array expansion

 

1) Accessing Elements

[root@oel01db ~]# fruits=("Apple" "Banana" "Cherry")
[root@oel01db ~]#
[root@oel01db ~]# echo "${fruits[@]}"
Apple Banana Cherry
[root@oel01db ~]#

To print each element on a new line:

[root@oel01db ~]# printf '%s\n' "${fruits[@]}"
Apple
Banana
Cherry
[root@oel01db ~]#

How printf "%s\n" "${fruits[@]}" Works

The Template

• %s → Placeholder for a string

• \n → Newline

The Data

${fruits[@]} contains:

• Apple

• Banana

• Cherry

The Execution (Implicit Loop Behavior)

printf automatically loops over all arguments:

• Apple → inserted into %s → newline

• Banana → inserted into %s → newline

• Cherry → inserted into %s → newline

This is called the implicit loop of printf.

Common Format Specifiers

Specifier	Data Type	Example	Output
%s	String	printf "%s" "Hello"	Hello
%d	Integer	printf "%d" 10	10
%f	Float	printf "%f" 1.5	1.500000
%q	Quoted String	printf "%q" "Space Word"	Space\ Word

---

2) Counting Elements and Lengths

Number of Elements

[root@oel01db ~]# echo ${#fruits[@]}
3
[root@oel01db ~]#

${#array[@]} → Returns total number of elements.

---

Length of a Specific Element

[root@oel01db ~]# echo ${#fruits[1]}
6
[root@oel01db ~]#

Index 1 = "Banana" → length = 6.

${#array[index]} → Returns string length of that element.

---

3) Array Slicing

Syntax:

${array[@]:start:length}

Length is optional. If omitted → Bash returns everything from start to end.

---

Print Full Array

[root@oel01db ~]# printf '%s\n' "${fruits[@]}"
Apple
Banana
Cherry
[root@oel01db ~]#

---

From Index 1 to End

[root@oel01db ~]# printf '%s\n' "${fruits[@]:1}"
Banana
Cherry
[root@oel01db ~]#

Explanation

• Index 0 → Apple (Skipped)

• Index 1 → Banana (Included)

• Index 2 → Cherry (Included)

---

Exactly One Element from Index 1

[root@oel01db ~]# printf '%s\n' "${fruits[@]:1:1}"
Banana
[root@oel01db ~]#

---

First Two Elements

[root@oel01db ~]# printf '%s\n' "${fruits[@]:0:2}"
Apple
Banana
[root@oel01db ~]#

---

Last Element

[root@oel01db ~]# printf '%s\n' "${fruits[@]: -1}"
Cherry
[root@oel01db ~]#

Note: There must be a space before -1.

---

4) Search and Replace in Arrays

Syntax:

${array[@]/search/replace}

Example:

[root@oel01db ~]# apps=("vscode" "vlc" "vim")
[root@oel01db ~]#
[root@oel01db ~]# echo ${apps[@]/v/X}
Xscode Xlc Xim
[root@oel01db ~]#

Using printf:

[root@oel01db ~]# printf '%s\n' "${apps[@]/v/X}"
Xscode
Xlc
Xim
[root@oel01db ~]#

This replaces the first occurrence of v in every element.

---

5) Cutting Characters (Pattern Removal)

• # → Remove from front

• % → Remove from back

---

Remove from End

[root@oel01db ~]# printf '%s\n' "${fruits[@]%x}"
Apple
Banana
Cherry
[root@oel01db ~]#

Remove e from end:

[root@oel01db ~]# printf '%s\n' "${fruits[@]%e}"
Appl
Banana
Cherry
[root@oel01db ~]#

---

Remove from Beginning

[root@oel01db ~]# printf '%s\n' "${fruits[@]#A}"
pple
Banana
Cherry
[root@oel01db ~]#

---

6) Getting Indexes and Keys

${fruits[@]} → Values
${!fruits[@]} → Index numbers (or keys)

---

Indexed Array Example

[root@oel01db ~]# fruits=(Apple Banana Cherry)
[root@oel01db ~]#
[root@oel01db ~]# printf 'Index: %s\n' "${!fruits[@]}"
Index: 0
Index: 1
Index: 2
[root@oel01db ~]#

---

Associative Arrays (Key-Value Pairs)

[root@oel01db ~]# unset colors
[root@oel01db ~]#
[root@oel01db ~]# declare -A colors
[root@oel01db ~]# colors[apple]="red"
[root@oel01db ~]# colors[banana]="yellow"
[root@oel01db ~]# colors[cherry]="dark red"
[root@oel01db ~]#
[root@oel01db ~]# printf 'Value: %s\n' "${colors[@]}"
Value: dark red
Value: red
Value: yellow
[root@oel01db ~]#
[root@oel01db ~]# printf 'Key: %s\n' "${!colors[@]}"
Key: cherry
Key: apple
Key: banana
[root@oel01db ~]#

In associative arrays:

• ${colors[@]} → Values

• ${!colors[@]} → Keys