,/END/p' file

Add a line after every line matching a pattern:

sed '/pattern/a\new line to insert' file

BSD versus GNU sed: the key differences

macOS ships BSD sed, while Linux typically has GNU sed. The most common points of divergence:

Feature	GNU sed	BSD sed (macOS)
In-place edit	`sed -i 's/x/y/' file`	`sed -i '' 's/x/y/' file`
Extended regex	`-E` or `-r`	`-E` only
Step address	`1~2` (every other line)	not supported
Multiline mode `m` flag	supported	not supported
`\w` in regex	supported	not supported

If a sed one-liner works on Linux but fails on macOS, check for GNU-only features and use gsed (install via Homebrew) on macOS to get GNU behaviour.

Common mistakes to avoid

Testing without -i: always run the command without -i first to verify the output, then add -i once confirmed. A wrong regex with -i overwrites the original file immediately.
Forgetting g flag: s/old/new/ only replaces the first occurrence per line. Add g for all occurrences.
Delimiter clashes: if your pattern contains /, either escape it as \/ or choose a different delimiter — s|/old/|/new/|g uses | instead and is much easier to read.
Hold-space confusion: h copies pattern space to hold space (overwriting it); H appends. g copies hold to pattern; G appends. Mixing these up silently drops lines.