sed and awk

sed basics

Substitute

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

# Replace all occurrences per line
sed 's/old/new/g' file.txt

# Case-insensitive replace (GNU sed)
sed 's/old/new/gI' file.txt

Delete lines

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

# Delete lines 5 through 10
sed '5,10d' file.txt

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

# Delete lines matching a pattern
sed '/DEBUG/d' file.txt

Insert, append, and in-place editing

# Insert line before line 3
sed '3i\new line here' file.txt

# Append line after line 3
sed '3a\new line here' file.txt

# GNU sed: edit file in place
sed -i 's/old/new/g' file.txt

# BSD sed (macOS): requires backup extension
sed -i '' 's/old/new/g' file.txt

# Both: use backup extension for safety
sed -i.bak 's/old/new/g' file.txt

sed addresses

sed '3s/old/new/' file.txt             # Line number
sed '3,7s/old/new/' file.txt           # Line range
sed '/START/,$s/old/new/' file.txt     # Pattern to end of file
sed '/BEGIN/,/END/s/old/new/' file.txt # Between two patterns
sed '1~2s/old/new/' file.txt           # Every 2nd line (GNU only)
sed '/KEEP/!d' file.txt                # Negate: all lines except match

sed with regex

# Capture groups and backreferences (basic regex)
sed 's/\(foo\)\(bar\)/\2\1/' file.txt

# Extended regex (-E flag) avoids escaping parens
sed -E 's/(foo)(bar)/\2\1/' file.txt

# Reformat dates: 2024-01-15 -> 01/15/2024
sed -E 's/([0-9]{4})-([0-9]{2})-([0-9]{2})/\2\/\3\/\1/' file.txt

# Strip HTML tags
sed -E 's/<[^>]+>//g' file.html

# Trim leading and trailing whitespace
sed -E 's/^[[:space:]]+//; s/[[:space:]]+$//' file.txt

Multiple commands

# Chain with -e or semicolons
sed -e 's/foo/bar/' -e 's/baz/qux/' file.txt
sed 's/foo/bar/; s/baz/qux/' file.txt

awk basics

Print columns

# Print second column (whitespace-separated by default)
awk '{print $2}' file.txt

# Print first and third columns
awk '{print $1, $3}' file.txt

# Print last column
awk '{print $NF}' file.txt

Field separator

# CSV: use comma as separator
awk -F',' '{print $2}' data.csv

# Colon-separated (e.g., /etc/passwd)
awk -F':' '{print $1, $3}' /etc/passwd

# Multiple separators
awk -F'[,;]' '{print $1, $2}' file.txt

# Set output separator
awk -F',' -v OFS='\t' '{print $1, $3}' data.csv

Built-in variables

# NR: current line number; NF: number of fields in current line
awk '{print NR, NF, $0}' file.txt

# FS/OFS: input/output field separators
awk 'BEGIN{FS=","; OFS="\t"} {print $1, $2}' data.csv

# FILENAME: current input file
awk '{print FILENAME, NR, $0}' *.txt

awk patterns and actions

BEGIN and END blocks

# Header and footer
awk 'BEGIN{print "Name\tScore"} {print $1, $2} END{print "---done---"}' file.txt

# Count lines
awk 'END{print NR, "lines"}' file.txt

Conditionals

# Print lines where column 3 > 100
awk '$3 > 100' file.txt

# Pattern match
awk '/ERROR/' file.txt

# Negated match
awk '!/DEBUG/' file.txt

# Conditional with field match
awk '$1 == "admin" {print $2}' file.txt

# Regex match on a field
awk '$2 ~ /^192\.168/' file.txt

# If-else
awk '{if ($3 > 90) print $1, "pass"; else print $1, "fail"}' file.txt

Arithmetic

# Sum a column
awk '{sum += $2} END{print sum}' file.txt

# Average
awk '{sum += $2; n++} END{print sum/n}' file.txt

# Max value in column 3
awk 'NR==1 || $3 > max {max=$3} END{print max}' file.txt

awk string functions

# split string into array
awk '{split($0, parts, ":"); print parts[1]}' file.txt

# substr (1-indexed)
awk '{print substr($1, 1, 3)}' file.txt

# gsub: global substitution; sub: first occurrence only
awk '{gsub(/foo/, "bar"); print}' file.txt

# match: find pattern, then extract via RSTART/RLENGTH
awk '{if (match($0, /[0-9]+/)) print substr($0, RSTART, RLENGTH)}' file.txt

# printf for formatted output
awk '{printf "%-20s %10.2f\n", $1, $2}' file.txt

# length, tolower, toupper
awk 'length($0) > 80' file.txt
awk '{print tolower($1)}' file.txt

Common recipes

Extract column from CSV

# Simple CSV
awk -F',' '{print $3}' data.csv

# CSV with quoted fields -- use a proper parser for complex cases
awk -F'","' '{gsub(/^"|"$/, "", $2); print $2}' data.csv

Reformat log lines

# Apache log: extract IP and URL
awk '{print $1, $7}' access.log

# Extract timestamp and message from syslog
sed -E 's/^([A-Z][a-z]+ [0-9]+ [0-9:]+) [^ ]+ (.+)/\1 | \2/' /var/log/syslog

Bulk rename patterns across files

# Replace in all matching files
find . -name '*.py' -exec sed -i '' 's/old_func/new_func/g' {} +

# Preview first (no -i flag)
grep -rl 'old_func' --include='*.py' . | xargs sed 's/old_func/new_func/g' | head -20

Swap fields

# Swap columns 1 and 2
awk '{temp=$1; $1=$2; $2=temp; print}' file.txt

# Swap with sed capture groups
sed -E 's/^([^ ]+) ([^ ]+)/\2 \1/' file.txt

Remove duplicate lines

# Remove all duplicates, preserving order
awk '!seen[$0]++' file.txt

# Remove duplicates based on a specific column
awk '!seen[$2]++' file.txt

Sum a column

awk '{s+=$2} END{print s}' file.txt

# Sum matching rows from a CSV
awk -F',' '/SALE/ {s+=$4} END{printf "%.2f\n", s}' data.csv

More one-liners

# Print lines between two markers (exclusive)
sed -n '/START/,/END/{/START/d;/END/d;p}' file.txt

# Join lines with commas
paste -sd',' file.txt

# Add a prefix to every line
sed 's/^/PREFIX: /' file.txt

# Remove trailing carriage returns (dos2unix)
sed 's/\r$//' file.txt

# Extract values between quotes
sed -n 's/.*"\([^"]*\)".*/\1/p' file.txt

# Unique values from a column, sorted by frequency
awk '{print $1}' file.txt | sort | uniq -c | sort -rn

sed vs awk: when to use which

Task	Use
Find-and-replace	sed
Delete/filter lines by pattern	sed (or grep)
In-place file editing	sed
Column extraction	awk
Arithmetic, aggregation	awk
Conditional logic per line	awk
Reformatting structured text	awk

Rule of thumb: if you need columns or math, use awk. If you need search-and-replace or line deletion, use sed.

Gotchas

BSD vs GNU sed

Feature	GNU sed (Linux)	BSD sed (macOS)
In-place edit	sed -i 's/.../.../'	sed -i '' 's/.../.../'
\t in replacement	Supported	Not supported (use literal tab)
Case-insensitive flag	s/old/new/I	Not available
Line stepping 1~2	Supported	Not available
\+, \? in basic regex	Supported	Not supported (use -E)

Portable approach: always use -E for extended regex and -i.bak for in-place editing on both platforms.

Quoting in shell

# Use single quotes to prevent shell expansion
sed 's/$HOME/replaced/' file.txt       # $HOME is literal
sed "s/$HOME/replaced/" file.txt       # $HOME is expanded by shell

# To use shell variables in sed, use double quotes
name="world"
sed "s/hello/$name/" file.txt

# Escape slashes with alternate delimiters
sed 's|/usr/local/bin|/opt/bin|g' file.txt
sed 's#old/path#new/path#g' file.txt

# Pass shell variables to awk with -v
name="admin"
awk -v user="$name" '$1 == user {print $2}' file.txt

# Never embed shell variables directly in awk single-quoted blocks
# This does NOT work:
awk '$1 == "$name"' file.txt   # literal string "$name"