sed

sed [options] 'command' [file...]

sed [options] -e 'command1' -e 'command2' [file...]

Command / Option	Description
s/before/after/	Replaces the first match on each line.
s/before/after/g	Replaces all matches on each line (global).
s/before/after/i	Replaces matches case-insensitively.
-i file	Edits the file in place (directly).
-i.bak file	Saves a backup to .bak before editing in place.
-n	Suppresses default output (used together with the p command).
/pattern/d	Deletes lines matching the pattern.
/pattern/p	Prints lines matching the pattern (used together with -n).
Nd	Deletes line N.
N,Md	Deletes lines N through M.
/pattern/a text	Appends text after each matching line.
/pattern/i text	Inserts text before each matching line.

The following files are used as examples.

data.txt

Hello World foo foo
This is a foo test
Another line here

config.txt

# Database settings
host=localhost
# Port number
port=3306

s/before/after/ replaces the first match on each line. Adding g replaces all matches.

sed 's/foo/bar/' data.txt
Hello World bar foo
This is a bar test
Another line here

sed 's/foo/bar/g' data.txt
Hello World bar bar
This is a bar test
Another line here

/pattern/d deletes lines matching the pattern. The example below removes comment lines (lines starting with #).

sed '/^#/d' config.txt
host=localhost
port=3306

/^$/d removes blank lines.

printf "line1\n\nline2\n\nline3\n" | sed '/^$/d'
line1
line2
line3

Combining -n and the p command extracts only specific lines.

sed -n '2,4p' config.txt
host=localhost
# Port number
port=3306

-e runs multiple substitutions at once.

sed -e 's/foo/bar/g' -e 's/World/Earth/' data.txt
Hello Earth bar bar
This is a bar test
Another line here

-i edits the file directly (in-place substitution). -i.bak creates a backup before editing.

sed -i 's/old/new/g' file.txt

sed -i.bak 's/old/new/g' file.txt

Removes leading and trailing whitespace (trim).

echo "  hello world  " | sed 's/^[[:space:]]*//; s/[[:space:]]*$//'
hello world

Deletes the first line (useful for skipping CSV headers, for example).

echo -e "name,age\nalice,30\nbob,25" | sed '1d'
alice,30
bob,25

When using sed -i for in-place editing, it is strongly recommended to create a backup first or work within a git-managed repository. On macOS, BSD sed is used instead of GNU sed, so some option syntax differs (for example, macOS requires an empty string: sed -i "").

For more complex text processing, use awk. For simple searches, see also grep.