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

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

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