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Sed - Stream Editor

· 2 min read
Max Kaido
Max Kaido
Architect

Sed is a powerful stream editor for filtering and transforming text. It's especially useful for automated text manipulation and find-and-replace operations.

Key Features​

  • Line-based text processing
  • Regular expression support
  • In-place file editing
  • Multiple commands in sequence
  • Automated text transformations

Common Usage​

Basic Substitution​

sed 's/old/new/'           # Replace first occurrence in each line
sed 's/old/new/g'          # Replace all occurrences
sed 's/old/new/gi'         # Replace all occurrences, case-insensitive

In-Place Editing​

sed -i 's/old/new/g' file.txt     # Edit file in-place
sed -i.bak 's/old/new/g' file.txt # Create backup before editing

Line Operations​

sed '1d'                   # Delete first line
sed '$d'                   # Delete last line
sed '1,5d'                # Delete lines 1-5
sed '/pattern/d'          # Delete lines matching pattern

Advanced Features​

sed -n '5p'               # Print only line 5
sed -n '5,10p'            # Print lines 5-10
sed '/start/,/end/p'      # Print lines between patterns

Pro Tips​

  1. Multiple Commands:

    sed -e 's/old/new/' -e 's/foo/bar/'  # Multiple substitutions
    sed 's/old/new/; s/foo/bar/'         # Alternative syntax

  2. Using Variables:

    pattern="old"
    sed "s/$pattern/new/g"               # Use double quotes for variables

  3. Working with Files:

    sed 's/old/new/g' file1 file2        # Process multiple files
    sed -i.$(date +%Y%m%d) 's/old/new/g' # Dated backups

Common Patterns​

Text Manipulation​

# Add text
sed 's/^/prefix /'        # Add prefix to each line
sed 's/$/ suffix/'        # Add suffix to each line

# Remove text
sed 's/pattern//g'        # Remove all occurrences
sed 's/^[ \t]*//'        # Remove leading whitespace
sed 's/[ \t]*$//'        # Remove trailing whitespace

# Transform text
sed 's/\([0-9]\)/[\1]/g' # Wrap digits in brackets
sed 's/.*/\U&/'          # Convert line to uppercase

Working with Code​

# Comment/Uncomment
sed 's/^#//'             # Uncomment lines
sed 's/^/#/'             # Comment lines
sed '/^$/d'              # Remove empty lines

# JSON manipulation
sed 's/"/\\"/g'          # Escape quotes
sed 's/\\n/\n/g'         # Convert \n to newlines

Complex Operations​

# Conditional replacement
sed '/pattern/s/old/new/' # Replace only in lines matching pattern

# Multi-line operations
sed ':a;N;$!ba;s/\n/ /g' # Join all lines
sed 'N;s/\n/ /'          # Join pairs of lines

Advanced Techniques​

Using Address Ranges​

sed '2,5s/old/new/'      # Lines 2-5
sed '/start/,/end/s/old/new/' # Between patterns
sed '2,+3s/old/new/'     # Line 2 and next 3 lines

Using Hold Space​

# Reverse line order
sed '1!G;h;$!d'

# Double space output
sed 'G'

# Number lines
sed = file | sed 'N;s/\n/\t/'

Regular Expression Features​

sed 's/[0-9]\{3\}/XXX/'  # Replace 3 digits with XXX
sed 's/\(foo\).*\(bar\)/\2 \1/' # Backreferences
sed 's/[[:space:]]\+/ /g' # Normalize whitespace

Integration Examples​

With Find​

find . -type f -name "*.txt" -exec sed -i 's/old/new/g' {} +

With Git​

git ls-files '*.js' | xargs sed -i 's/old/new/g'

In Scripts​

#!/bin/bash
# Safe sed with error checking
if ! sed -i.bak 's/old/new/g' file.txt; then
    echo "Sed operation failed"
    mv file.txt.bak file.txt
    exit 1
fi