What are the common string operations in Shell scripts? How to perform string concatenation, slicing, and replacement?

Common string operations in Shell scripts include concatenation, slicing, replacement, comparison, and length calculation.

String Definition

Basic Definition

bash
# Define strings
str1="Hello World"
str2='Hello World'
str3=Hello

# Multi-line string
str4="Line 1
Line 2
Line 3"

String Concatenation

Simple Concatenation

bash
# Direct concatenation
str1="Hello"
str2="World"
str3=$str1" "$str2
echo $str3  # Output: Hello World

# Use quotes
str4="${str1} ${str2}"
echo $str4  # Output: Hello World

# Multiple string concatenation
str5="Hello" " " "World"
echo $str5  # Output: Hello World

Dynamic Concatenation

bash
# Concatenate variables and text
name="John"
greeting="Hello, $name!"
echo $greeting  # Output: Hello, John!

# Concatenate command output
date_str="Current date: $(date)"
echo $date_str

# Concatenate array elements
arr=("apple" "banana" "cherry")
str="${arr[0]}, ${arr[1]}, ${arr[2]}"
echo $str  # Output: apple, banana, cherry

String Length

Calculate Length

bash
# Calculate string length
str="Hello World"
echo ${#str}  # Output: 11

# Use expr
expr length "$str"  # Output: 11

# Use awk
echo "$str" | awk '{print length}'

String Slicing

Basic Slicing

bash
str="Hello World"

# Slice from specified position
echo ${str:0}    # Output: Hello World
echo ${str:6}    # Output: World

# Slice specified length from position
echo ${str:0:5}  # Output: Hello
echo ${str:6:5}  # Output: World

# Slice from end
echo ${str: -5}  # Output: World
echo ${str: -5:3}  # Output: Wor

Delete Substrings

bash
str="Hello World"

# Delete shortest matching prefix
echo ${str#He}    # Output: llo World
echo ${str#*o}    # Output:  World

# Delete longest matching prefix
echo ${str##*o}   # Output: rld

# Delete shortest matching suffix
echo ${str%ld}    # Output: Hello Wor
echo ${str%o*}    # Output: Hello W

# Delete longest matching suffix
echo ${str%%o*}   # Output: Hell

Extract Filename and Path

bash
filepath="/path/to/file.txt"

# Extract filename
filename=${filepath##*/}
echo $filename  # Output: file.txt

# Extract directory
dirname=${filepath%/*}
echo $dirname   # Output: /path/to

# Extract extension
extension=${filepath##*.}
echo $extension  # Output: txt

# Remove extension
basename=${filename%.*}
echo $basename   # Output: file

String Replacement

Basic Replacement

bash
str="Hello World"

# Replace first match
echo ${str/World/Bash}  # Output: Hello Bash

# Replace all matches
echo ${str//o/O}        # Output: HellO WOrld

# Delete matches
echo ${str/o/}          # Output: Hell World
echo ${str//o/}         # Output: Hell Wrld

Prefix and Suffix Replacement

bash
str="Hello World"

# Replace prefix
echo ${str/#Hello/Hi}   # Output: Hi World

# Replace suffix
echo ${str/%World/Bash} # Output: Hello Bash

Case Conversion

bash
str="Hello World"

# Convert to uppercase
echo ${str^^}  # Output: HELLO WORLD

# Convert to lowercase
echo ${str,,}  # Output: hello world

# Capitalize first letter
echo ${str^}   # Output: Hello world

String Comparison

Basic Comparison

bash
str1="Hello"
str2="World"

# Equality comparison
if [ "$str1" = "$str2" ]; then
    echo "Strings are equal"
fi

# Inequality comparison
if [ "$str1" != "$str2" ]; then
    echo "Strings are not equal"
fi

# Use [[ ]]
if [[ "$str1" == "$str2" ]]; then
    echo "Strings are equal"
fi

Pattern Matching

bash
str="Hello World"

# Check if starts with specified string
if [[ "$str" == Hello* ]]; then
    echo "Starts with Hello"
fi

# Check if ends with specified string
if [[ "$str" == *World ]]; then
    echo "Ends with World"
fi

# Check if contains specified string
if [[ "$str" == *lo* ]]; then
    echo "Contains 'lo'"
fi

Regular Expression Matching

bash
str="Hello123"

# Use =~ for regex matching
if [[ "$str" =~ ^[A-Za-z]+$ ]]; then
    echo "Only letters"
fi

if [[ "$str" =~ ^[A-Za-z0-9]+$ ]]; then
    echo "Letters and numbers"
fi

# Extract matched substring
if [[ "$str" =~ ([A-Za-z]+)([0-9]+) ]]; then
    echo "Letters: ${BASH_REMATCH[1]}"
    echo "Numbers: ${BASH_REMATCH[2]}"
fi

String Splitting

Split Using IFS

bash
str="apple,banana,cherry"

# Set IFS and split
IFS=',' read -ra arr <<< "$str"
echo "${arr[0]}"  # Output: apple
echo "${arr[1]}"  # Output: banana
echo "${arr[2]}"  # Output: cherry

# Iterate over split array
for item in "${arr[@]}"; do
    echo "Item: $item"
done

Split Using cut

bash
str="apple:banana:cherry"

# Split by delimiter
echo "$str" | cut -d: -f1  # Output: apple
echo "$str" | cut -d: -f2  # Output: banana
echo "$str" | cut -d: -f3  # Output: cherry

String Whitespace Removal

Remove Whitespace Characters

bash
str="  Hello World  "

# Remove leading whitespace
str="${str#"${str%%[![:space:]]*}"}"

# Remove trailing whitespace
str="${str%"${str##*[![:space:]]}"}"

echo "$str"  # Output: Hello World

Remove Whitespace Using sed

bash
str="  Hello World  "

# Remove leading whitespace
echo "$str" | sed 's/^[[:space:]]*//'

# Remove trailing whitespace
echo "$str" | sed 's/[[:space:]]*$//'

# Remove all whitespace
echo "$str" | sed 's/[[:space:]]//g'

Practical Application Examples

Validate Input

bash
# Validate email format
validate_email() {
    local email="$1"
    if [[ "$email" =~ ^[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,}$ ]]; then
        echo "Valid email"
        return 0
    else
        echo "Invalid email"
        return 1
    fi
}

validate_email "user@example.com"

Format Output

bash
# Format string
format_string() {
    local name="$1"
    local age="$2"
    local city="$3"
    
    printf "Name: %-20s Age: %3d City: %s\n" "$name" "$age" "$city"
}

format_string "John Doe" 25 "New York"

Batch Rename

bash
# Batch change file extensions
for file in *.txt; do
    new_name="${file%.txt}.bak"
    mv "$file" "$new_name"
done

Log Parsing

bash
# Parse log line
log_line="[2024-01-01 10:00:00] [INFO] User logged in"

# Extract time
time="${log_line#\[}"
time="${time%%\]*}"
echo "Time: $time"

# Extract log level
level="${log_line#*\[}"
level="${level%%\]*}"
echo "Level: $level"

# Extract message
message="${log_line#*\] }"
echo "Message: $message"

String Operation Best Practices

1. Always use quotes: Prevent issues with spaces and special characters
2. Use ${} instead of $(): More efficient for string operations
3. Check string length: Avoid empty string errors
4. Use regex to validate input: Improve data quality
5. Use printf for formatted output: More flexible than echo
6. Be aware of case sensitivity: Use ^ and , for conversion
7. Use arrays to store split results: Easier for subsequent processing