Mastering sed: Essential Commands and Real-World Examples for Text Processing

Introduction

Linux users are familiar with the "three swords" grep, awk, and sed. This article focuses on sed (stream editor), a non‑interactive text editor that operates via commands and supports regular expressions for powerful text manipulation.

How sed Works

sed

reads input line by line, stores each line in a temporary buffer called the pattern space, applies the specified commands to that buffer, outputs the result, and then proceeds to the next line. This cycle repeats until the end of the file unless output redirection or the -i option is used to modify the file in place.

Regular Expressions Overview

sed relies heavily on regular expressions. The article briefly reviews basic regex symbols ( ., *, ^, $, {}, []) and introduces extended regex (enabled with -r) which adds ?, +, |, (), and unescaped {}.

Basic Syntax

The general command form is: sed [options] 'command' filename Common options include -n (quiet mode), -e (multiple commands), -i (in‑place editing), -f (script file), and -r (extended regex).

Addressing

sed can target specific lines using numeric or regex addresses.

Numeric Addressing

# Replace "hello" with "A" on line 4
sed '4s/hello/A/g' file.txt
# Replace on lines 2‑4
sed '2,4s/hello/A/g' file.txt
# From line 2, next 4 lines (2‑6)
sed '2,+4s/hello/A/g' file.txt
# Last line
sed '$s/hello/A/g' file.txt
# All but line 1
sed '1!s/hello/A/g' file.txt

Regex Addressing

# Delete lines containing "hello"
sed '/hello/d' file.txt
# Delete empty lines
sed '/^$/d' file.txt
# Delete from a line starting with "ts" to a line starting with "te"
sed '/^ts/,/^te/d' file.txt

Mixed Addressing

# Delete from line 1 to the first line starting with "ts"
sed '1,/^ts/d' file.txt

Sub‑Commands

Substitute ( s )

# Replace first occurrence of "hello" with "HELLO"
sed 's/hello/HELLO/' file.txt
# Replace all occurrences (global)
sed 's/hello/HELLO/g' file.txt
# Replace the 2nd occurrence only
sed 's/hello/A/2' file.txt
# Replace from the 2nd occurrence onward
sed 's/hello/A/2g' file.txt
# Add "#" at line start
sed 's/^/#/g' file.txt
# Append "xxx" at line end
sed 's/$/xxx/g' file.txt

Append ( a )

# Append a line after every line
sed 'a A' file.txt
# Append after lines 1‑2
sed '1,2a A' file.txt

Insert ( i )

# Insert a line before lines 1‑2
sed '1,2i A' file.txt

Change ( c )

# Replace every line with "A"
sed 'c A' file.txt
# Replace lines 1‑2 with a single "A"
sed '1,2c A' file.txt
# Replace lines 1‑2 with two lines "A" and "A"
sed '1,2c A
A' file.txt

Delete ( d )

# Delete lines 1‑3
sed '1,3d' file.txt
# Delete lines starting with "This"
sed '/^This/d' file.txt

Print Line Number ( = )

# Print line numbers for lines 1‑2
sed '1,2=' file.txt
# Print line numbers at line start
sed '=' file.txt | sed 'N;s/
/\t/'

Read Next Line ( N )

# Merge even lines into odd lines
sed 'N;s/
//' file.txt

Practical Exercises

Delete the second character of each line: sed -r 's/(.)(.)(.*)$/\1\3/' file.txt Swap the first two characters of each line: sed -r 's/(.)(.)(.*)/\2\1\3/' file.txt Remove all digits: sed 's/[0-9]//g' file.txt Replace spaces with tabs: sed -r 's/ +/\t/g' file.txt Wrap uppercase letters in parentheses: sed -r 's/([A-Z])/(\1)/g' file.txt Delete every other line: sed '0~2{d}' file.txt Delete all blank lines: sed '/^$/d' file.txt By practicing these commands and mastering regular expressions, readers can dramatically improve text‑processing efficiency in everyday Linux tasks.