String Operations

Bash provides powerful built-in string manipulation capabilities. Learn to extract, measure, and transform strings without external tools.

String Length

Get the length of a string:

#!/bin/bash
STR="Hello, World!"
echo ${#STR}    # 13

# Works with variables
NAME="Alice"
echo "Name has ${#NAME} characters"    # 5

Substring Extraction

Extract parts of a string:

#!/bin/bash
STR="Hello, World!"

# ${string:position}
echo "${STR:7}"        # World!

# ${string:position:length}
echo "${STR:0:5}"      # Hello
echo "${STR:7:5}"      # World

# Negative position (from end)
echo "${STR: -6}"      # World!
echo "${STR: -6:5}"    # World

Note: Space before negative number is required!

Exercise: Extract Substring

Get part of a string:

String Replacement

Replace parts of strings:

#!/bin/bash
STR="hello world world"

# Replace first occurrence
echo "${STR/world/universe}"    # hello universe world

# Replace all occurrences
echo "${STR//world/universe}"   # hello universe universe

# Replace at beginning
echo "${STR/#hello/hi}"         # hi world world

# Replace at end
echo "${STR/%world/earth}"      # hello world earth

Removing Substrings

Remove from beginning or end:

#!/bin/bash
FILE="archive.tar.gz"

# Remove shortest match from beginning
echo "${FILE#*.}"      # tar.gz

# Remove longest match from beginning
echo "${FILE##*.}"     # gz

# Remove shortest match from end
echo "${FILE%.*}"      # archive.tar

# Remove longest match from end
echo "${FILE%%.*}"     # archive

Memory Trick

• # removes from the beginning (# is before $ on keyboard)
• % removes from the end (% is after numbers)
• Single = shortest match, Double = longest match

Exercise: Extract Filename

Get filename from path:

Case Conversion

Change string case (Bash 4+):

#!/bin/bash
STR="Hello World"

# Lowercase
echo "${STR,,}"        # hello world
echo "${STR,}"         # hello World (first char only)

# Uppercase
echo "${STR^^}"        # HELLO WORLD
echo "${STR^}"         # Hello World (first char only)

# Toggle case (Bash 4+)
echo "${STR~~}"        # hELLO wORLD

Default Values

Provide defaults for empty/unset variables:

#!/bin/bash

# Use default if unset or empty
echo "${NAME:-Unknown}"    # Unknown if NAME is empty

# Assign default if unset or empty
echo "${NAME:=Default}"    # Sets NAME to Default

# Display error if unset or empty
echo "${NAME:?Error: NAME is required}"

# Use alternative if set
echo "${NAME:+Name is: $NAME}"

Syntax	Meaning
${VAR:-default}	Use default if VAR is unset/empty
${VAR:=default}	Assign default if VAR is unset/empty
${VAR:?error}	Exit with error if VAR is unset/empty
${VAR:+value}	Use value if VAR is set

Concatenation

Join strings together:

#!/bin/bash
FIRST="Hello"
LAST="World"

# Simple concatenation
FULL="$FIRST $LAST"
echo "$FULL"    # Hello World

# Concatenation with braces
PREFIX="pre"
echo "${PREFIX}fix"    # prefix

# Append to variable
STR="Hello"
STR+=" World"
echo "$STR"    # Hello World

Exercise: String Default

Use default value:

Indirect Variable Reference

Access variable by name:

#!/bin/bash
VAR_NAME="greeting"
greeting="Hello, World!"

# Using indirect reference
echo "${!VAR_NAME}"    # Hello, World!

# List all matching variables
echo "${!BASH*}"    # All vars starting with BASH

String Splitting

Split into array:

#!/bin/bash
STR="one,two,three"

# Using IFS
IFS=',' read -ra PARTS <<< "$STR"
echo "${PARTS[0]}"    # one
echo "${PARTS[1]}"    # two
echo "${PARTS[2]}"    # three

# Loop through parts
for part in "${PARTS[@]}"; do
    echo "Part: $part"
done

Quick Reference

# Length
${#string}

# Substring
${string:start}
${string:start:length}

# Replacement
${string/old/new}       # First
${string//old/new}      # All
${string/#old/new}      # Beginning
${string/%old/new}      # End

# Removal
${string#pattern}       # Beginning (shortest)
${string##pattern}      # Beginning (longest)
${string%pattern}       # End (shortest)
${string%%pattern}      # End (longest)

# Case
${string,,}             # Lowercase
${string^^}             # Uppercase

# Defaults
${string:-default}      # Use default
${string:=default}      # Assign default

Key Takeaways

• ${#VAR} gives string length
• ${VAR:start:length} extracts substrings
• ${VAR/old/new} replaces first match, // replaces all
• # removes from beginning, % removes from end
• ${VAR,,} lowercases, ${VAR^^} uppercases
• ${VAR:-default} provides default for empty variables
• Use += to append to strings
• These are all built-in - no external commands needed!