sed Command in Linux Explained with Examples

Quick take: The sed command is a stream editor for transforming text. Its most common use is substitution: sed 's/old/new/g' file. Add -i to edit the file in place and -n with p to print selected lines.

Introduction

The sed command (stream editor) reads text line by line and applies editing commands — substituting, deleting, inserting, or printing. It is the standard tool for non-interactive find-and-replace and for transforming files in scripts where opening an editor is impractical.

This guide focuses on the operations you will actually use: substitution, in-place editing, deleting and printing lines, and applying changes only to matching lines.

Syntax

The basic syntax of the sed command is:

sed [OPTIONS] 'COMMAND' [FILE...]

Common Options and Parameters

The most useful options and parameters for the sed command:

Practical Examples

Real sed commands you can run today:

# Replace the first occurrence per line
sed 's/error/ERROR/' app.log
# Replace every occurrence (global)
sed 's/foo/bar/g' data.txt
# Edit a file in place, keeping a backup
sed -i.bak 's/8080/80/g' nginx.conf
# Delete blank lines
sed '/^$/d' notes.txt
# Delete lines matching a pattern
sed '/DEBUG/d' app.log
# Print only lines 5 to 10
sed -n '5,10p' file.txt
# Replace only on lines matching a pattern
sed '/^server/s/listen 80/listen 443/' site.conf

Advanced sed: Ranges and Multiple Edits

Beyond single substitutions, sed can target line ranges, chain several commands, and act only on lines that match a pattern. This makes it a precise editor for configuration files.

# Substitute only between two markers
sed '/BEGIN/,/END/s/old/new/g' file

# Run several edits in one pass
sed -e 's/foo/bar/g' -e '/^#/d' -e 's/  */ /g' file

# Substitute only on lines that contain 'listen'
sed '/listen/s/80/443/' nginx.conf

# Insert a line after every match
sed '/\[mysqld\]/a max_connections = 200' my.cnf

Combining an address (a line number or /pattern/) with the s command is the key to surgical edits that leave the rest of the file untouched.

Common sed Pitfalls

The biggest risk with sed is the -i flag: it edits files in place with no undo. Always test the command by letting it print to the screen first, and use -i.bak so sed keeps the original as file.bak before changing anything.

The second pitfall is escaping slashes. When your pattern or replacement contains / — as paths and URLs do — switch the delimiter instead of escaping every slash: sed 's#/old/path#/new/path#g' is far more readable than the backslash-heavy alternative. sed accepts almost any character as the delimiter right after the s.

Tips and Best Practices

• Always test without -i first — run the substitution to stdout, confirm the result, then add -i to write it back.
• Use -i.bak so sed saves the original as file.bak before editing, giving you a safety net.
• Pick a different delimiter when your text contains slashes: sed 's#/old/path#/new/path#g' avoids escaping.

Final Thoughts

sed is the fastest way to transform text without opening an editor — indispensable in scripts and one-off fixes alike. Master the substitution command s/old/new/g, the in-place flag -i, and line selection, and you can reshape configuration files and logs in a single line. It works beautifully alongside grep and awk.