10 ‐ List & Arrays - CloudScope/DevOpsWithCloudScope GitHub Wiki

In shell scripting, arrays and lists are essential data structures that allow you to store and manipulate collections of items. Here’s a breakdown of their usage and key points:

Arrays in Shell

1. Definition:

   • In shell (particularly Bash), arrays are zero-indexed collections of elements that can be accessed using indices.
   • Bash supports one-dimensional arrays.

2. Declaring Arrays:

   • Arrays can be declared explicitly using parentheses or assigned values using the declare command.
   • Examples:

     # Explicit declaration
     array=(item1 item2 item3)
     
     # Using declare command
     declare -a array=(item1 item2 item3)
     

3. Accessing Array Elements:

   • Individual elements can be accessed using their index.
   • Example:

     echo ${array[0]}  # Outputs: item1
     

4. Modifying Arrays:

   • You can update elements by specifying their index.
   • Example:

     array[1]="newItem"  # Changes item2 to newItem
     

5. Array Operations:

   • Length: To get the number of elements in an array:

     echo ${#array[@]}  # Outputs the length of the array
     

   • All Elements: To access all elements:

     echo ${array[@]}  # Prints all elements in the array
     

   • Iterating over Arrays:

     for item in "${array[@]}"; do
         echo $item
     done
     

6. Associative Arrays (Bash 4+):

   • Associative arrays use strings as keys.
   • Declaring associative arrays:

     declare -A assocArray
     assocArray[key1]="value1"
     assocArray[key2]="value2"
     

7. Accessing Associative Array Elements:

   • Example:

     echo ${assocArray[key1]}  # Outputs: value1
     

Lists in Shell

In shell scripting, lists are often just space-separated strings or newline-separated items that can be processed with commands like for, while, or tools like read and xargs.

1. Creating Lists:

   • Lists are typically created by space-separated values or by using commands like seq.
   • Example:

     list="item1 item2 item3"
     

2. Accessing and Iterating over Lists:

   • Use loops to iterate:

     for item in $list; do
         echo $item
     done
     

3. Converting Strings to Arrays:

   • You can convert a list (string) to an array by splitting on whitespace.
   • Example:

     array=($list)
     echo ${array[0]}  # Outputs: item1
     

4. Handling Lists with Commands:

   • To handle multi-line output, use commands like mapfile (or readarray in Bash).
   • Example:

     mapfile -t lines < <(command)
     for line in "${lines[@]}"; do
         echo $line
     done
     

Key Points

• Arrays provide indexed access and are more structured compared to simple lists.
• Associative arrays allow mapping of keys to values and are available in newer versions of Bash.
• Lists are more flexible for iteration and are commonly used in shell scripts for handling command outputs.