How to define and use arrays in Shell scripts? What are the common array operations?

Array operations in Shell scripts include array definition, access, iteration, and common operations.

Array Definition

Regular Arrays

bash
# Define empty array
arr=()

# Define array (space separated)
arr=(apple banana cherry)

# Define individually
arr[0]="apple"
arr[1]="banana"
arr[2]="cherry"

# Define using command output
arr=($(ls *.txt))

Associative Arrays (Bash 4.0+)

bash
# Declare associative array
declare -A arr

# Define associative array
arr[name]="John"
arr[age]=25
arr[city]="Beijing"

# Define all at once
declare -A arr=([name]="John" [age]=25 [city]="Beijing")

Array Access

Access Single Element

bash
arr=(apple banana cherry)

# Access element at specific index
echo ${arr[0]}    # Output: apple
echo ${arr[1]}    # Output: banana
echo ${arr[2]}    # Output: cherry

# Access last element
echo ${arr[-1]}   # Output: cherry

# Access non-existent index
echo ${arr[10]}   # Output: (empty)

Access All Elements

bash
# Access all elements
echo ${arr[@]}    # Output: apple banana cherry
echo ${arr[*]}    # Output: apple banana cherry

# Access all element indices
echo ${!arr[@]}   # Output: 0 1 2

# Access array length
echo ${#arr[@]}   # Output: 3
echo ${#arr[*]}   # Output: 3

Access Partial Elements

bash
# Access all elements starting from index 1
echo ${arr[@]:1}  # Output: banana cherry

# Access 2 elements starting from index 1
echo ${arr[@]:1:2}  # Output: banana cherry

# Access starting from 2nd to last element
echo ${arr[@]: -2}  # Output: banana cherry

Array Iteration

Iterate All Elements

bash
arr=(apple banana cherry)

# Method 1: Use for loop
for item in "${arr[@]}"; do
    echo "Item: $item"
done

# Method 2: Use index iteration
for i in "${!arr[@]}"; do
    echo "Index $i: ${arr[$i]}"
done

# Method 3: Use C-style loop
for ((i=0; i<${#arr[@]}; i++)); do
    echo "Index $i: ${arr[$i]}"
done

Iterate Associative Array

bash
declare -A arr=([name]="John" [age]=25 [city]="Beijing")

# Iterate keys
for key in "${!arr[@]}"; do
    echo "Key: $key"
done

# Iterate key-value pairs
for key in "${!arr[@]}"; do
    echo "$key: ${arr[$key]}"
done

Array Operations

Add Elements

bash
arr=(apple banana)

# Add to end
arr+=(cherry)
arr+=("date" "fig")

# Add to specific position
arr[2]="cherry"  # Replace or add

# Add using index
arr[${#arr[@]}]="grape"  # Add to end

Delete Elements

bash
arr=(apple banana cherry date fig)

# Delete element at specific index
unset arr[2]  # Delete element at index 2

# Delete entire array
unset arr

# Delete associative array element
declare -A arr=([name]="John" [age]=25)
unset arr[name]

Modify Elements

bash
arr=(apple banana cherry)

# Modify element at specific index
arr[0]="orange"
arr[1]="pear"

# Batch modify
for i in "${!arr[@]}"; do
    arr[$i]="${arr[$i]}_modified"
done

Array Slicing

bash
arr=(one two three four five)

# Slicing
echo ${arr[@]:1:3}  # Output: two three four

# Slice and assign
new_arr=(${arr[@]:1:3})

Array Sorting

Simple Sorting

bash
arr=(banana apple cherry date)

# Lexicographic sort
sorted=($(echo "${arr[@]}" | tr ' ' '\n' | sort))
echo "${sorted[@]}"

# Numeric sort
nums=(5 2 8 1 9)
sorted_nums=($(echo "${nums[@]}" | tr ' ' '\n' | sort -n))
echo "${sorted_nums[@]}"

# Reverse sort
sorted_rev=($(echo "${arr[@]}" | tr ' ' '\n' | sort -r))
echo "${sorted_rev[@]}"

Deduplication

bash
arr=(apple banana apple cherry banana)

# Remove duplicates
unique=($(echo "${arr[@]}" | tr ' ' '\n' | sort -u))
echo "${unique[@]}"

Array Search

Find Element

bash
arr=(apple banana cherry)

# Check if element exists
if [[ " ${arr[@]} " =~ " banana " ]]; then
    echo "Found banana"
fi

# Use function to find
contains_element() {
    local e match="$1"
    shift
    for e; do [[ "$e" == "$match" ]] && return 0; done
    return 1
}

if contains_element "banana" "${arr[@]}"; then
    echo "Found banana"
fi

Find Index

bash
arr=(apple banana cherry)

# Find element index
get_index() {
    local element=$1
    shift
    local arr=("$@")
    for i in "${!arr[@]}"; do
        if [[ "${arr[$i]}" == "$element" ]]; then
            echo $i
            return 0
        fi
    done
    return 1
}

index=$(get_index "banana" "${arr[@]}")
echo "Index of banana: $index"

Practical Application Examples

File Processing

bash
# Get all .txt files
files=($(ls *.txt))

# Iterate and process
for file in "${files[@]}"; do
    echo "Processing: $file"
    # Process file
done

# Check if files exist
if [ ${#files[@]} -eq 0 ]; then
    echo "No .txt files found"
fi

Parameter Processing

bash
# Store parameters in array
args=("$@")

# Iterate parameters
for arg in "${args[@]}"; do
    echo "Argument: $arg"
done

# Check for specific parameter
if [[ " ${args[@]} " =~ " --verbose " ]]; then
    verbose=true
fi

Data Statistics

bash
# Read data into array
readarray -t lines < data.txt

# Count lines
echo "Total lines: ${#lines[@]}"

# Count lines containing specific content
count=0
for line in "${lines[@]}"; do
    if [[ "$line" =~ "pattern" ]]; then
        ((count++))
    fi
done
echo "Matching lines: $count"

Configuration Management

bash
# Parse config file to associative array
declare -A config
while IFS='=' read -r key value; do
    config[$key]="$value"
done < config.txt

# Access configuration
echo "Database: ${config[db_host]}"
echo "Port: ${config[db_port]}"

Array Best Practices

1. Always use quotes: "${arr[@]}" prevents issues with spaces and special characters
2. Use ${#arr[@]} for length: Instead of ${#arr}
3. Use ${!arr[@]} for indices: Safer when iterating
4. Declare associative arrays: Use declare -A for associative arrays
5. Avoid uninitialized indices: Check if index exists
6. Encapsulate complex operations in functions: Improve code readability
7. Note array indices start at 0: Consistent with other programming languages