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.

sed Commands Reference

sed

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.

Tips and 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

Test changes by viewing output first, and only add -i once you are confident, since in-place editing rewrites the original file.

sed Commands Reference

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sed

How it works

Tips and examples