How to Use sed for Stream Editing

What is sed?

sed (Stream Editor) is a non-interactive text editor that processes input line by line. It is ideal for automated text transformations, search-and-replace operations, and scripted file modifications on your Breeze.

Basic Syntax

sed [options] 'command' filename

Search and Replace

# Replace first occurrence per line
sed 's/old/new/' file.txt

# Replace ALL occurrences per line (global flag)
sed 's/old/new/g' file.txt

# Case-insensitive replacement
sed 's/error/warning/Ig' logfile.txt

# Edit file in place
sed -i 's/old/new/g' config.conf

# Create a backup before in-place editing
sed -i.bak 's/old/new/g' config.conf

Deleting Lines

# Delete line 5
sed '5d' file.txt

# Delete lines 10 through 20
sed '10,20d' file.txt

# Delete blank lines
sed '/^$/d' file.txt

# Delete lines matching a pattern
sed '/^#/d' config.conf    # Remove comments
sed '/DEBUG/d' app.log      # Remove debug lines

Inserting and Appending

# Insert a line before line 3
sed '3i\New line inserted here' file.txt

# Append a line after line 5
sed '5a\Appended after line 5' file.txt

# Insert before a matching pattern
sed '/\[mysqld\]/i\# Custom MySQL settings' my.cnf

Printing Specific Lines

# Print only line 10
sed -n '10p' file.txt

# Print lines 5 through 15
sed -n '5,15p' file.txt

# Print lines matching a pattern
sed -n '/error/p' logfile.txt

# Print lines between two patterns
sed -n '/START/,/END/p' file.txt

Using Regular Expressions

# Extended regex with -E
sed -E 's/[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}/REDACTED/g' access.log

# Capture groups
sed -E 's/^([a-z]+):([0-9]+)/User: \1, Port: \2/' data.txt

# Remove HTML tags
sed -E 's/]+>//g' page.html

Multiple Commands

# Using -e flag
sed -e 's/foo/bar/g' -e 's/baz/qux/g' file.txt

# Using semicolons
sed 's/foo/bar/g; s/baz/qux/g' file.txt

# Using a sed script file
sed -f commands.sed file.txt

Practical Server Examples

# Update a config value
sed -i 's/^listen_port=.*/listen_port=8080/' app.conf

# Comment out a line
sed -i 's/^dangerous_setting/#&/' config.conf

# Uncomment a line
sed -i 's/^#\(wanted_setting\)/\1/' config.conf

# Remove trailing whitespace
sed -i 's/[[:space:]]*$//' script.sh

# Add a line after a match (e.g., add a server block entry)
sed -i '/^upstream backend/a\    server 10.0.0.5:8080;' nginx.conf

Important Notes

• Always test without -i first to preview changes
• Use -i.bak to create backups when editing in place
• On macOS/BSD, sed -i requires an explicit backup extension: sed -i '' 's/old/new/g'
• Combine sed with find for bulk operations: find . -name "*.conf" -exec sed -i 's/old/new/g' {} +