Bash Tutorial · Pipes and Text Processing · Lesson 10 of 30

Bash sed: Edit Text Streams

Substitute, delete and print lines with sed, and edit files in place safely.

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Substitution

sed 's/old/new/' replaces the first match on each line; add g for all matches. Any character can be the delimiter, which helps with paths: s|/usr/local|/opt|.

Substitutions and capture groups

echo "Hello World, hello world" | sed 's/hello/Hi/'
echo "Hello World, hello world" | sed 's/hello/Hi/gI'
echo "/usr/local/bin/app" | sed 's|/usr/local|/opt|'
echo "2026-01-15" | sed -E 's/([0-9]{4})-([0-9]{2})-([0-9]{2})/\3.\2.\1/'

Output

Hello World, Hi world
Hi World, Hi world
/opt/bin/app
15.01.2026

Run this on your computer — see “Get Started” for setup.

Selecting lines

sed -n '3p' prints line 3; sed -n '/ERROR/p' prints matching lines.
sed '/^#/d' deletes comment lines; sed '1d' deletes the first line.

Editing files in place

sed -i 's/a/b/' file changes the file itself (GNU sed). On macOS use sed -i '' .... Keep a backup with -i.bak when in doubt.

In-place edit with a backup

cat > app.conf <<'EOF'
# database settings
db_host=localhost
db_port=5432
# debug
debug=true
EOF
sed -i.bak 's/^debug=true/debug=false/' app.conf
sed '/^#/d' app.conf
ls app.conf*

Output

db_host=localhost
db_port=5432
debug=false
app.conf
app.conf.bak

Run this on your computer — see “Get Started” for setup.

Test yourself

What does the g flag in s/a/b/g do?

Makes the match case-insensitive
Replaces every match on the line, not just the first
Edits the file in place
Prints only changed lines

Show answer

Answer: Replaces every match on the line, not just the first

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