sed Line Addressing: Numbers, $, and Regex

A sed address before a command restricts that command to matching lines: a number, the $ symbol, or a regex.

Every sed command can carry an address. Master the three single-line forms and you control exactly where substitutions and deletes apply.

What it does

An address precedes a command and limits it to specific lines. A bare number N matches line N, $ matches the last line, and /regex/ matches any line the regex hits. With no address, the command runs on every line.

Common usage

Terminal

sed '1s/^/# /' file              # prefix only line 1
sed '$s/$/ EOF/' file            # append to the last line
sed '/^server:/s/dev/prod/' app.yml  # change only under server:
sed -n '/BEGIN/=' file           # print line numbers of matches

Options

Address	What it matches
N	Exactly line number N
$	The last line of the input
/regex/	Every line matching the regular expression
0,/regex/ (GNU)	From the start until the first regex match
N~M (GNU)	Every Mth line starting at line N (step)
addr!	Invert: run the command on non-matching lines

In CI

Number and $ addressing are POSIX and portable. The step form N~M and the 0,/regex/ start address are GNU extensions, so they fail on BSD sed; avoid them in macOS matrix jobs.

Common errors in CI

sed: -e expression #1, char N: unexpected `,' usually means a bad range address. On BSD sed, a GNU-only address such as 1~2p raises sed: 1: "1~2p": invalid command code ~. Use a regex or explicit range that both implementations accept.