Linux sed Command with Examples: Find and Replace

sed (stream editor) edits text line by line without opening an editor. It is often used in scripts to change configuration files.

cat app.conf

# Server settings
host=localhost
port=8080

debug=true
# Database
db_host=localhost
db_port=5432

Find and replace#

The form is s/old/new/:

sed 's/localhost/10.0.0.5/' app.conf | head -n 3

# Server settings
host=10.0.0.5
port=8080

By default, sed prints the result and does not change the file. It replaces only the first match on each line; add g for all matches:

echo "cat cat cat" | sed 's/cat/dog/'
echo "cat cat cat" | sed 's/cat/dog/g'
echo "Cat CAT" | sed 's/cat/dog/gi'

dog cat cat
dog dog dog
dog dog

i ignores case. If your text contains slashes, use another separator:

echo "/usr/local/bin" | sed 's#/usr/local#/opt#'

/opt/bin

Edit a file in place#

-i changes the file. -i.bak keeps a backup first:

sed -i.bak 's/port=8080/port=9090/' app.conf
grep -n '^port' app.conf app.conf.bak

app.conf:3:port=9090
app.conf.bak:3:port=8080

Delete and print lines#

d deletes matching lines. Here we remove comments and empty lines, which is a handy way to read a config file:

sed '/^#/d; /^$/d' app.conf

host=localhost
port=9090
debug=true
db_host=localhost
db_port=5432

-n turns off normal printing, and p prints chosen lines:

sed -n '2,3p' app.conf
sed -n '/^db/p' app.conf

host=localhost
port=9090
db_host=localhost
db_port=5432

Target particular lines#

Put an address before a command:

sed '5s/true/false/' app.conf | sed -n '5p'
sed '/debug/s/true/false/' app.conf | grep debug

debug=false
debug=false

Insert lines#

i inserts before, and a appends after:

sed '/^port/a timeout=30' app.conf | sed -n '2,4p'

host=localhost
port=9090
timeout=30

Groups#

With -E, parentheses capture text, and \1 and \2 reuse it. This swaps the name and value:

echo "host=localhost" | sed -E 's/(.+)=(.+)/\2 is the \1/'
echo "2026-11-05" | sed -E 's/([0-9]+)-([0-9]+)-([0-9]+)/\3\/\2\/\1/'

localhost is the host
05/11/2026

Common mistakes#

Practice#

sed '/^#/d; s/localhost/db.local/g' app.conf

host=db.local
port=9090

debug=true
db_host=db.local
db_port=5432

Key takeaway#

Next: Working with Columns in awk.